Classifications

• • 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
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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 steel materials and tanks for transporting liquid carbon dioxide.
• Steel can be used as a constituent material 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 applications at low temperatures.
• 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 9).
• 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
• Low temperature steels used in low temperature pressure vessels are expected to achieve both high strength and low temperature toughness.
• low temperature pressure vessels are manufactured by welding steel materials, and post-weld heat treatment (sometimes called PWHT) is sometimes performed to remove residual stresses caused by welding.
• PWHT post-weld heat treatment
• the objective of this disclosure is to provide a steel material and a tank for transporting liquid carbon dioxide that are suitable for low-temperature applications, having high tensile strength and good low-temperature toughness regardless of whether or not they are subjected to post-weld heat treatment.
• the gist of the present disclosure is as follows. ⁇ 1> 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, and has a chemical composition in which ⁇ represented by the following formula (1) is 4.0 or more and 16.0 or less, The tens
• ⁇ 4> The steel material according to any one of ⁇ 1> to ⁇ 3>, wherein the chemical composition includes the following Group C: [Group C] one or more selected from the group consisting of Mg: 0.0003% or more, 0.0200% or less, Ca: 0.0003% or more, 0.0200% or less, and REM: 0.0003% or more, 0.0200% or less.
• Group C [Group C] one or more selected from the group consisting of Mg: 0.0003% or more, 0.0200% or less, Ca: 0.0003% or more, 0.0200% or less, and REM: 0.0003% or more, 0.0200% or less.
• ⁇ 5> The steel material according to any one of ⁇ 1> to ⁇ 4>, wherein an aspect ratio of prior austenite grains at a portion that is 1 ⁇ 4 of the thickness from a surface of the steel material in a thickness direction is 1.5 or more.
• ⁇ 6> The steel material according to any one of ⁇ 1> to ⁇ 4>, wherein the aspect ratio of prior austenite grains at a portion from the surface of the steel material to 1/4 of the thickness in the thickness direction is less than 1.5.
• ⁇ 7> The steel material according to any one of ⁇ 1> to ⁇ 6>, wherein the microstructure at a portion of the steel material that is 1 ⁇ 4 of the thickness from the surface in the thickness direction has an average crystal grain size of 20.0 ⁇ m or less.
• ⁇ 8> The steel material according to any one of ⁇ 1> to ⁇ 7>, having a Charpy impact absorption energy of 150 J or more at ⁇ 100° C.
• ⁇ 9> The steel material according to any one of ⁇ 1> to ⁇ 8>, wherein when the steel material 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 steel material is held at 600°C for 2 hours, the Charpy impact absorption energy at -100°C in a place subjected to the heat treatment is 150 J or more.
• a tank for transporting liquid carbon dioxide, comprising the steel material according to any one of ⁇ 1> to ⁇ 9>.
• the present disclosure makes it possible to provide a steel material and a tank for transporting liquid carbon dioxide that are suitable for low-temperature applications, having high tensile strength and good low-temperature toughness regardless of whether the steel is subjected to post-weld heat treatment or not.
• FIG. 1 is a diagram showing an example of a discrimination result of a microstructure.
• 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 material or “base material” refers to a steel material portion that does not include a surface treatment layer such as a plating layer, a coating film, etc.
• a surface treatment layer such as a plating layer, a coating film, etc. may be formed on the surface of the steel material according to the present disclosure.
• 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.
• 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 reheating and quenching, performed tensile tests, and observed the microstructure.
• the microstructure of the 1/4t portion of steel materials with tensile strengths 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 improve the toughness of steel materials.
• the toughness of steel materials is ensured by the composition of the microstructure.
• the inventors of the present disclosure have taken samples from the 1/4t section of steel materials that have been reheated and quenched, and further subjected to intermediate heat treatment, conducted Charpy impact tests, and observed the microstructure.
• steel materials with a Charpy impact absorption energy of 150 J or more at -100°C have a total area ratio of lower bainite and martensite of 15.0% or more, and an area ratio of retained austenite of 0.2% or more and less than 5.0%.
• the total area ratio of lower bainite and martensite was measured using EBSD.
