Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K9/00—Arc welding or cutting
  • B23K9/23—Arc welding or cutting taking account of the properties of the materials to be welded
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
• • 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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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

Definitions

• This disclosure relates to welded joints.
• Steel can be used for welded structures such as buildings, bridges, ships, line pipes, marine structures, pressure vessels, and tanks. Steel has excellent strength and low-temperature toughness, making it effective for low-temperature applications.
• Low-temperature steels are used for low-temperature pressure vessels such as storage tanks for liquefied gas.
• Low-temperature steels include Al-killed steel, nickel steel, high Mn steel, and austenitic stainless steel, depending on the operating temperature.
• nickel steels such as 3.5% Ni steel are used as materials for tanks that carry liquefied ethane and liquefied ethylene, which have operating temperatures of around -100°C.
• Ni is often included in steel materials that require low-temperature toughness, such as this 3.5% Ni steel, such as low-temperature pressure vessels.
• Patent Document 1 proposes a nickel-containing steel material for low temperature use having excellent toughness, which has a specific chemical composition containing 2.7% or more and 5.0% or less of Ni, a prior austenite grain size during quenching heating of 20 ⁇ m or less, an effective crystal grain size after heat treatment of 12 ⁇ m or less, and a tensile strength of 450 MPa or more and 690 MPa or less. Furthermore, various steel materials with defined chemical compositions and microstructures (metal structures) have been proposed with the aim of achieving low-temperature toughness and high strength (see, for example, Patent Documents 2 to 11).
• Patent Document 1 JP 2019-81930 A Patent Document 2: International Publication No. 2014/103629 Patent Document 3: JP 52-156121 A Patent Document 4: JP 55-104427 A Patent Document 5: JP 58-73717 A Patent Document 6: JP 7-331328 A Patent Document 7: JP 2001-123222 A Patent Document 8: JP 2001-123245 A Patent Document 9: JP 2007-46096 A Patent Document 10: JP 2-254120 A Patent Document 11: JP 2002-224835 A
• the low-temperature steel used in low-temperature pressure vessels is expected to have both high strength and low-temperature toughness.
• Cryogenic pressure vessels are manufactured by welding steel materials, and may be subjected to post-weld heat treatment (sometimes called PWHT) to remove residual stress caused by welding.
• PWHT post-weld heat treatment
• the objective of this disclosure is to provide a welded joint suitable for low-temperature applications, which uses a steel material with high tensile strength as the base material and has good low-temperature toughness regardless of whether it is before or after post-weld heat treatment.
• the gist of the present disclosure is as follows. ⁇ 1> A welded joint having a base material and a weld made of a steel material, wherein the chemical composition of the base material is, in mass%, C: 0.03% or more, 0.20% or less, Si: 0.01% or more, 0.50% or less, Mn: 0.10% or more, 1.65% or less, P: 0.025% or less, S: 0.0250% or less, Ni: 2.65% or more, 4.45% or less, Al: 0.001% or more, 0.100% or less, O: 0.0100% or less, N: 0.0100% or less, Cu: 0 to 1.50%, Cr: 0-3.00%, Mo: 0-2.00%, B: 0 to 0.0050%, Nb: 0 to 0.050%, Ti: 0 to 0.050%, V: 0 to 0.10%, Mg: 0 to 0.0200%, Ca: 0-0.0200%, REM: 0-0.0200%, The balance is Fe and impurities,
• ⁇ 6> The steel material according to any one of ⁇ 1> to ⁇ 5>, wherein, when the welded joint is subjected to a heat treatment in which the heating rate and the cooling rate are 55°C/h in a temperature range of 425°C or more and the welded joint is held at 600°C for 2 hours, the Charpy impact absorption energy at -100°C of the welded heat affected zone at the location where the heat treatment is performed is 70J or more.
• a welded joint suitable for low-temperature applications which uses a steel material with high tensile strength as the base material and has good low-temperature toughness regardless of whether it is before or after post-weld heat treatment.
• FIG. 1 is a diagram showing an example of a discrimination result of a microstructure.
• FIG. 1 is a schematic diagram showing an example of a portion from which a test specimen used for the Charpy impact absorption energy of a weld heat affected zone is taken from a L-shaped weld joint.
• FIG. 1 is a schematic diagram showing another example of a portion from which a test specimen used for the Charpy impact absorption energy of a weld heat affected zone is taken from a square weld joint.
• FIG. 1 is a schematic diagram showing an example of a portion from which a test specimen used for Charpy impact absorption energy of a weld heat affected zone is taken from a K-type weld joint.
• FIG. 1 is a diagram showing an example of a discrimination result of a microstructure.
