WO2024071422A1 - Tôle d'acier - Google Patents

Tôle d'acier Download PDF

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Publication number
WO2024071422A1
WO2024071422A1 PCT/JP2023/035791 JP2023035791W WO2024071422A1 WO 2024071422 A1 WO2024071422 A1 WO 2024071422A1 JP 2023035791 W JP2023035791 W JP 2023035791W WO 2024071422 A1 WO2024071422 A1 WO 2024071422A1
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Prior art keywords
less
mass
steel plate
value
steel
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PCT/JP2023/035791
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English (en)
Japanese (ja)
Inventor
信幸 吉村
遼太郎 白石
竜一 本間
史寿 高峰
弘宜 若松
武史 大久保
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日本製鉄株式会社
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Application filed by 日本製鉄株式会社 filed Critical 日本製鉄株式会社
Priority to JP2024501495A priority Critical patent/JP7469734B1/ja
Publication of WO2024071422A1 publication Critical patent/WO2024071422A1/fr

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/38—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling sheets of limited length, e.g. folded sheets, superimposed sheets, pack rolling
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C1/00—Refining of pig-iron; Cast iron
    • C21C1/02—Dephosphorising or desulfurising
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

  • the present invention relates to a steel sheet.
  • This application claims priority based on Japanese Patent Application No. 2022-157410, filed on September 30, 2022, the contents of which are incorporated herein by reference.
  • CCS Carbon Dioxide Capture and Storage
  • the steel plate used as the material for the transport tank is required to have high strength and excellent low-temperature toughness.
  • the strength is required to have a tensile strength of 780 N/ mm2 or more.
  • the low-temperature toughness is required to be excellent in the most severe condition, evaluated by a Charpy test at minus 65°C (-65°C), for a plate thickness of 20 to 60 mm used as a transport tank, although this depends on the plate thickness.
  • the test temperature of the Charpy test is minus 65°C because the Charpy test is a small-scale test and is generally evaluated at a certain temperature lower than the usage temperature depending on the plate thickness.
  • stress relief annealing may be performed on the welds to reduce the possibility of fracture.
  • Stress relief annealing is a heat treatment method in which the welds of a welded structure are heated to a temperature below the Ac1 transformation point and then slowly cooled in order to reduce the residual stress caused by welding.
  • stress relief annealing is applied to high-tensile steel with a tensile strength of 780 N/mm2 or more , alloy carbides are selectively precipitated at the grain boundaries, and these alloy carbides cause grain boundary embrittlement, which significantly reduces the toughness of the area where stress relief annealing is performed. This phenomenon is generally called SR (Stress Relieving) embrittlement.
  • SR embrittlement is highly likely to occur in high-tensile steel that contains B and is manufactured by quenching and tempering.
  • high-tensile steel not only the embrittlement of the base material but also the embrittlement of the weld heat-affected zone obtained when a welded joint is created using this high-tensile steel is significant.
  • the base material and welds (especially the weld heat-affected zones) have excellent low-temperature toughness even after stress relief annealing is performed.
  • Patent Document 1 shows a high-strength steel plate characterized by adjusting the chemical composition and setting the average crystal grain size to 15 ⁇ m or less.
  • the low-temperature toughness at -65°C of the steel plate described in Patent Document 1 has not been evaluated, and there is room for further improvement in low-temperature toughness.
  • the present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a steel plate suitable for a liquefied CO2 transport tank, which is excellent in the strength of the base material and the low-temperature toughness of the base material and the welded heat-affected zone, and further excellent in the strength of the base material after stress relief annealing and the low-temperature toughness of the base material and the welded heat-affected zone.
  • a steel sheet according to one embodiment of the present invention is The chemical composition, in mass%, is C: 0.07 to 0.11%, Si: 0.10 to 0.15%, Mn: 0.70 to 1.20%, Ni: 1.00 to 2.50%, Cr: 0.20 to 0.80%, Mo: 0.20 to 0.80%, V: 0.005 to 0.070%, Al: 0.010 to 0.100%, B: 0.0005 to 0.0030%, N: 0.0015 to 0.0050%, P: 0.006% or less, S: 0.0030% or less, Cu: 0 to 1.00%, Nb: 0 to 0.030%, Ti: 0 to 0.010%, Ca: 0 to 0.0030%, Mg: 0 to 0.0030%, REM: 0 to 0.0030%, O: 0.0040% or less, The balance is Fe and impurities.
  • an ⁇ value defined by the following formula (1) of 1.00 to 1.50 mass% A ⁇ value defined by the following formula (2) is 10.0 to 15.0
  • a ⁇ value defined by the following formula (3) is 0.70 to 1.50 mass%
  • the Ceq value defined by the following formula (4) is 0.550 to 0.620 mass%
  • Yield strength is 670 to 870 N/mm 2
  • Tensile strength of 780 to 940 N/mm 2 Charpy absorbed energy at -65°C is 100 J or more
  • the average hardness at 121 measurement points is 265 Hv to 290 Hv, with a standard deviation of 20 or less.
  • the plate thickness is 10 to 60 mm.
  • [C], [Si], [P], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb], [Ti], and [V] represent the contents (mass %) of C, Si, P, Mn, Cu, Ni, Cr, Mo, Nb, Ti, and V, respectively, and elements that are not contained, including the amount of elements mixed in as impurities, are substituted with 0.