• the area ratio of retained austenite was measured by X-ray diffraction.
• the volume ratio of retained austenite measured by X-ray diffraction can be considered as an area ratio.
• the inventors of the present disclosure have conducted research to ensure the toughness of the steel material.
• the toughness of the steel material is ensured by reducing the area surrounded by the high-angle grain boundaries, where the difference in crystal orientation is 15° or more.
• the inventors of the present disclosure collected samples from the 1/4t part of the steel material after reheating and quenching, and measured the circle-equivalent diameter of the area surrounded by the high-angle grain boundaries by EBSD.
• the circle-equivalent diameter of the area surrounded by the high-angle grain boundaries is referred to as the grain size.
• the samples were mechanically polished and electrolytically polished, and an analysis was performed in an area of 4 mm 2 using an EBSD device attached to an FE-SEM (field emission scanning electron microscope).
• the average grain size (sometimes referred to as the "effective grain size") was calculated by the area-weighted average of the grain sizes measured in the area of 4 mm 2, weighted by the area of each grain. It has been found that if the average crystal grain size of the 1 ⁇ 4 t portion of the steel material is 20.0 ⁇ m or less, the toughness of the steel material tends to be further improved regardless of whether it is before or after post-weld heat treatment.
• the inventors of this disclosure also investigated steel material that had been subjected to direct quenching, in which the material was water-cooled after hot rolling, and then to intermediate heat treatment, and obtained results similar to those obtained with steel material that had been reheated and quenched.
• 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 set to 0.50% or less in the present disclosure.
• the Si content is preferably set to 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. 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. Although there is no lower limit for the N content, in terms of manufacturing costs, in the present disclosure, 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 according to 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.
• the B content is preferably 0.0030% or less, or 0.0020% or less.
• the steel material according to 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 is an element that has the effect of refining the metal structure and improving the strength of the steel material. Therefore, in the present disclosure, the Nb content may be 0.001% or more.
• the toughness of the HAZ 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 of the steel material 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 and the weldability may be affected. From the viewpoint of suppressing deterioration of the mechanical properties, the Ti content is 0.050% or less, and preferably 0.040% or less, 0.030% or less, or 0.020% or less. Particularly from the viewpoint of ensuring the toughness of the steel material 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 in the present disclosure is set to 0.0200% or less.
• 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 the steel material and the 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, 0.0060% or less, or 0.0040% or less.
• the balance of the chemical composition of the steel material according to 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 range of the ⁇ value is set to 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 part 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 of the steel material according to the present disclosure at a portion from the surface to 1 ⁇ 4 of the thickness in the thickness direction includes lower bainite, martensite, and retained austenite.
• the bainite may include upper bainite in addition to lower bainite.
• “Bainite” is a structure that contains 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 that contains retained austenite or MA phase (mixed martensite-austenite phase) between the laths, and upper bainite that contains carbides between the laths.
• “Lower bainite” is lath-shaped lower bainite that contains 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. From the viewpoint of ensuring the toughness of the steel material, the area ratios of the lower bainite and martensite in the 1/4t portion are 15.0% or more.
• the area ratios of the lower bainite and martensite in the 1/4t portion are preferably 20.0% or more, or 30.0% or more.
• the sum of the area ratios of the lower bainite and martensite in the 1/4t portion may be 95.0% or more.
• Total area ratio of upper bainite, lower bainite and martensite 90.0% or more
• the total area ratio of the upper bainite, lower bainite and martensite in the 1/4t portion is 90.0% or more.
• the total area ratio of the upper bainite, lower bainite and martensite in the 1/4t portion may be 95.0% or more.
• the upper bainite in the 1/4t portion may be 1.0% or more.
• the retained austenite increases the toughness of the steel material.
• the area ratio of the retained austenite in the 1/4t portion is 0.2% or more.
• the area ratio of the retained austenite in the 1/4t portion is preferably 0.3% or more, or 0.5% or more.
• the area ratio of the retained austenite in the 1/4t portion of the steel plate according to the present disclosure is at most less than 5%.