• FIG. 1 is a schematic diagram showing an example of a portion from which a test specimen used for the Charpy impact absorption energy of a weld heat affected zone is taken from
• FIG. 1 is a schematic diagram showing another example of a portion from which a test specimen used for the Charpy impact absorption energy of a weld heat affected zone is taken from a K-type welded joint.
• FIG. 2B is a schematic perspective view showing a notch shape of the test piece in FIG. 2A.
• post-weld heat treatment refers to a post-weld heat treatment conforming to the contents specified in JIS Z 3700:2009 "Post-weld heat treatment method".
• steel or base material refers to a steel portion that does not include a surface treatment layer such as a plating layer or a coating film. However, a surface treatment layer such as a plating layer or a coating film may be formed on the surface of the steel and welded joint in this disclosure.
• base material refers to a steel portion that is not affected by welding, in contrast to the welded portion (weld metal and welded heat affected zone) in a welded joint.
• Welded heat affected zone refers to a steel portion that is thermally affected by welding.
• a numerical range expressed using “to” means a range that includes the numerical values before and after “to” as the lower and upper limits. However, when the numerical values before and after “to” are followed by “more than” or “less than,” the numerical range does not include these numerical values as the lower or upper limit. With respect to the contents of elements in chemical compositions, “%” means “mass %".
• process includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.
• the inventors of the present disclosure have conducted research to improve the strength of steel materials used in the manufacture of welded joints.
• the tensile strength of steel materials is ensured by the composition of the microstructure.
• the inventors of the present disclosure collected samples from the 1/4t portion (t: thickness of steel material), which is a portion of 1/4 of the thickness in the thickness direction from the surface of the steel material after hot rolling and accelerated cooling, performed tensile tests, and observed the microstructure.
• the microstructure of the 1/4t portion of steel materials with a tensile strength of 590 MPa or more and 930 MPa or less has an area ratio of ferrite of less than 10.0%, and the total area ratio of upper bainite, lower bainite, and martensite of 90.0% or more.
• the total area ratio of upper bainite, lower bainite, and martensite was measured using electron backscatter diffraction (hereinafter referred to as "EBSD").
• the inventors of the present disclosure have conducted studies to ensure the toughness of welded joints using the above steel material.
• the toughness of a welded joint is ensured by reducing the area surrounded by high-angle grain boundaries in which the difference in crystal orientation is 15° or more.
• the inventors of the present disclosure have taken samples from the heat-affected zone of a welded joint, and mechanical polishing and electrolytic polishing have been performed on the samples.
• the circular equivalent diameter of the area surrounded by the high-angle grain boundaries was measured using an EBSD device attached to an FE-SEM (field emission scanning electron microscope).
• the circular equivalent diameter of the area surrounded by the high-angle grain boundaries is referred to as the grain size.
• the area where the grain size is measured is the area between the fusion line (hereinafter sometimes referred to as "FL") position and a position 1 mm away from the fusion line (hereinafter "FL + 1 mm").
• the grain size was measured in an area of 4 mm2 along the fusion line.
• the effective grain size (sometimes referred to as the "effective grain size in the weld heat affected zone” in this disclosure) was calculated as the average of the top 10 largest grain sizes among those measured in a 4 mm2 region. It was found that if the effective grain size in the weld heat affected zone of a weld joint is 100.0 ⁇ m or less, the toughness of the weld joint tends to be further improved regardless of whether it is before or after post-weld heat treatment.
• the inventors of the present disclosure have conducted research to reduce the effective grain size in the weld heat affected zone of a welded joint.
• the effective grain size in the weld heat affected zone of a welded joint is ensured by the microstructural configuration of the base material made of steel.
• the inventors of the present disclosure collected samples from the 1/4t part of the steel, observed the microstructure, and also conducted a Charpy impact test on the weld heat affected zone of a welded joint produced by welding steel. As a result, it was found that the microstructure of steel with a Charpy impact absorption energy of 70 J or more at -100°C in the weld heat affected zone has a total area ratio of lower bainite and martensite of 15.0% or more. The total area ratio of lower bainite and martensite was measured using EBSD.
• C is an element that increases the strength of steel materials. From the viewpoint of ensuring the strength of steel materials used in structures, the C content is 0.03% or more in the present disclosure.
• the C content is preferably
• C is an element that reduces the toughness of the weld heat affected zone (hereinafter sometimes referred to as "HAZ"). From the viewpoint of ensuring toughness, the C content is 0.20% or less in the present disclosure.
• the C content is preferably 0.16% or less, 0.14% or less, or 0.12% or less. be.
• Silicon is used as a deoxidizer and is an element that dissolves in steel to increase strength. From the viewpoint of controlling the O concentration in the molten steel, the present disclosure specifies a silicon content of 0.01 % or more.
• the Si content is preferably 0.03% or more, 0.05% or more, or 0.10% or more.