  • [fB] calculated by the following formulas (A) to (E) may be 0.0003 mass% or more.
  • [fB] [B] - 0.77 x [fN] ...
  • [fN] [N] - 0.29 x [fTi] - 0.52 x [fAl] ...
  • [fTi] [Ti] - 2 ⁇ [fO] ...
  • [fAl] [Al] - 1.125 ⁇ [fO] ...
  • [fO] [O] - 0.4 x [Ca] - 0.66 x [Mg] - 0.11 x [REM] ...
  • [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] represent the contents (mass%) of B, N, Ti, Al, O, Ca, Mg, and REM, respectively. Elements that are not contained, including the amount of elements mixed in as impurities, are substituted with 0, and when the calculated values of [fN], [fTi], [fAl], and [fO] are less than 0%, 0 is substituted.
  • the steel sheet when the steel sheet is subjected to stress relief annealing at a holding temperature of 600°C for 2 hours and a heating rate and a heating rate of 55°C/hr or less in a temperature range of 425°C or more, the steel sheet may have a yield strength of 670 to 870 N/ mm2 , a tensile strength of 780 to 940 N/ mm2 , and a Charpy absorbed energy at -40°C of 27 J or more at a location where the stress relief annealing has been performed.
  • a steel plate having excellent strength of the base material and low-temperature toughness of the base material and the welded heat affected zone, and further having excellent strength of the base material and low-temperature toughness of the base material and the welded heat affected zone after stress relief annealing.
  • This steel plate is suitable for use in liquefied CO2 transport tanks.
  • stress relief annealing means stress relief annealing conforming to the contents specified in JIS Z 3700:2022 "Post-weld heat treatment method” unless otherwise specified.
  • welding means welding with a welding heat input of 1.1 to 4.5 kJ/mm unless otherwise specified.
  • C is an element that improves the strength of the base material.
  • the C content is set to 0.07% or more.
  • the C content is preferably 0.08% or more.
  • the C content is set to 0.11% or less, preferably 0.10% or less, and more preferably less than 0.10%.
  • Silicon is an element that is generally contained in steel in many cases as a deoxidizing element. In order to contain silicon for the purpose of deoxidization, the silicon content is set to 0.10% or more. On the other hand, Si is an element that reduces the toughness of steel after stress relief annealing. In addition, in order to suppress the decrease in the toughness of the welded heat affected zone after stress relief annealing (SR), it is preferable that the Si content is low. Therefore, in the steel plate according to this embodiment, the Si content is set to 0.15% or less. The Si content is preferably set to 0.14% or less, more preferably 0.13% or less, and even more preferably 0.12% or less.
  • Mn 0.70 to 1.20%
  • Mn is an element effective for deoxidization and also improves the strength of steel, therefore the Mn content is set to 0.70% or more, preferably 0.90% or more.
  • the Mn content is set to 1.20% or less, and preferably 1.10% or less.
  • Ni is an element effective for improving the hardenability and toughness of steel, and therefore the Ni content is set to 1.00% or more, and preferably 1.20% or more.
  • the Ni content is set to 2.50% or less.
  • the Ni content is preferably set to 2.00% or less.
  • Cr 0.20 to 0.80%
  • Cr is an element effective for improving the hardenability of steel and improving the strength of steel by precipitation strengthening during tempering. Therefore, the Cr content is set to 0.20% or more, and preferably 0.40% or more. On the other hand, if Cr is excessively contained, there is a risk that the toughness of the base metal and the welded heat affected zone after stress relief annealing may decrease. Therefore, the Cr content is set to 0.80% or less, and preferably 0.70% or less.
  • Mo is an element effective for improving hardenability and strength of steel by precipitation strengthening during tempering. Therefore, the Mo content is set to 0.20% or more.
  • the Mo content is preferably set to 0.30% or more, more preferably 0.35% or more, and further preferably 0.40% or more.
  • Mo carbides may precipitate at grain boundaries after stress relief annealing, reducing the toughness of the base material and the welded heat affected zone, and the impact on the welded heat affected zone is particularly large. Therefore, the Mo content is set to 0.80% or less, and preferably 0.60% or less.
  • V 0.005 to 0.070%
  • Cr and Mo is an element effective for improving hardenability and improving the strength of steel by precipitation strengthening during tempering. Therefore, the V content is set to 0.005% or more, and preferably 0.010% or more.
  • the V content is set to 0.070% or less, and preferably 0.050% or less.
  • Al 0.010 to 0.100%
  • Al is an element useful for deoxidization, and also an element that refines the grain size during quenching by forming nitrides. In addition, it is an essential element for ensuring [fB] by forming nitrides. Therefore, in the steel sheet according to the present embodiment, the Al content is set to 0.010% or more. When the N content is high, the Al content is preferably 0.030% or more, more preferably 0.040% or more, in order to fix N and ensure fB. On the other hand, if Al is contained in excess, Al may form coarse nitrides, which may reduce the toughness of the base material and the welded heat affected zone. Therefore, the Al content is set to 0.100% or less, and preferably 0.080% or less.
  • B is an element that improves the hardenability of the steel by being contained in a small amount. Therefore, the B content is set to 0.0005% or more.
