WO2023162896A1 - 原油油槽用鋼材 - Google Patents

原油油槽用鋼材 Download PDF

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Publication number
WO2023162896A1
WO2023162896A1 PCT/JP2023/005840 JP2023005840W WO2023162896A1 WO 2023162896 A1 WO2023162896 A1 WO 2023162896A1 JP 2023005840 W JP2023005840 W JP 2023005840W WO 2023162896 A1 WO2023162896 A1 WO 2023162896A1
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crude oil
corrosion
steel material
steel
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PCT/JP2023/005840
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English (en)
French (fr)
Japanese (ja)
Inventor
実 伊藤
和幸 鹿島
道郎 金子
啓介 中井
大貴 今城
隆行 米澤
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Nippon Steel Corp
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Nippon Steel Corp
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Priority to JP2024503116A priority Critical patent/JP7769263B2/ja
Priority to CN202380014675.9A priority patent/CN118302557A/zh
Priority to KR1020247017333A priority patent/KR20240090929A/ko
Publication of WO2023162896A1 publication Critical patent/WO2023162896A1/ja
Anticipated expiration legal-status Critical
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to steel materials for crude oil tanks.
  • Crude oil tankers used steel for welded structures with excellent strength and weldability, but many pitting-like localized corrosion (called pits) with a relatively high rate of progress were generated.
  • the diameter of these pits is about 10-30 mm, and the rate of growth reaches 2-3 mm/year. This value far exceeds 0.1 mm/year, which is the average rate of wastage due to corrosion that is taken into consideration when designing the hull.
  • Crude oil contains salt water with a concentration of about 10% called brine, and the brine, which has a higher specific gravity than crude oil, deposits on the bottom plate of the oil tank during transportation.
  • the oil tank bottom plate is covered with a high-viscosity oil layer (hereinafter also referred to as "oil coat") adhered to the steel plate and has the same anti-corrosion effect as painting, so corrosion by brine does not occur.
  • oil coat a high-viscosity oil layer
  • the corroded portion is hydrolyzed and acidified due to corrosion, which further accelerates the corrosion, resulting in pitting-like corrosion. In other words, it is caused by the corrosion of the defective part of the oil coat by the brine.
  • the reason why the pit corrosion that had occurred up to that time due to docking did not progress again and stopped is as follows.
  • the tank is cleaned to check for cracks and pits in the tank.
  • the corrosive liquid inside the pits is also removed in order to determine the pit level and measure the depth.
  • an oil coat is formed when the crude oil is loaded.
  • the pitted area is covered with a thicker oil coat than the surrounding area, so it is protected from corrosion. After that, corrosion does not progress.
  • Patent Document 1 proposes steel for cargo oil pipes, which is used in an environment where crude oil and seawater are alternately or simultaneously exposed.
  • Patent Literature 2 proposes a corrosion-resistant steel plate for a cargo oil tank, which takes into account the corrosion resistance of welded portions.
  • Patent Document 3 proposes a corrosion-resistant steel containing Cu, Ni, Cr, Mo, Sb, and Sn for use in crude oil and heavy oil storage.
  • Patent Literature 4 proposes corrosion-resistant steel for use in tanks for transporting and storing crude oil.
  • Patent Document 1 since the steel for cargo oil pipes described in Patent Document 1 contains more than 0.1% Cr in a crude oil tank environment, there is still room for improvement in terms of local corrosiveness, weldability, and economic efficiency. ing.
  • Patent Document 3 The corrosion-resistant steel for crude oil and heavy oil storage described in Patent Document 3 requires the addition of a large amount of alloying elements in order to obtain excellent corrosion resistance, so there is still room for improvement in terms of economy and weldability.
  • the basic components are Cu: 0.5-1.5%, Ni: 0.5-3.0%, and Cr: 0.5-2. Since it contains 0%, it is necessary to add a large amount of alloying elements for the manifestation of the effect, and there is a problem that the economy and weldability are inferior. In addition, since Cr is excessively contained in the environment of the bottom plate of a crude oil tank, the rate of progress of local corrosion occurring in the bottom plate is not reduced, and there is a problem that corrosion resistance commensurate with the total amount of alloy addition cannot be obtained.