• the Si content is 0.50% or less in the present disclosure.
• the Si content is preferably 0.30% or less, Or, it is 0.20% or less.
• Mn 0.10% or more, 1.65% or less
• Mn is used as a deoxidizer and is an element that enhances the hardenability of steel and contributes to high strength.
• the Mn content is 0. . 10% or more.
• 0.10% or more of Mn forms MnS, reducing the amount of dissolved S and preventing hot cracking.
• the Mn content is preferably 0.30% or more, or 0.50% or more.
• the Mn content is 1.65% or less in the present disclosure.
• the Mn content is preferably 1.50% or less, 1.25% or less, or 1.10% or less.
• P 0.025% or less
• the P content may be 0.001% or more.
• the P content is 0.025% or less. Preferably, it is 0.016% or less, 0.012% or less, or 0.008% or less.
• S is an impurity element.
• the S content may be 0.0001% or more.
• the S content is preferably 0.0100% or less, or 0.0050% or less.
• Ni 2.65% or more, 4.45% or less
• the Ni content is 2.65% or more.
• the Ni content is preferably 3.00% or more.
• the Ni content is 3.20% or more.
• the Ni content is 4.45% or less.
• the Ni content is preferably 4.10% or less, or 3.80% or less.
• Al 0.001% or more, 0.100% or less
• Al is an element useful for deoxidization and also forms nitrides to refine the grain size during quenching. Therefore, in this disclosure, the Al content is set to 0.001%. However, if Al is contained in excess, Al may form coarse nitrides, which may reduce the toughness of the steel material and the HAZ. Therefore, the Al content is set to 0.100% or less.
• the Al content is preferably 0.080% or less, or 0.050% or less.
• O is an impurity element.
• the O content may be 0.0001% or more.
• the O content is excessive, If the content of O is too high, coarse oxides are generated, and the toughness and ductility of the steel material and the HAZ may deteriorate. From the viewpoint of ensuring the toughness and ductility of the steel material and the HAZ, the O content is 0.0100% or less.
• the O content is preferably 0.0060% or less, or 0.0040% or less.
• N is an impurity element.
• the N content may be 0.0001% or more.
• Steel properties and toughness of HAZ From the viewpoint of ensuring this, in the present disclosure, the N content is 0.0100% or less.
• the N content is preferably 0.0050% or less, or 0.0040% or less.
• the steel material according to the present disclosure may contain other elements (selective elements) in place of a portion of the Fe.
• selective elements from groups A to C may be included, but the content of these elements may be 0%.
• the steel material in the present disclosure may contain, as necessary, one or more of the optional elements Cu, Cr, Mo, and B shown below, which have the effect of improving hardenability.
• Cu is an element that may be mixed into steel materials during the manufacturing process.
• the lower limit of the Cu content is not limited and may be 0%.
• Cu also has an adverse effect on weldability and HAZ toughness. Since Cu has little adverse effect and has the effect of increasing the hardenability of steel, it is also an element that improves the strength of the steel material. Therefore, in the present disclosure, the Cu content may be 0.01% or more.
• the Cu content is The Cu content is preferably 0.10% or more. However, from the viewpoint of suppressing the occurrence of Cu cracks during hot rolling of the steel material, the Cu content is 1.50% or less in the present disclosure.
• the Cu content is It is preferably 1.00% or less, 0.80% or less, 0.60% or less, or 0.50% or less.
• Cr 3.00% or less
• Cr is an element that may be mixed into steel during the manufacturing process.
• the lower limit of the Cr content is not limited and may be 0%. Cr also has the effect of improving the hardenability of steel. Therefore, in the present disclosure, the Cr content may be 0.01% or more.
• the Cr content is preferably 0.10% or more.
• the Cr content is 3.00% or less in the present disclosure.
• the Cr content is preferably 2.20% or less, 1.40% or less, Or, it is 0.80% or less.
• Mo is an element that may be mixed into steel during the manufacturing process.
• the lower limit of the Mo content is not limited and may be 0%.
• Mo is also an element that improves the strength of steel because it has the effect of increasing the hardenability of steel. Therefore, in the present disclosure, the Mo content may be 0.01% or more.
• the Mo content is preferably 0.05% or more, 0.10% or more, 0.20% or more, or 0.30% or more.
• the Mo content is 2.00% or less.
• the Mo content is preferably 1.20% or less, or 0.80% or less.
• B is an element that may be mixed into steel during the manufacturing process.
• the lower limit of the B content is not limited and may be 0%.
• B also has a significant effect of improving the hardenability of steel. Therefore, in the present disclosure, the B content may be 0.0003% or more.
• the B content is 0.0050% or less in the present disclosure.