  • the B content may be set to 0.0006% or more, 0.0008% or more, or 0.0010% or more.
  • B may form coarse nitrides and/or carbides, which may reduce the toughness of the base metal. Therefore, the B content is set to 0.0030% or less.
  • the B content may also be set to 0.0020% or less or 0.0010% or less.
  • N is an element that forms nitrides to refine the crystal grain size of the base material and improve toughness. Therefore, the N content is set to 0.0015% or more. The N content may be set to 0.0030% or more or 0.0035% or more. On the other hand, if N is contained excessively, the nitrides become coarse and the toughness of the weld heat affected zone in the as-welded state decreases. Therefore, the N content is set to 0.0050% or less.
  • P and S are impurity elements contained in steel, and the lower the content, the better. Therefore, the lower limit of the P content and the S content is 0%.
  • the P content is set to 0.006% or less
  • the S content is set to 0.0030% or less.
  • the P content is preferably set to 0.005% or less.
  • the S content may be set to 0.0020% or less.
  • Cu (Cu: 0 to 1.00%) Cu is not an essential element in the steel plate according to this embodiment, so the lower limit of the Cu content is 0%. However, since Cu has an effect of improving the strength of steel, it can be contained as necessary. When Cu is contained, in order to utilize the effect, the Cu content is preferably 0.10% or more, and more preferably 0.20% or more. If necessary, the Cu content may be 0.25% or more or 0.30% or more. On the other hand, if Cu is contained excessively, there is a risk that the toughness of the base material will decrease due to the occurrence of cracks on the steel sheet surface and the precipitation of Cu. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably set to 0.80% or less. If necessary, the Cu content may be set to 0.70% or less, 0.60% or less, 0.50% or less, or 0.40% or less.
  • Nb 0 to 0.030% Since Nb is not an essential element in the steel sheet according to this embodiment, the lower limit of the Nb content is 0%. However, since Nb is an element that refines crystal grains during quenching, it can be contained as necessary. When Nb is contained, in order to utilize its effect, the Nb content is preferably 0.001% or more. On the other hand, if Nb is contained in excess, Nb may form coarse carbonitrides and reduce the toughness of the base material. Therefore, the Nb content is set to 0.030% or less. Since the toughness of the welded heat affected zone is improved with a smaller Nb content, the Nb content may be set to 0.020% or less, 0.010% or less, or 0.005% or less.
  • Ti is not an essential element in the steel plate according to the present embodiment, so the lower limit of the Ti content is 0%. However, Ti may refine the crystal grains when the steel is heated to a high temperature by slab heating, etc., so it may be contained as necessary. When Ti is contained, in order to utilize its effect, it is preferable that the Ti content is 0.001% or more. On the other hand, if Ti is contained excessively, Ti may form coarse carbonitrides, as with Nb, and may reduce the toughness of the base material. Therefore, the Ti content is set to 0.010% or less. If necessary, the Ti content may be set to 0.005% or less or 0.002% or less.
  • the steel sheet according to the present embodiment may contain one or more of Ca, Mg, and REM. Since Ca, Mg, and REM are not essential elements, the lower limit of the content of Ca, Mg, and REM is 0%.
  • Ca has the effect of spheroidizing sulfides in the steel sheet, thereby reducing the influence of MnS that reduces the toughness of the steel sheet.
  • the Ca content may be set to 0.0001% or more.
  • the Ca content is set to 0.0030% or less. If necessary, the Ca content may be set to 0.0015% or less, 0.0010% or less, 0.0005% or less, or 0.0002% or less.
  • Mg and REM form oxides to improve the toughness of the weld heat affected zone.
  • the Mg and REM contents may each be 0.0001% or more.
  • the Mg content and the REM content are each set to 0.0030% or less. If necessary, the Mg content and the REM content may be set to 0.015% or less, 0.010% or less, 0.005% or less, or 0.002% or less, or less than 0.0015%.
  • REM is a general term for rare earth metals including Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Compared to other additive elements, REM is characterized by its strong deoxidizing effect and forms stable oxides in steel.
  • Oxygen (O) is an impurity element contained in steel, and in many cases, it forms oxides of several ⁇ m to several tens of ⁇ m in size together with Ca, Mg, REM, Al, Ti, etc., which have strong deoxidizing power. When coarse oxides are contained or when the number density of oxides is high, they may become the starting point of brittle fracture, so the lower the O content, the more preferable it is. For this reason, the lower limit of the O content is 0%. In the steel plate according to this embodiment, the O content is set to 0.0040% or less, preferably 0.0030% or less, in order to improve the toughness of the welded portion.
  • the steel sheet according to the present embodiment is composed of the above-mentioned components, with the balance being Fe and impurities.
  • the impurities refer to components that are mixed in due to various factors in raw materials such as ores or scraps, or in the manufacturing process, during industrial production of steel material, and are permissible within a range that does not adversely affect the present invention.
  • [fB] calculated by the following formulas (A) to (E) is 0.0003 mass% or more.
  • [fB] represents the amount of B dissolved in the steel.
  • [fB] By setting [fB] to 0.0003 mass% or more, the hardenability of the steel can be improved in high-tensile steel having a yield strength of 670 to 870 N/mm 2 and a tensile strength of 780 to 940 N/mm 2.