  • the present invention has been made to solve the above problems, and an object of the present invention is to provide a crude oil tank steel material that exhibits excellent corrosion resistance in the environment of the bottom plate of a crude oil tank.
  • the present invention has been made to solve the above problems, and the gist of the present invention is the following steel material for crude oil tanks.
  • the chemical composition of the crude oil tank steel material is % by mass, C: 0.03 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.60-2.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.001 to 0.050%, N: Contains 0.001 to 0.010%, and Cu: 0.01 to 0.60%, Mo: 0.01 to 0.20%, W: 0.01 to 0.20%, Sn: 0.01-0.20%, and Sb: 0.01-0.20% Contains one or more selected from balance: Fe and impurities,
  • the CRI value defined by the following formula (i) is 0.50 or more
  • the total area ratio of bainite and/or ashular ferrite is 20 to 95% in the metal structure in the cross section parallel to the rolling direction and thickness direction of the steel material for crude oil tanks,
  • the area ratio of martensite is 2.0% or less, and Vickers hardness is 210HV10 or less, Steel for crude oil tanks.
  • CRI Cu+6
  • the chemical composition, instead of part of the Fe, is mass %, Nb: 0.002 to 0.200%, V: 0.005 to 0.500%, Ti: 0.002 to 0.200%, Ta: 0.005 to 0.500%, Zr: 0.005-0.500%, and B: 0.0002-0.0050% It contains one or more selected from The steel material for crude oil tanks according to any one of (1) to (3) above.
  • the chemical composition instead of a part of the Fe, by mass%, Mg: 0.0001-0.01%, Ca: 0.0005 to 0.01%, Y: 0.0001 to 0.1%, La: 0.005-0.1%, and Ce: 0.005-0.1% It contains one or more selected from The steel material for crude oil tanks according to any one of (1) to (4) above.
  • the present inventors conducted a corrosion test for a COT (crude oil tank) bottom plate stipulated by the IMO (International Maritime Organization) simulating the environment in a pit. Using a steel material containing W, Sn, and Sb within a concentration range usable as shipbuilding steel, the optimum conditions were investigated.
  • the C content is 0.06-0.16%
  • the Si content is 0.10-0.30%
  • the Mn content is 0.60-1.60%
  • the P content is 0.60-1.60%.
  • 005-0.020% S content 0.004-0.030%
  • Al content 0.01-0.05%
  • a steel material was melted and rolled into a steel plate TP1 having a thickness of 6 mm.
  • Cu 0.01 to 0.60%
  • Mo 0.01 to 0.20%
  • W 0.01 to 0.20%
  • Sn 0.01 to 0.20 %
  • Sb one or more selected from 0.01 to 0.20%.
  • Table 1 shows the chemical compositions of TP1 to TP24.
  • the air-cooled test pieces were slightly flattened ferrite and pearlite, and the ferrite grain size was even larger and granular at the center of the plate thickness.
  • the water-cooled test pieces with good corrosion resistance contained 20% or more of the total area of bainite and/or acicular ferrite at any depth position.
  • the total area ratio of bainite and / or acicular ferrite is 20 to 95%, and the area ratio of martensite is 2. It was found that a metallographic structure of 0.0% or less was obtained, thereby ensuring good corrosion resistance. Moreover, it was found that when the Vickers hardness exceeds 210HV10, the corrosion resistance deteriorates.
  • the CRI value is set to 0.50 or more, and the metal structure has a total area ratio of bainite and/or ascular ferrite of 20 to 95% throughout the plate thickness, and an area ratio of martensite of 2.0. % or less and the Vickers hardness is 210HV10 or less, the corrosion resistance of the steel sheet is improved over the entire thickness.
  • C 0.03-0.20%
  • C is an effective element for increasing the strength.
  • the C content is less than 0.03%, industrial economic efficiency is remarkably impaired.