• the B content is preferably 0.0030% or less, or 0.0020% or less.
• the steel material in the present disclosure may contain, as necessary, one or more of the optional elements Nb, Ti, and V shown below, which have the effect of increasing the strength of the steel material by forming precipitates such as carbides and nitrides.
• Nb is an element that may be mixed into steel during the manufacturing process.
• the lower limit of the Nb content is not limited and may be 0%.
• Nb also forms carbides and nitrides.
• Nb has the effect of refining the metal structure and is also an element that improves the strength of the steel material. Therefore, in the present disclosure, the Nb content may be 0.001% or more.
• the toughness and From the viewpoint of suppressing deterioration of weldability the Nb content is 0.050% or less.
• the Nb content is preferably 0.040% or less, or 0.030% or less.
• the Nb content in the HAZ after PWHT is 0.050% or less. From the viewpoint of ensuring toughness, the Nb content may be 0.004% or less.
• Ti 0.050% or less
• Ti is an element that may be mixed into steel during the manufacturing process.
• the lower limit of the Ti content is not limited and may be 0%.
• Ti also forms carbides and nitrides.
• Ti has the effect of refining the metal structure and is also an element that improves the strength of the steel material. Therefore, in the present disclosure, the Ti content may be 0.001% or more.
• the toughness of the HAZ or the weldability may be deteriorated. From the viewpoint of suppressing deterioration of the properties, the Ti content is 0.050% or less.
• the Ti content is preferably 0.040% or less, 0.030% or less, or 0.020% or less. From the viewpoint of ensuring the HAZ toughness after PWHT, the Ti content may be 0.004% or less, or 0.002% or less.
• V is an element that may be mixed into steel during the manufacturing process.
• the lower limit of the V content is not limited and may be 0%.
• V also forms carbides and nitrides.
• V is also an element that improves the strength of steel materials. Therefore, in the present disclosure, the V content may be 0.01% or more.
• the V content is 0.10% or less, preferably 0.08% or less, or 0.05% or less.
• the steel material according to the present disclosure may contain one or more of the optional elements Mg, Ca, and REM shown below, as necessary.
• Mg is an element that may be mixed into steel during the manufacturing process.
• the lower limit of the Mg content is not limited and may be 0%.
• Mg forms an oxide, It is also an element that improves the toughness of the weld heat affected zone. Therefore, in the present disclosure, the Mg content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more. If the content is excessive, coarse oxides are formed, which may reduce the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, the Mg content is set to 0.0200% or less in the present disclosure.
• the Mg content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.
• Ca is an element that may be mixed into steel during the manufacturing process.
• the lower limit of the Ca content is not limited and may be 0%.
• Ca also dissolves sulfides in the steel into spherical particles. It is also an element that reduces the effect of MnS, which reduces the toughness of steel and weld heat affected zone, by increasing the Ca content. Therefore, in the present disclosure, the Ca content is set to 0.0003% or more, 0.0006% or more, or 0.
• the Ca content is 0.0200% or less.
• the Ca content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.
• Rare earth metals are a collective term for 17 elements, including two elements, Sc and Y, and 15 lanthanoid elements, such as La, Ce, and Nd.
• the REM content is the total content of the 17 elements.
• REM is an element that may be mixed into steel during the manufacturing process.
• the lower limit of the REM content is not limited and may be 0%.
• REM also forms oxides.
• REM is also an element that improves the toughness of the weld heat affected zone. Therefore, in the present disclosure, the REM content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more.
• the REM content in the present disclosure is set to 0.0200%.
• the REM content is preferably 0.0100% or less, 0.006% or less, or 0.0040% or less.
• the balance of the chemical composition of the steel material in the present disclosure is iron (Fe) and impurities.
• the impurities refer to components that are mixed in due to raw materials such as ores and scraps or other factors during industrial production of the steel material.
• ⁇ value ( ⁇ value: 4.0 or more and 16.0 or less)
• [C], [Si], [Mn], [Cu], [Ni], [Cr] and [Mo] are the contents (mass%) of C, Si, Mn, Cu, Ni, Cr and Mo in the steel. If the corresponding element is not contained, substitute zero. Note that ⁇ [C] is synonymous with [C] 1/2 .
• the ⁇ value is in the range of 4.0 to 16.0. This is an index showing the hardenability of the steel material, and the higher the ⁇ value, the more the lower bainite and martensite structures with a favorable balance of strength and toughness can be formed.
• ⁇ is in the appropriate range, the ratio of the lower bainite and martensite structures with a favorable balance of strength and toughness in the HAZ structure is high, and HAZ toughness can be ensured.
• ⁇ is 4.0 or more, the hardenability of the base material is ensured, the ratio of the lower bainite and martensite structures with a favorable balance of strength and toughness increases, and toughness deterioration is suppressed.