  • B easily forms nitrides in the steel.
  • Ti and Al easily form nitrides and oxides. Therefore, the amount of B dissolved in the steel [fB] is calculated by the following formulas (A) to (E).
  • [fB] may be 0.0005 mass% or more, or may be 0.0015 mass% or more.
  • [fB] may be 0.0025 mass% or less, or may be 0.0018 mass% or less.
  • [fB] [B] - 0.77 x [fN] ...
  • [fN] [N] - 0.29 x [fTi] - 0.52 x [fAl] ...
  • [fTi] [Ti] - 2 ⁇ [fO] ...
  • [fAl] [Al] - 1.125 ⁇ [fO] ...
  • [fO] [O] - 0.4 x [Ca] - 0.66 x [Mg] - 0.11 x [REM] ... (E)
  • [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] respectively represent the content (mass%) of B, N, Ti, Al, O, Ca, Mg, and REM in the steel plate. Elements that are not contained, including the amount of elements mixed in as impurities, are substituted with 0, and when the calculated values of [fN], [fTi], [fAl], and [fO] are less than 0%, 0 is substituted.
  • the ranges of the ⁇ , ⁇ , and ⁇ values calculated from the content of each element are also limited as follows for the steel plate according to this embodiment.
  • ⁇ value ( ⁇ value: 1.00 to 1.50% by mass)
  • the ⁇ value is expressed by the following formula (1).
  • ⁇ [C] + 6 ⁇ [Si] + 100 ⁇ [P] ...
  • [C], [Si] and [P] are the contents (mass%) of C, Si and P in the steel plate, respectively.
  • the ⁇ value is set to 1.50 mass% or less. This is a necessary condition for improving the toughness of the coarse-grained portion of the base material or the welded heat-affected zone after stress relief annealing, and it is necessary to adjust C, Si, and P within a range that satisfies this condition.
  • the grain boundary segregation concentration of P increases, making brittle fracture at the grain boundaries more likely to occur, but brittle fracture can be controlled by P, C, and Si.
  • P is inevitably contained in steel, and since it significantly reduces grain boundary strength due to grain boundary segregation, it is a representative element that causes SR embrittlement and has the highest coefficient.
  • C and Si are also elements that are inevitably contained in steel, and if these elements increase, they will cause embrittlement due to cementite formed at the grain boundaries. It is desirable to reduce both elements, but there are cases where a certain amount is contained due to characteristics or specifications.
  • the ⁇ value is 1.00 mass% or more.
  • This lower limit is a range determined by the component constraints in the specifications of the field of application and the limits of element control in manufacturing, and is calculated by substituting the above-mentioned lower limits of the C, Si, and P contents and the realistic minimum values in manufacturing into formula (1).
  • the preferred lower limit of the ⁇ value can be calculated from the preferred lower limits of the C, Si, and P contents.
  • the ⁇ value may be more than 1.10 mass%, or may be 1.30 mass% or more.
  • ⁇ value ( ⁇ value: 10.0 to 15.0)
  • the ⁇ value is calculated by the following formula (2).
  • ⁇ 0.65 ⁇ [C] 1/2 ⁇ (1 + 0.64 ⁇ [Si]) ⁇ (1 + 4.10 ⁇ [Mn]) ⁇ (1 + 0.27 ⁇ [Cu]) ⁇ (1 + 0.52 ⁇ [Ni]) ⁇ (1 + 2.33 ⁇ [Cr]) ⁇ (1 + 3.14 ⁇ [Mo]) ...
  • [C], [Si], [Mn], [Cu], [Ni], [Cr] and [Mo] are the contents (mass%) of C, Si, Mn, Cu, Ni, Cr and Mo in the steel.
  • the ⁇ value is in the range of 10.0 to 15.0.
  • the ⁇ value is an index showing the hardenability of the steel material, and the higher the ⁇ value, the more reliably the formation of upper bainite structure, which has a poor balance of strength and toughness, can be avoided, but if the ⁇ value is too high, the strength of the steel material increases and the toughness deteriorates. In other words, it also serves as an index showing the target range of the content of alloy elements required to improve the toughness of the welded heat-affected zone as it is. If necessary, the ⁇ value may be 11.0 or more. At the same time, the ⁇ value may be 14.0 or less.
  • the ⁇ value is calculated by the following formula (3).
  • ⁇ [Mn] + 20 ⁇ [Nb] + 36 ⁇ [Ti] ...
  • [Mn], [Nb], and [Ti] are the contents (mass%) of Mn, Nb, and Ti in the steel plate.
  • the ⁇ value is in the range of 0.70 to 1.50 mass%.
  • Mn, Nb, and Ti are all elements that promote grain boundary embrittlement after stress relief annealing, and by setting the ⁇ value to 1.50 mass% or less, it is possible to suppress the decrease in the toughness of the base material and the welded heat affected zone after stress relief annealing.
  • the ⁇ value may be 1.40 mass% or less.
  • Mn, Nb and Ti in order to ensure a certain degree of hardenability and obtain a microstructure having an advantageous balance of strength and toughness, it is preferable to add certain amounts of Mn, Nb and Ti, and the ⁇ value is set to 0.70 mass% or more.