  • an excessive C content deteriorates weldability and joint toughness, which is not preferable as a steel material for welded structures. Therefore, the C content should be 0.03 to 0.20%.
  • the C content is preferably 0.05% or more, or 0.08% or more, and preferably 0.18% or less, or 0.16% or less.
  • Si 0.05-0.50% Si is required as a deoxidizing element. On the other hand, an excessive Si content deteriorates the toughness of the base material. Therefore, the Si content should be 0.05 to 0.50%. If the requirements for weldability and joint toughness are severe, the Si content is preferably 0.10% or more, or 0.20% or more, and 0.40% or less, or 0.30% or less. is preferred.
  • Mn 0.60-2.00%
  • Mn is effective as an element that improves the strength of steel materials.
  • an excessive Mn content deteriorates the toughness of the base material. Therefore, the Mn content should be 0.60 to 2.00%.
  • the Mn content is preferably 0.75% or more, or 0.90% or more.
  • the Mn content is preferably 1.80% or less, or 1.60% or less.
  • P 0.030% or less
  • P is an impurity element
  • the P content is made 0.030% or less in order to reduce the rate of local corrosion progress and improve weldability.
  • the P content is preferably 0.020% or less.
  • the P content is more preferably 0.010% or less.
  • the lower limit of the P content does not need to be specified in particular, that is, the P content may be 0%, but an extreme reduction leads to an increase in steelmaking costs. Therefore, the P content may be 0.001% or more, or 0.005% or more.
  • S 0.030% or less
  • S is an impurity element, and the S content is made 0.030% or less in order to reduce the rate of local corrosion progress and improve mechanical properties, particularly ductility. Also, in order to ensure corrosion resistance and mechanical properties, the lower the S content, the better, preferably 0.020% or less, or 0.010% or less.
  • the lower limit of the S content does not need to be specified in particular, that is, the S content may be 0%, but an extreme reduction leads to an increase in steelmaking costs. Therefore, the S content may be 0.001% or more, or 0.003% or more.
  • Al 0.001-0.050%
  • Al is an element effective in refining the austenite grain size of the base material by forming AlN.
  • the Al content is set to 0.001 to 0.050%.
  • the Al content is preferably 0.005% or more, or 0.010% or more, and preferably 0.040% or less, or 0.030% or less.
  • N 0.001 to 0.010%
  • N combines with V, Al, and Ti and is effective for austenite grain refinement and precipitation strengthening.
  • the N content is set to 0.001 to 0.010%.
  • the N content is preferably 0.002% or more, or 0.003% or more, and preferably 0.008% or less, or 0.006% or less.
  • Cu, Mo, W, Sn, and Sb are elements that improve general corrosion resistance. On the other hand, if the content of these elements is excessive, it not only impairs economy but also causes deterioration of mechanical properties. Therefore, Cu: 0.01 to 0.60%, Mo: 0.01 to 0.20%, W: 0.01 to 0.20%, Sn: 0.01 to 0.20%, Sb: 0.01 to 0.20%. 1 or 2 or more selected from 01 to 0.20%.
  • the Cu content is preferably 0.05% or more, or 0.08% or more, and preferably 0.55% or less, or 0.50% or less.
  • the contents of Mo, W, Sn, and Sb are each preferably 0.02% or more or 0.03% or more, and preferably 0.18% or less or 0.15% or less. preferable.
  • the CRI value represented by formula (i) is less than 0.50, sufficient corrosion resistance cannot be obtained. On the other hand, when it is 0.50 or more, excellent corrosion resistance can be exhibited stably. Therefore, the CRI value should be 0.50 or more.
  • the CRI value is preferably 0.55 or higher, more preferably 0.60 or higher.
  • the upper limit of the CRI value is not particularly set, the upper limit of the CRI value is 2.40 in the chemical composition of the present invention.
  • CRI Cu+6 ⁇ Mo+2 ⁇ W+0.5 ⁇ Sn+0.5 ⁇ Sb (i)
  • each element symbol in the formula represents the content (% by mass) of each element contained in the steel material, and is set to 0 when not contained.