• the ratio of the lower bainite and martensite in the structure of the HAZ portion is likely to increase, and the HAZ toughness is also improved.
• the ⁇ value is 16.0 or less, the strength of the steel material does not become too high, and toughness can be ensured.
• the ⁇ value is 16.0 or less, toughness after PWHT can be ensured.
• the HAZ does not become too hard, and HAZ toughness can be ensured.
• the ⁇ value is preferably 4.5 or more, or 5.0 or more.
• the ⁇ value is preferably 15.5 or less, or 15.0 or less.
• the microstructure at a portion of the steel material from the surface to 1 ⁇ 4 of the thickness in the thickness direction in the present disclosure includes lower bainite and martensite.
• the bainite may include upper bainite in addition to lower bainite.
• Boinite is a structure containing bainitic ferrite ( ⁇ °B) with a substructure within the grains, and is a general term for upper bainite and lower bainite.
• Upper bainite is either or both of upper bainite containing retained austenite or MA phase (mixed martensite-austenite phase) between the laths, and upper bainite containing carbides between the laths.
• Lower bainite is lath-shaped lower bainite containing carbides within the laths.
• Lath martensite exists in four forms: lath, butterfly, lens, and thin plate, but the components disclosed in this disclosure mainly form lath martensite.
• Lath martensite is composed of packets and blocks consisting of groups of laths in a specific arrangement, and is a structure in which one austenite grain is divided into several packets.
• the lower bainite and martensite are hard phases and increase the toughness of the steel material and the HAZ. From the viewpoint of ensuring HAZ toughness, the area ratio of the lower bainite and martensite in the 1/4t portion of the steel material is 15.0% or more.
• the area ratio of the lower bainite and martensite in the 1/4t portion is preferably 20.0% or more, or 30.0% or more.
• the sum of the area ratio of the lower bainite and the area ratio of the martensite in the 1/4t portion may be 100%.
• Total area ratio of upper bainite, lower bainite and martensite in 1/4t part of steel 90.0% or more
• the total area ratio of the upper bainite, lower bainite and martensite in the 1/4t portion of the steel material is 90.0% or more.
• the total area ratio of the upper bainite, lower bainite and martensite in the 1/4t portion may be 100%.
• the upper bainite in the 1/4t portion may be 1.0% or more.
• Observation of the microstructure of steel is carried out using a sample with the 1/4t part of the steel as the observation surface.
• Two types of samples are prepared: (a) electrolytic polishing, and (b) nital etching. Measurements are taken at three locations for each of samples (a) and (b) using the method described below, and the average of the three locations is taken as the area ratio of the microstructure of the steel. Three samples each of (a) and (b) may be prepared and the average taken for each sample, or measurements may be taken at three locations within a single sample and the average taken.
• the total area ratio of upper bainite, lower bainite and martensite is measured by EBSD using electrolytically polished samples that are mechanically polished to a mirror finish and then electrolytically polished to remove the distorted layer caused by mechanical polishing.
• the measurement magnification is 200 times, and measurements are made over an area of 400 ⁇ m x 400 ⁇ m at a pitch of 0.4 ⁇ m.
• the measurement is performed with an electron beam diameter of 0.4 ⁇ m or less.
• the confidence index (CI value) is set to 0.1 or more.
• the judgment between ferrite and upper bainite, lower bainite and martensite is performed by setting the threshold value of Grain Average Misorientation (GAM) to 0.5.
• GBM Grain Average Misorientation
• the GAM value is an index defined in OIM-Analysis (EBSD crystal orientation analysis software manufactured by TSL, USA).
• the region where the GAM is 0.5 or less is ferrite, and the region where the GAM is more than 0.5 is upper bainite, lower bainite, or martensite.
• upper bainite, lower bainite, and martensite are determined using the EBSD GAM as a threshold value, and therefore include not only upper bainite, lower bainite, and martensite, but also tempered upper bainite, tempered lower bainite, and tempered martensite.
• the region surrounded by a white line is upper bainite (Bu), and the other regions are lower bainite + martensite (BL + M).
• the part determined to be upper bainite (Bu) has sparse carbides that look white and coarse and dense regions are mixed.
• carbides are densely and uniformly present.
• the total area fraction of lower bainite and martensite is obtained by subtracting the area fraction of upper bainite from the total area fraction of upper bainite, lower bainite, and martensite measured above. When the sum of the area ratios of lower bainite and martensite is more than 0% by the above structure determination, lower bainite and martensite are usually included.
• Lower bainite and martensite can be distinguished by SEM or TEM (transmission electron microscope), and the presence of each structure can be confirmed.
• the effective grain size of the welded heat affected zone is 100.0 ⁇ m or less.