  • the ⁇ value may be 0.75 mass% or more.
  • the steel plate according to the present embodiment it is possible to widen the allowable range of the ⁇ value by newly setting a ⁇ value, so that it is possible to provide a steel with excellent low-temperature toughness of the welded portion both as-welded and after stress relief annealing, even in fields where the chemical composition regulations of the applicable standards are strict as described above.
  • the carbon equivalent Ceq which is an index showing the hardenability of steel and is calculated by the following formula (4), is set to 0.550 to 0.620 mass%.
  • Ceq is less than 0.550 mass%, the strength of the steel plate may be insufficient. If necessary, Ceq may be set to 0.570 mass% or more, or 0.600 mass% or more. If Ceq exceeds 0.620 mass%, the toughness of the steel plate may decrease. If necessary, Ceq may be set to 0.600 mass% or less.
  • yield strength 670 to 870 N/ mm2
  • tensile strength 780 to 940 N/ mm2
  • the yield strength is set to 670 to 870 N/mm 2
  • the tensile strength of the steel plate is set to 780 to 940 N/mm 2.
  • the steel plate selected for such applications is a steel plate having the above-mentioned yield strength and tensile strength, so the yield strength and tensile strength of the steel plate according to this embodiment are also set to the above ranges.
  • the yield strength may be set to 690 N/mm 2 to 830 N/mm 2.
  • the tensile strength may be set to 800 N/mm 2 to 900 N/mm 2 .
  • the Charpy absorbed energy at -65°C is a value measured at a position that is 1/4 of the plate thickness from the surface in the plate thickness direction (sometimes referred to as the t/4 position or 1/4 thickness position).
  • ⁇ c crack opening displacement
  • the Charpy impact test has been used as a method for evaluating the brittle fracture resistance of a material.
  • the value obtained from the Charpy impact test represents the average toughness of the evaluation target area.
  • the CTOD test even if the average toughness of the evaluation target area is good, if there is even a slightly brittle part in the evaluation target area, the existence of the part is reflected in ⁇ c.
  • the steel plate according to this embodiment preferably has a ⁇ value of 0.10 mm or more in a CTOD test at ⁇ 35° C. In this case, the safety of a transportation tank made of the steel plate according to this embodiment is further improved.
  • the average value of the hardness at 121 measurement positions must be 265Hv to 290Hv and the standard deviation must be 20 or less in the hardness distribution measurement at 0.05mm pitch in the range of 0.5mm x 0.5mm at the 1/4 thickness position.
  • the structure of the steel plate according to the present embodiment is preferably a mixed structure of martensite structure and lower bainite structure, which is superior in strength-toughness balance, but due to localized ⁇ grain size and microsegregation variations, the hardenability may be partially reduced, and an upper martensite structure with inferior strength-toughness balance may be formed.
  • the distribution of hardness may become nonuniform, and the toughness of the base material may deteriorate. If the average value of the hardness is less than 265Hv or the standard deviation exceeds 20, the upper bainite structure may be included, and the toughness of the base material cannot be ensured. On the other hand, if the average value exceeds 290Hv, the strength may become too high, and the toughness may decrease.
  • the above-mentioned hardness distribution measurement is performed by taking a micro sample with a surface (L-section) parallel to the rolling direction and thickness direction of the steel plate as the observation surface, and using a micro Vickers hardness tester.
  • the measurement area is a 0.5 mm x 0.5 mm range centered on any t/4 position within the micro observation surface, with a measurement pitch of 0.05 mm and a measurement load of 25 gf, and measurements are taken at 11 vertical points x 11 horizontal points, for a total of 121 points.
  • the average and standard deviation are calculated from the obtained measurement values.
  • the steel plate according to the present embodiment is intended for steel plates that require SR, and therefore the thickness is set to 10 mm or more.
  • the thickness is preferably 25 mm or more.
  • a steel plate with a thickness of more than 60 mm is not preferred because it makes little contribution to reducing the weight of the transport tank to which it is applied. Therefore, the thickness of the steel plate according to the present embodiment is set to 60 mm or less.
  • steel plate according to this embodiment may have the configuration described below.
  • the steel plate according to this embodiment preferably has a structure at a 1/4 thickness position of the cross section in the plate thickness direction that is a mixed structure of martensite and lower bainite.
  • the martensite and lower bainite structures may occupy 85 area % or more in total.
  • the average grain size at the 1/4 thickness position may be 15.0 ⁇ m or less.
  • the average grain size may be 14.5 ⁇ m or less, or 14.0 ⁇ m or less, as necessary. Since it is preferable that the average grain size at the 1/4 thickness position of the steel plate is small, there is no need to specify a lower limit. Usually, the average grain size is about 10.0 ⁇ m at the smallest.
  • the average crystal grain size is defined as follows. A sample in which the L-section of the steel sheet can be observed is prepared, and the 1/4 thickness position of the L-section is used as the observation area. A crystal orientation analysis using an electron beam backscatter diffraction pattern analysis method (EBSD method) using a scanning electron microscope is performed in a range of 200 ⁇ m in the sheet thickness direction and 250 ⁇ m in the rolling direction at a pitch of 0.5 ⁇ m.