• the area ratio of the retained austenite in the 1/4t portion is preferably 3.0% or less, or 2.0% or less.
• 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, martensite, and retained austenite 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, upper bainite, lower bainite, martensite, and retained austenite 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 GAM is 0.5 or less is ferrite, and the region where GAM is more than 0.5 is upper bainite, lower bainite, martensite, or retained austenite.
• upper bainite, lower bainite, martensite, and retained austenite are determined using the EBSD GAM as a threshold value, and therefore include not only upper bainite, lower bainite, martensite, and retained austenite, but also tempered upper bainite, tempered lower bainite, and tempered martensite. Comparing the structure of direct quenching (DQ) and that followed by tempering (DQT), after tempering, decomposition of MA and coarsening of carbides occur, but the appearance of the structure does not change significantly.
• DQ direct quenching
• DQT tempering
• the region surrounded by a white line is upper bainite (Bu), and the other regions are lower bainite + martensite (B L +M).
• the part determined to be upper bainite (Bu) has sparse carbides that look white and coarse and dense regions are mixed.
• carbides are dense and uniformly present.
• the total area fraction of lower bainite, martensite, and retained austenite is obtained by subtracting the area fraction of upper bainite from the total area fraction of upper bainite, lower bainite, martensite, and retained austenite measured above.
• the total area fraction of lower bainite and martensite is obtained by determining the area fraction of retained austenite using a measurement method described later and subtracting it from the total area fraction of lower bainite, martensite, and retained austenite.
• 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 area ratio of the retained austenite is measured by X-ray diffraction.
• the area ratio of the retained austenite is measured using a sample with a measurement surface at a portion of 1/4 of the thickness from the surface of the steel material in the thickness direction (also referred to as "1/4t portion" in this specification).
• the sample is a 2 mm thick test piece, taken from a position of 1/4 of the width from the end of the width direction (direction perpendicular to the rolling direction and thickness direction) of the steel material, chemically polished, and used to measure the volume ratio of the retained austenite by X-ray diffraction using a Mo tube.
• Quantitative analysis was performed based on the ratio of the integrated intensity of the (200) and (211) diffraction peaks of the ferrite phase to the integrated intensity of the (200), (220), and (311) diffraction peaks of the austenite phase, and the average value of six combinations was adopted.
• the integrated intensity of the diffraction peak is obtained by fitting the background based on the signals before and after the peak and subtracting the signal.
• the volume ratio measured by X-ray diffraction is regarded as the area ratio.
• the toughness of the steel material can be further improved. This is because flattening the prior austenite grains increases the grain boundary area, which substantially refines the austenite grains and is effective in refining the average crystal grain size.
• the aspect ratio of the prior austenite grains is usually 4.0 or less, and may be 3.5 or less.
• the aspect ratio of the prior austenite grains in the 1/4t portion may be less than 1.5 from the viewpoint of ensuring homogeneity of the microstructure.
• the aspect ratio of the prior austenite grains in the 1/4t portion may be preferably 1.4 or less, or 1.3 or less.
• the aspect ratio of prior austenite crystal grains (sometimes referred to as prior austenite grains) of a steel material is determined as follows: First, an L-section (a section parallel to the rolling direction and thickness direction of the steel material) at a location 1/4 of the thickness from the surface of the steel material in the thickness direction is mirror-polished, and then etched with an etchant based on a saturated aqueous solution of 2 to 4% picric acid to reveal prior austenite grain boundaries in an arbitrary region of 1.0 mm in the rolling direction ⁇ 0.5 mm in the thickness direction. Next, the long and short diameters of each prior austenite grain are measured, and the aspect ratio of each prior austenite grain is calculated by dividing the long diameter by the short diameter.
• the arithmetic average of all the aspect ratios of the prior austenite grains thus calculated is determined as the "aspect ratio of the prior austenite grains.”
• the long diameter is the maximum length of the prior austenite grain, and the short diameter is the maximum distance between two lines parallel to the long diameter direction that contact the grain.
• the average grain size (effective grain size) of the 1/4t part of the steel material is preferably 20.0 ⁇ m or less. This is because it has been found that if the average grain size of the 1/4t part of the steel material is 20.0 ⁇ m or less, the toughness of the steel material tends to be further improved regardless of whether it is before or after PWHT.