• the effective grain size of the welded heat affected zone is preferably 95.0 ⁇ m or less, or 90.0 ⁇ m or less.
• the lower limit of the effective grain size of the welded heat affected zone is not particularly limited, but the effective grain size of the welded heat affected zone may be, for example, 50.0 ⁇ m or more, or 60.0 ⁇ m or more.
• the welded joint having the base material and the welded portion made of the steel material in the present disclosure has mechanical properties that combine the strength of the steel material and the low-temperature toughness of the welded heat-affected zone.
• the welded heat-affected zone has excellent toughness at -100°C, and can also exhibit excellent low-temperature toughness even after PWHT.
• the tensile strength of the steel material is set to 590 to 930 MPa.
• a steel material that can ensure the strength of the structure even if it is thin is required.
• the steel material selected for such applications is a steel material having the above-mentioned tensile strength, so the steel material in the present disclosure is also manufactured to have the above-mentioned tensile strength.
• the Charpy impact absorption energy of the welded heat affected zone at -100°C is preferably 70 J or more. Since the welded joint of the present disclosure has low-temperature toughness with the Charpy impact absorption energy of the welded heat affected zone at -100°C of 70 J or more, a transport tank made of the welded joint of the present disclosure can be suitably used for transporting liquid carbon dioxide, for example.
• the Charpy impact absorption energy of the welded heat affected zone at -100°C is a value measured using samples 14 taken from a region including the fusion line position (FL) of the welded zone 12 and a region including a position 1 mm away from the fusion line (FL+1 mm) at the 1/4t portion of the base material 10 of the welded joint 20, as shown in Figures 2A, 2B, 3A, and 3B.
• 15 is the weld metal
• 16 is the welded heat affected zone (HAZ).
• the notch 18 of the test piece 14 is formed so that the length direction of the notch 18 is parallel to the thickness direction Y of the base material 10 and the depth direction of the notch 18 is parallel to the length direction Z of the base material 10.
• the length direction Z of the base material 10 is the rolling direction
• the width direction X is perpendicular to the rolling direction Z and the thickness direction Y.
• PWHT Charpy impact absorption energy at -100°C of weld heat affected zone after PWHT
• PWHT may be performed on the welded portion after assembly into a transport tank.
• the time during which the welded joint is heated to a temperature range of 425°C or higher is long, the HAZ toughness tends to decrease.
• PWHT may reduce HAZ toughness. The reason is unclear, but it is presumed that P (phosphorus) and Mn diffuse to the grain boundaries and that growth or aggregation of inclusions occurs in the structure, reducing brittleness and reducing toughness. The reduction in toughness due to PWHT can be suppressed by limiting the P and Mn content and reducing the effective grain size of the weld heat affected zone.
• the tensile strength (TS) and the yield strength (YS) in the examples are measured by a tensile test in accordance with JIS Z2241:2011.
• a JIS 14A test piece is used, which is taken from the 1/4 thickness position and has a longitudinal direction parallel to the width direction of the steel material (C direction).
• TS and YS are measured using three test pieces, and are calculated by averaging them.
• the toughness of the steel material is evaluated by the brittle-ductile transition temperature (vTrs) by a Charpy impact test using an impact blade with a radius of 2 mm in accordance with the provisions of JIS Z2242:2018.
• the Charpy impact test is performed on three pieces at five temperatures, the brittle fracture rate is measured, and the vTrs is calculated.
• a V-notch test piece is used, which is taken from the 1/4t position of the steel material and has a longitudinal direction parallel to the width direction of the steel material (C direction).
• the Charpy impact absorption energy of the weld heat affected zone is measured by a Charpy impact test at -100°C using an impact blade with a radius of 2 mm in accordance with the provisions of JIS Z2242:2018.
• the Charpy impact absorption energy is measured using three test pieces and calculated by averaging them.
• V-notch test piece 14 is used, which is cut from a weld joint 20 at a position corresponding to the 1/4t part of the steel material, as shown in Figures 2A, 2B, 3A, 3B, and 4.
• the shape of the steel material in this disclosure is not particularly limited, and may be steel plate, steel strip, shaped steel, steel pipe, etc.
• steel pipe and shaped steel include steel material formed by joining steel plates, for example, welded steel pipe and welded shaped steel, as well as shaped steel joined with rivets.
• the thickness of steel material such as steel plate, steel strip, shaped steel, steel pipe, etc. (thickness of flange for shaped steel) is not particularly limited, and is usually 3 mm or more and 150 mm or less.
• the thickness of the steel material may be 6 mm or more, 10 mm or more, 15 mm or more, or 30 mm or more.
• the thickness of the steel material may also be 100 mm or less, 80 mm or less, or 60 mm or less.