  • EBSD method electron beam backscatter diffraction pattern analysis method
  • the region surrounded by grain boundaries with a crystal orientation difference of 15° or more is defined as a crystal grain
  • the circle-equivalent grain size of the crystal grain is defined as the crystal grain size
  • the value calculated by the area-weighted average weighted by the area of each crystal grain is defined as the average crystal grain size.
  • the steel plate according to this embodiment is subjected to stress relief annealing of the welded portion after assembly into a transport tank, during which not only the welded portion but also the base material is heated.
  • the toughness of the base material tends to decrease.
  • the steel plate according to this embodiment preferably has a Charpy absorbed energy of 27 J or more at -40°C after stress relief annealing. In this case, safety can be further improved.
  • the Charpy absorbed energy at -40°C after stress relief annealing is measured at the location where stress relief annealing was performed when the steel plate is subjected to stress relief annealing at a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and cooling rate of 55°C/hr or less in a temperature range of 425°C or higher.
  • the steel plate according to this embodiment preferably has a yield strength of 670 to 870 N/mm 2 and a tensile strength of 780 to 940 N/mm 2 after stress relief annealing. This ensures sufficient strength in a transport tank for liquefied CO 2 that has been subjected to stress relief annealing.
  • the steel sheet according to this embodiment has a ⁇ value of 0.10 mm or more in a CTOD test at -35°C, even after stress relief annealing. In this case, safety is further improved.
  • the steel plate according to this embodiment has the above-mentioned configuration, and therefore has excellent toughness in the weld heat affected zone (as welded and after stress relief annealing).
  • the toughness of the weld heat affected zone is not limited, but as a target value, in the as-welded case, the Charpy absorbed energy at -65°C is preferably 70 J or more, and in the case after stress relief annealing, the Charpy absorbed energy at -65°C is preferably 70 J or more.
  • the weld heat affected zone has a ⁇ value of 0.10 mm or more in a CTOD test at ⁇ 35° C., whether as welded or after stress relief annealing.
  • a commonly used method for producing steel products may be used. That is, for example, steel produced by a converter process or an electric furnace process and refined in a secondary refining facility is made into a slab by continuous casting or ingot casting.
  • the slab thickness is sufficient as long as it can reduce segregation and improve the material properties by reducing porosity, and for this purpose, the slab thickness is preferably 150 mm or more. There is no particular upper limit for the slab thickness, but the slab thickness may be, for example, 600 mm or less or 400 mm or less.
  • the slab is then heated to about 950 to 1250°C in a slab heating furnace, and is then rolled to a predetermined thickness by hot rolling under the conditions described below to obtain a steel sheet.
  • This steel sheet is then quenched and tempered to obtain a steel sheet (final steel sheet) having the desired properties.
  • the steel plate according to this embodiment needs to have its P content reduced to 0.006% or less. There are cases where it is not possible to reduce the P content to 0.006% or less using normal dephosphorization methods, but in such cases measures can be taken such as extending the dephosphorization treatment time.
  • the cumulative reduction ratio When hot rolling, it is desirable to set the cumulative reduction ratio at 50% or more when the rolling temperature is in the range of 1150 to 900°C. There is no particular need to specify an upper limit for the cumulative reduction ratio in the above temperature range, but the cumulative reduction ratio may be 80% or less, or 70% or less.
  • the reheating and quenching process described below may be omitted by performing a direct quenching process in which the plate is immediately water-cooled after hot rolling.
  • the cooling start temperature is set to Ar3 point or more, and water cooling is performed to 300 ° C or less.
  • the average cooling rate during water cooling is preferably 5 ° C / sec or more in the range of 700 ° C to 300 ° C in the cooling temperature history of the front and back surfaces of the steel plate.
  • the average cooling rate may be, for example, 100 ° C / sec or less, 50 ° C / sec or less, or 20 ° C / sec or less.
  • reheating may be performed after direct quenching, and further quenching may be performed.
  • the Ar3 point is calculated using the following formula.
  • Ar3 910 - 310 x [C] - 8 x [Mn] - 20 x [Cu] - 15 x [Cr] - 55 x [Ni] - 80 x [Mo] + 0.35 x (t-8)
  • [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are the contents of C, Mn, Cu, Cr, Ni, and Mo in the steel plate, respectively, in mass%
  • t is the thickness of the steel plate in mm.
  • quenching For plate thicknesses of 50 mm or more, it is preferable to perform quenching by cooling the steel plate once after rolling and then reheating it. For plate thicknesses of 50 mm or more, if reheating and quenching is performed, direct quenching after hot rolling may be omitted, or quenching may be performed directly.
  • the heating temperature during quenching is desirably 925°C or less, and may be 920°C or less, 915°C or less, or 910°C or less. This is because the metal structure of a thick steel plate may not be sufficiently refined after rolling. If the quenching temperature for a steel plate in which the metal structure is not sufficiently refined exceeds 925°C, the reverse transformation ⁇ structure formed by heating becomes coarse, and the average crystal grain size of the final structure after ⁇ / ⁇ transformation by subsequent cooling also becomes coarse.