• the average grain size of the 1/4t part of the steel material may be more than 20.0 ⁇ m. The smaller the average grain size of the steel material, the more preferable it is, so the lower limit is not limited. Usually, the average grain size is 10 ⁇ m or more.
• the effective grain size is calculated by weighted averaging.
• the steel material according to the present disclosure has mechanical properties that balance strength and low-temperature toughness, and is particularly excellent in 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.
• the steel material selected for such applications is a steel material having the above-mentioned tensile strength, so the steel material according to the present disclosure is also manufactured to have the above-mentioned tensile strength.
• the steel material of the present disclosure preferably has a Charpy impact absorption energy of 150 J or more at -100°C. Since the steel material of the present disclosure has low-temperature toughness with a Charpy impact absorption energy of 150 J or more at -100°C, a transport tank made of the steel material of the present disclosure can be suitably used for transporting liquid carbon dioxide, for example.
• the Charpy impact absorption energy at -100°C is a value measured using a sample taken from a 1/4 position of the thickness.
• PWHT Charpy impact absorption energy at -100°C after PWHT
• PWHT may be performed on the welded parts after assembly into a transport tank. At this time, not only the welded parts but also the base material part of the steel material that is not affected by welding (also simply referred to as the base material) are heated. If the time for which the base material is heated to a temperature range of 425°C or more is long, the toughness of the base material tends to decrease.
• the toughness of the part where the PWHT was performed is preferably 150J or more in Charpy impact absorption energy at -100°C.
• the Charpy impact absorption energy at -100°C after PWHT may be 100J or more.
• the Charpy impact absorption energy at -100°C after PWHT is also a value measured using a sample taken from the 1/4 position of the thickness.
• the steel material of the present disclosure preferably has a Charpy impact absorption energy of 50 J or more at -100 ° C. after the thermal cycle. Since the steel material of the present disclosure has a low-temperature toughness of 50 J or more at -100 ° C. after the thermal cycle, a transport tank made of the steel material of the present disclosure can be suitably used, for example, for transporting liquid carbon dioxide.
• the Charpy impact absorption energy at -100 ° C. after the thermal cycle may be 40 J or more.
• thermal cycle is a value measured by using a sample taken from a 1/4 position of the thickness of the steel material as a thermal cycle test piece, heating it to 1350 ° C. at 60 ° C. / s, holding it at 1350 ° C. for 1 s, and then cooling it to room temperature at 20 ° C. / s, and then taking a Charpy test piece from there.
• PWHT Charpy impact absorption energy at -100°C after thermal cycle
• the steel material of the present disclosure is subjected to PWHT in which the heating rate and cooling rate are 55°C/h in a temperature range of 425°C or higher and the steel is held at 600°C for 2 hours, and then Charpy test pieces are taken and measured.
• the toughness of the portion where the PWHT was performed is preferably 50 J or more in terms of Charpy impact absorption energy at -100°C.
• the Charpy impact absorption energy at -100°C of the portion where the PWHT was performed after the thermal cycle test may be 40 J or more.
• PWHT can sometimes reduce the toughness of steel. The reason for this is not clear, but it is presumed that the diffusion of P (phosphorus) and Mn to grain boundaries and the growth or aggregation of inclusions in the structure reduce brittleness and therefore toughness. The reduction in toughness due to PWHT can be suppressed by limiting the P and Mn content and reducing the average crystal grain size of the steel.
• 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 Charpy impact absorption energy 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 For the Charpy impact test, a V-notch test piece is used, which is taken from a 1/4 thickness position of the steel material and has a longitudinal direction parallel to the width direction of the steel material (C direction).
• the V-notch of the test piece is formed so that the longitudinal direction of the notch is the thickness direction of the steel material and the depth direction of the notch is the rolling direction of the steel material.
• the shape of the steel material according to the present 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.
• the steel material disclosed herein has mechanical properties that combine strength and low-temperature toughness, and is particularly capable of exhibiting excellent low-temperature toughness even after PWHT, making it suitable for use as a constituent material for tanks that store and transport liquefied gases, particularly liquid carbon dioxide.