• welded joints have mechanical properties that combine strength and low-temperature toughness, and can exhibit excellent low-temperature toughness even after PWHT, making them suitable for use as tanks for storing and transporting liquefied gases, particularly liquid carbon dioxide.
• the manufacturing method of the steel material in the present disclosure is not particularly limited, but the steel material in the present disclosure is, for example, produced by melting steel satisfying the above-mentioned chemical composition and then continuously casting a steel slab.
• the steel slab is heated, hot-rolled, and then directly water-cooled (direct quenching (DQ)), or allowed to cool, reheated, and water-cooled (reheat quenching (RQ)) to produce a steel material.
• DQ direct quenching
• RQ reheat quenching
• intermediate heat treatment L
• tempering tempering
• the manufacturing process after hot rolling is selected from the above combinations of DQ, RQ, L, and T, and is, for example, DQT, RQT, DQLT, and RQLT.
• DQT Direct Quenching (DQ), Tempering (T)
• RQT Natural cooling, reheat quenching (RQ), tempering (T)
• DQLT Direct quenching (DQ), intermediate heat treatment (L), tempering (T)
• RQLT Natural cooling, reheat quenching (RQ), intermediate heat treatment (L), tempering (T)
• DQT is preferred for manufacturing the steel materials in this disclosure, and an example of a preferred manufacturing process is shown below.
• the heating temperature of the steel slab to be hot rolled is Ac 3 or more from the viewpoint of performing hot rolling in a temperature range where the metal structure of the rolled material is austenite.
• the heating temperature of the steel slab is preferably 1000°C or more from the viewpoint of reducing the deformation resistance.
• the heating temperature of the hot rolling is 1250°C or less from the viewpoint of suppressing the coarsening of heated ⁇ grains.
• the heating temperature of the hot rolling is preferably 1200°C or less.
• Ac 3 is a value calculated by the following formula.
• Hot rolling may be performed by rolling in a temperature range where recrystallization occurs (recrystallization temperature range rolling) and rolling in a temperature range where recrystallization is suppressed (non-recrystallization temperature range rolling).
• Recrystallization temperature region rolling is hot rolling performed at a temperature of 900°C or higher during rolling.
• the cumulative reduction in recrystallization temperature region rolling is preferably 20% or more, and more preferably 30% or more, from the viewpoint of refining the austenite grain size of the steel material.
• Non-recrystallization temperature region rolling is hot rolling performed at a temperature of the rolled material during rolling of less than 900° C.
• the cumulative reduction in non-recrystallization temperature region rolling is preferably 20% or more, more preferably 30% or more, from the viewpoint of refining the grain size of the steel material.
• the end temperature of the hot rolling is Ar 3 or more from the viewpoint of suppressing the formation of ferrite that reduces strength.
• the steel material is subjected to accelerated cooling such as water cooling.
• the start temperature of the accelerated cooling is Ar 3 or more from the viewpoint of suppressing the formation of ferrite that reduces strength.
• Ar 3 is a value calculated by the following formula.
• Ar 3 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo+0.35(t-8)
• the element symbols in the formula represent the content (mass%) of each element contained in the steel material, and t represents the thickness (mm) of the steel material.
• the cooling rate is 1.0°C/s or more.
• the cooling rate of the accelerated cooling is preferably 5.0°C/s or more, or 10.0°C/s or more.
• the cooling rate is a value calculated by simulating the cooling rate at a 1/4 position of the thickness using heat transfer calculation.
• the accelerated cooling stop temperature is 400°C or lower from the viewpoint of improving the strength of the steel by ensuring upper bainite, lower bainite, and martensite.
• the accelerated cooling stop temperature is preferably 350°C or lower. Accelerated cooling may be performed down to room temperature.
• the accelerated cooling stop temperature is preferably 100°C or higher from the viewpoint of dehydrogenating the steel.
• the steel After accelerated cooling, the steel may be subjected to a tempering treatment.
• the heating temperature of the tempering treatment is preferably 650°C or less, 620°C or less, or 590°C or less.
• the heating temperature of the tempering treatment is preferably 350°C or more, or 400°C or more.
• a groove is formed at the end of the steel material (base material) in the present disclosure described above.
• the shape of the groove is any one of a single bevel groove, a single J groove, a single U groove, a double bevel groove, a double J groove, and a double U groove.
• the grooves are butted together and welding is performed using a welding material.
• the welding material is not particularly limited and is appropriately determined according to the characteristics of the desired welded joint.
• the welding method is a covered arc welding, a gas-shielded arc welding, a submerged arc welding, a TIG welding, etc.
• the welding heat input and the number of welding passes are appropriately determined according to the welding method and the plate thickness.