  • the lower limit of the quenching temperature is not preferable if it is slightly above the Ac3 point (for example, within a temperature range of not less than the Ac3 point and not more than the Ac3 point + 20°C), because this may cause variations in reverse transformation ⁇ grain size and insufficient solid solution of carbides containing B, resulting in insufficient hardenability. Therefore, the quenching temperature is preferably 880°C or higher, and more preferably 890°C or higher. In the above description of the quenching treatment conditions, it is assumed that the steel plate has a thickness of 50 mm or more, but these quenching treatment conditions are also applied when reheating and quenching are performed on a steel plate having a thickness of less than 50 mm.
  • tempering is performed after quenching (after direct quenching or reheating quenching, or after reheating quenching if both are performed).
  • the heating temperature during tempering i.e., the tempering temperature
  • the tempering temperature is desirably 660°C or lower. If the tempering temperature exceeds 660°C, the tempering effect becomes excessive, making it difficult to ensure the yield stress and tensile strength, and toughness may decrease.
  • the tempering temperature is 500°C or higher, preferably 600°C or higher. If the tempering temperature is too low, tempering becomes insufficient, making it difficult to ensure the specified yield stress and tensile strength.
  • the steel plate When cooling is performed after reheating and quenching or tempering, it is preferable to cool the steel plate by water cooling (accelerated cooling) rather than air cooling in order to prevent a decrease in the toughness of the base material due to temper embrittlement. In this case, it is preferable to set the average cooling rate to 300°C to 0.1°C/sec or more or 0.5°C/sec or more.
  • the steel plate according to the present embodiment is suitable as a steel plate for a liquefied CO2 transport tank.
  • it can be used as a transport tank mounted on a ship.
  • liquefied CO2 is filled into a transport tank installed on the ship and transported, but in order to prevent CO2 from solidifying (becoming dry ice) in the transport tank, it is preferable to transport it while maintaining a pressure of about 2 MPa.
  • CO2 in order to maintain CO2 in a liquid state at a pressure of about 2 MPa, it is preferable to keep CO2 at about minus 35°C.
  • the steel plate according to the present embodiment can be suitably used for such applications.
  • the conditions in the embodiment are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions.
  • Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.
  • the molten iron that had completed the blast furnace treatment was tapped into a molten iron ladle, and after preliminary treatment such as desulfurization was carried out, the molten iron was then inserted into a converter. Next, dephosphorization was carried out in the converter, and the P content was adjusted to 0.006% or less.
  • the dephosphorized molten steel was further subjected to composition adjustment, and then slabs having the chemical compositions shown in Tables 1A and 1B were cast. Thereafter, the slab was heated in a heating furnace to the heating temperature shown in the table, and then hot-rolled to a predetermined plate thickness to obtain a steel plate. Furthermore, the steel plate was quenched and tempered to obtain a steel plate (final steel plate) having the specified properties.
  • Table 2 shows the heating temperature before rolling, the cumulative reduction ratio of hot rolling at 1150 to 900°C, the plate thickness after rolling, the quenching temperature, and the tempering temperature. Cooling after reheating and quenching or after tempering was performed by water cooling, and the average cooling rate to 300°C was set to 0.1°C/sec or more. In addition, for some steel plates, direct quenching treatment was performed in which the steel plate was immediately water-cooled after hot rolling. The cooling start temperature, cooling end temperature, and average cooling rate in this case are shown in the table.
  • Tables 1A and 1B show the chemical composition, ⁇ value, ⁇ value, ⁇ value, fB value, and carbon equivalent Ceq of the steel plate.
  • the column before SR of the base material properties in Table 3A shows the average hardness (average Hv) of the base material at 121 measurement positions, the average grain size (EBSD grain size), the yield strength (MPa), the tensile strength (MPa), the yield ratio, the Charpy absorbed energy (J) at -65°C, and the ⁇ value (mm) of the CTOD test at -35°C.
  • the EBSD grain size was measured by preparing a sample in which the L-section of the steel sheet could be observed, observing the 1/4 thickness position of the L-section, and performing crystal orientation analysis using an electron beam backscatter diffraction pattern analysis method (EBSD method) using a scanning electron microscope in a range of 200 ⁇ m in the sheet thickness direction and 250 ⁇ m in the rolling direction at a pitch of 0.5 ⁇ m. From the results of the crystal orientation analysis, the region surrounded by grain boundaries with a crystal orientation difference of 15° or more was defined as a crystal grain, the circle-equivalent grain size of the crystal grain was defined as the crystal grain size, and the value calculated by the area-weighted average weighted by the area of each crystal grain was defined as the average crystal grain size.
  • EBSD method electron beam backscatter diffraction pattern analysis method
  • the tensile test was conducted in accordance with JIS Z 2241:2011, using JIS No. 4 round bar test pieces with a parallel section of ⁇ 14 mm taken from the 1/4 thickness position in the C direction.
  • the yield strength and tensile strength are each the average of two test pieces.
  • the yield strength was taken as 0.2% proof stress.
  • the yield ratio is the ratio of the yield strength YS to the tensile strength TS, and is expressed as a percentage, i.e., 100 x (YS/TS). The yield ratio is expressed in %.
  • a micro sample was taken with the L-section parallel to the rolling direction and thickness direction of the steel plate as the observation surface, the observation surface was wet polished, and then the mirror surface was buffed using 1.0 ⁇ m diamond particles, and measurements were taken using a micro Vickers hardness tester.