• a steel satisfying the above-mentioned chemical composition is melted, and then a steel slab is produced by continuous casting.
• the steel slab is heated and hot rolled, and then directly water-cooled and quenched (DQ), or is hot rolled, cooled, reheated, and water-cooled (RQ).
• DQ directly water-cooled and quenched
• RQ water-cooled
• L intermediate heat treatment
• T tempering
• the heating temperature of the rolled material 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 rolled material is preferably 1000°C or more from the viewpoint of reducing the deformation resistance.
• the heating temperature of the rolled material is 1250°C or less from the viewpoint of suppressing the coarsening of heated ⁇ grains.
• the heating temperature of the rolled material 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 30% or more, 40% or more, or 50% 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 30% or more, 40% or more, or 50% or more from the viewpoint of refining the average crystal grain size of the steel material.
• the cumulative reduction in non-recrystallization temperature region rolling is determined from the difference between the thickness of the rolled material at 900° C. and the thickness of the steel material after the rolling is completed.
• Cumulative reduction rate (%) of non-recrystallization temperature region rolling 100 x ([thickness of rolled material at 900 ° C.] - [thickness of steel material after rolling]) / [thickness of rolled material at 900 ° C.]
• 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 reheating temperature of the steel is Ac 3 or more because quenching is performed from an austenite single phase structure.
• the reheating temperature of the steel is preferably 850°C or more, 880°C or more, or 900°C or more.
• the upper limit of the reheating temperature is not particularly specified, but since heating to an excessively high temperature may cause austenite grains to coarsen and lead to a decrease in toughness, it is preferably 1000°C or less, 950°C or less, or 930°C or less.
• Ac 3 is a value calculated by the above-mentioned formula.
• the steel material is subjected to intermediate heat treatment (L).
• the intermediate heat treatment is held in a temperature range of Ac 1 or more and Ac 3 or less in order to ensure stable retained austenite even at low temperatures.
• the holding time is preferably 20 minutes or more.
• the holding time is preferably 120 minutes or less.
• Cooling is preferably water cooling (quenching).
• Ac 1 is a value calculated by the following formula.
• the steel may be subjected to a tempering treatment.
• the heating temperature in the tempering treatment is preferably 660°C or less, or 640°C or less, from the viewpoint of suppressing a decrease in strength.
• the heating temperature in the tempering treatment is preferably 400°C or more, 450°C or more, or 500°C or more.
• Bu upper bainite BL: lower bainite M: martensite Retained
• ⁇ retained austenite Toughness was measured by measuring the average value of the Charpy impact absorption energy at -100°C (KV2) 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.
• Nos. 1 to 19, 101 to 109 are examples of the present invention, and Nos. 20, 21, 22, and 110 ⁇ 116 are comparative examples.
• No. 20 since ⁇ was too small, sufficient hardenability, sufficient strength, and sufficient low-temperature toughness were not obtained.
• No. 21 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 ⁇ value was less than the lower limit of the present disclosure, and the hardenability was insufficient, the strength was insufficient, and sufficient low-temperature toughness was not obtained.
• Nos. 20 since ⁇ was too small, sufficient hardenability, sufficient strength, and sufficient low-temperature toughness were not obtained.
• No. 21 the Mn content was too high, and therefore sufficient low-temperature toughness was not obtained either before or after the PWHT.
• No. 22 the
• the ⁇ value exceeded the upper limit of the present disclosure, the hardenability was too high, and the strength was excessively high.
• the total area ratio of lower bainite and martensite was insufficient, and sufficient low-temperature toughness was not obtained either before or after PWHT.
• the chemical composition and microstructure of the steel material 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 at -100°C is high, and low-temperature toughness of 150 J or more is obtained.
• the steel material disclosed herein can be used primarily as a material for transport tanks for liquefied carbon dioxide.
• the steel material disclosed herein can also be used to manufacture other welded structures, such as buildings, bridges, ships, line pipes, marine structures, pressure vessels and tanks, etc.

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