• the welding heat input is, for example, about 1.0 to 6.0 kJ/mm. Specific welding conditions for gas shielded arc welding and submerged arc welding are shown below. In gas shielded arc welding, a mixture of Ar and CO2 gas is used, with the ratio of CO2 gas being 20%.
• the groove angle is 10R at the tip, 30° for both the upper and lower layers
• the preheat temperature is 100-150°C
• the interpass temperature is 100-150°C
• the gas flow rate is 15-20L/min for the lower layer and 18-22L/min for the upper layer
• the material that forms the weld metal is YM-69F (solid wire manufactured by Nippon Steel Welding Co., Ltd.).
• the current is 250-270A
• the voltage is 27-33V
• the heat input is 1.5-2.4kJ/mm
• the welding speed is 30-40cm/min.
• the number of passes is determined depending on the plate thickness.
• a notch is introduced at the fusion line (FL) or at a position 1 mm away from the fusion line (FL+1 mm) as shown in Figures 2A, 2B, 3A, 3B, and 4, and a Charpy impact test is performed.
• the groove shape is a square, J-shaped, U-shaped, K-shaped, double-sided J-shaped, or H-shaped
• the V-notch at FL is placed on the tangent line of the fusion line on the weld heat affected zone side, and includes 80% or more of the weld heat affected zone.
• the V-notch at FL+1 mm is placed at a position 1 mm away from the FL notch.
• the grain size of the metal structure near FL is roughly determined depending on the heat input of welding. Therefore, the absorbed energy of the weld heat affected zone is not dependent on the groove shape, but is largely dependent on the heat input.
• GMAW multi-layer gas shielded arc welding
• a mixture of Ar and CO2 gas is used, with the ratio of CO2 gas being 20%.
• the groove angle is 10R at the tip, 30° for both the upper and lower layers, with a preheat temperature of 100-150°C, an interpass temperature of 100-150°C, a gas flow rate of 17L/min for the lower layer, and 20L/min for the upper layer.
• the welding was performed using YM-69F (solid wire manufactured by Nippon Steel Welding Co., Ltd.) on the weld metal.
• the current was 260A
• the voltage was 30V
• the heat input was 2.0kJ/mm
• the welding speed was 35cm/min.
• the number of passes was changed depending on the plate thickness.
• the above steel material was used to prepare a R-shaped groove, and multi-layer submerged arc (SAW) welding was performed to produce a welded joint.
• SAW multi-layer submerged arc
• the toughness of the steel material was measured by vTrs.
• the toughness of the weld heat affected zone was measured by making a notch at FL or FL+1 mm on the I side of a K-shaped groove or a R-shaped groove, and performing a Charpy impact test at -100°C.
• the HAZ toughness was measured by measuring the average value of the Charpy impact absorption energy (KV2) at -100°C when the welded joint was as-produced (As-weld toughness), and the average value of the Charpy impact absorption energy at -100°C after PWHT with a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and a cooling rate of 55°C/h in a temperature range of 425°C or higher.
• KV2 Charpy impact absorption energy
• the toughness was measured for each of the following cases: (1) no heat treatment was performed on the steel (base material), (2) heat treatment was performed after welding, and (3) no heat treatment was performed after welding. In each case, the measurements were performed using samples taken from a portion at 1/4 of the thickness.
• (2) Welded joint toughness The average value of the Charpy impact absorption energy at -100°C after preparing a K-shaped groove or a L-shaped groove, performing multi-layer gas shielded arc welding (GMAW) with a heat input of 2.0 kJ/mm, or performing multi-layer submerged arc welding (SAW) with a heat input of 4.0 kJ/mm, and manufacturing a welded joint.
• GMAW multi-layer gas shielded arc welding
• SAW multi-layer submerged arc welding
• Nos. 1 to 19 are examples of the present invention, and Nos. 20, 21, and 22 are comparative examples.
• No. 20 since ⁇ was too small, sufficient hardenability, sufficient strength, and sufficient low-temperature toughness were not obtained.
• the Mn content was too high, and therefore sufficient low-temperature toughness was not obtained either before or after the PWHT.
• No. 22 the ⁇ value was too high, so the strength was excessive and sufficient low-temperature toughness was not obtained either before or after PWHT.
• the chemical composition and microstructure of the steel are appropriately controlled, and the tensile strength is within an appropriate range of 590 MPa or more and 930 MPa or less.
• the Charpy impact absorption energy of the weld heat-affected zone at -100°C is high, and a low-temperature HAZ toughness of 70 J or more is obtained, regardless of whether it is before or after PWHT.
• the welded joints disclosed herein are suitable for use in transport tanks for liquefied carbon dioxide.
• the welded joints disclosed herein can also be used in other welded structures, such as buildings, bridges, ships, line pipes, marine structures, pressure vessels and tanks, etc.

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