  • the measurement area was randomly selected within a 0.5 mm x 0.5 mm range centered on the 1/4t position within the micro observation surface, and measurements were taken at a total of 121 points (11 vertical x 11 horizontal) with a measurement pitch of 0.05 mm and a measurement load of 25 gf. The average value and standard deviation were calculated from the obtained measurements.
  • a semi-automatic welded joint with a weld line parallel to the rolling direction was produced and evaluated. Specifically, a K groove was produced, argon gas containing 20% CO2 was used as the shielding gas, the welding wire was YM-69F manufactured by Nippon Steel Welding Industry Co., Ltd., the heat input was set to 2.0 kJ/mm, and the preheating was set to 100°C, and multi-layer gas shielded arc welding (GMAW) was performed to produce a welded joint.
  • GMAW multi-layer gas shielded arc welding
  • stress relief annealing was performed on the base material and welded parts. Stress relief annealing was performed at a holding temperature of 600°C for 2 hours, with heating and cooling rates of 55°C/hr or less in the temperature range of 425°C or higher.
  • the yield strength and tensile strength of the base material after SR were determined in the same manner as before SR.
  • a Charpy test was performed at -40°C on a test piece taken in the C direction at the t/4 position of the base material after SR to determine the Charpy absorbed energy.
  • a CTOD test was also performed at -35°C to determine the ⁇ value.
  • the Charpy absorbed energy of the base material and welded joints was measured by taking three V-notch test pieces from each of the base material and welded joints and conducting a Charpy impact test at a specified temperature.
  • the V-notch test pieces were full-size test pieces as specified in JIS Z 2242:2005 taken in the C direction from each plate thickness position.
  • the Charpy impact test was also conducted in accordance with JIS Z 2242:2005.
  • the ⁇ value ( ⁇ c) in the CTOD test was measured in accordance with BS7448 (British Standard) Part 1 (1991) and BS7448 (British Standard) Part 2 (1997).
  • evaluation was performed in the C direction (sheet width direction) in which the longitudinal direction of the test piece was perpendicular to the rolling direction.
  • gas shielded arc welding was performed at a heat input of 35 kJ/mm on the butted steel plate of the K-groove, and the tip of the fatigue notch of the CTOD test piece of the weld was processed to be the center of the plate thickness of the I-side fusion line (FL) of the weld, and the CTOD test was performed at a specified temperature.
  • the welded joint was evaluated only in the L direction (rolling direction).
  • the test piece was taken so that the tip of the fatigue crack corresponds to the weld bond.
  • Three tests were performed at each test temperature, and the minimum value of the measured data obtained was taken as the ⁇ value of the CTOD test.
  • the unit of CDOD shown in Tables 3A and 3B is mm.
  • a steel plate having excellent strength and low-temperature toughness, and further having excellent strength and low-temperature toughness after stress relief annealing can be provided.
  • This steel plate is suitable for use in liquefied CO2 transport tanks and has high industrial applicability.

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Abstract

L'invention concerne une tôle d'acier ayant une composition chimique donnée et ayant une valeur α de 1,00 à 1,50 % en masse, une valeur β de 10,0 à 15,0, une valeur γ de 0,70 à 1,50 % en masse, une valeur Ceq de 0,550 à 0,620 % en masse, une limite d'élasticité de 670 à 870 N/mm2, une résistance à la traction de 780 à 940 N/mm2 et une énergie d'absorption de Charpy à -65 °C de 100 J ou plus. Lorsque la tôle d'acier est examinée pour une distribution de dureté à un pas de 0,05 mm par rapport à 121 parties de 1 mm × 1 mm situées à 1/4 de l'épaisseur de tôle, la dureté moyenne est de 265 à 290 Hv et l'écart-type est de 20 ou moins. La tôle d'acier a une épaisseur de 10 à 60 mm.
PCT/JP2023/035791 2022-09-30 2023-09-29 Tôle d'acier WO2024071422A1 (fr)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2015127444A (ja) * 2013-12-27 2015-07-09 Ｊｆｅスチール株式会社 耐疲労き裂伝ぱ特性に優れた鋼材およびその製造方法並びに耐疲労き裂伝ぱ特性に優れた鋼材の判定方法
JP2015196891A (ja) * 2014-04-02 2015-11-09 新日鐵住金株式会社 打抜き穴広げ性と低温靭性に優れた引張最大強度９８０ＭＰａ以上の高強度熱延鋼板及びその製造方法
WO2018020972A1 (fr) * 2016-07-28 2018-02-01 新日鐵住金株式会社 Tuyau et colonne montante en acier sans soudure de haute résistance

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2015127444A (ja) * 2013-12-27 2015-07-09 Ｊｆｅスチール株式会社 耐疲労き裂伝ぱ特性に優れた鋼材およびその製造方法並びに耐疲労き裂伝ぱ特性に優れた鋼材の判定方法
JP2015196891A (ja) * 2014-04-02 2015-11-09 新日鐵住金株式会社 打抜き穴広げ性と低温靭性に優れた引張最大強度９８０ＭＰａ以上の高強度熱延鋼板及びその製造方法
WO2018020972A1 (fr) * 2016-07-28 2018-02-01 新日鐵住金株式会社 Tuyau et colonne montante en acier sans soudure de haute résistance

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