  • the balance is Fe and impurities.
  • impurities are components that are mixed in by various factors in the manufacturing process, such as raw materials such as ores and scraps, when manufacturing steel materials industrially, and are allowed within a range that does not adversely affect the present invention. means something
  • one or more elements selected from the following elements may be contained within the ranges shown below instead of part of Fe.
  • the lower limit of the content is 0%. The reason for limiting each element will be explained.
  • Cr less than 0.10% Cr is effective in increasing the strength, so it may be contained as needed for strength adjustment.
  • Cr accelerates the rate of progress of local corrosion and deteriorates local corrosion resistance in a crude oil environment.
  • Cr may promote the generation of solid S. Therefore, the Cr content is set to less than 0.10%.
  • the Cr content is preferably 0.08% or less, or 0.05% or less.
  • the lower limit is not particularly limited and may be 0%, but if the above effect is desired, the Cr content is preferably 0.01% or more, or 0.03% or more.
  • the Ni and Co contents are preferably 0.08% or more, or 0.10% or more, and preferably 0.40% or less, or 0.30% or less, respectively.
  • Nb 0.002-0.200%
  • V 0.005-0.500%
  • Ti 0.002-0.200%
  • Ta 0.005-0.500%
  • Zr 0.005-0.500%
  • B 0.0002 to 0.0050%
  • Nb, V, Ti, Ta, Zr, and B are elements effective in increasing the strength of steel in small amounts, and may be contained as needed for strength adjustment.
  • Nb 0.002-0.200%
  • V 0.005-0.500%
  • Zr 0.005-0.500%. 005 to 0.500%
  • B 0.0002 to 0.0050%.
  • the content of Nb and Ti is preferably 0.005% or more, 0.010% or more, or 0.020% or more, respectively, and 0.180% or less, 0.150% or less, or 0.130% The following are preferred.
  • the contents of V, Ta, and Zr are preferably 0.010% or more, 0.020% or more, or 0.050% or more, respectively, Alternatively, it is preferably 0.200% or less.
  • the B content is preferably 0.0005% or more, or 0.0010% or more, and preferably 0.0040% or less, or 0.0030% or less.
  • the Mg content is preferably 0.0005% or more, 0.0010% or more, or 0.0020% or more, and preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less .
  • the Ca content is preferably 0.0008% or more, 0.0010% or more, or 0.0020% or more, and is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less.
  • the Y content is preferably 0.0005% or more, 0.0010% or more, or 0.0020% or more, and is 0.09% or less, 0.08% or less, or 0.07% or less. preferable.
  • the content of La and Ce is preferably 0.008% or more, 0.010% or more, or 0.015% or more, respectively, and 0.09% or less, 0.08% or less, or 0.07% The following are preferred.
  • the corrosion resistance of the steel material of the present invention is exhibited by the alloying elements contained. Mechanisms for this include the elution of these alloying elements in the pit environment and incorporation into corrosion products, or the deposition and coating of metals on the surface of the steel material.
  • bainite and acicular ferrite generate a slight potential difference with ferrite. Then, in the early stage of corrosion, it moderately accelerates the corrosion of alloying elements, promotes the incorporation of alloying elements into corrosion products, and promotes precipitation on the steel material surface as a metal, thereby exhibiting an effect of improving corrosion resistance. Therefore, the total area ratio of bainite and/or acicular ferrite is set to 20% or more.
  • the total area ratio of bainite and/or acicular ferrite should be 95% or less, preferably less than 90%.
  • bainite may also include tempered bainite, but no distinction is made in the present specification.
  • ashular ferrite is ferrite that transforms in a low temperature range, and refers to acicular ferrite or non-granular ferrite. It does not refer to granular ferrite with polyhedral grain boundaries, ie, polygonal ferrite seen in micrographs.
  • Martensite 2.0% or less Martensite forms a battery with ferrite in the pit environment and acts as a cathode. As a result, the progress of ferrite corrosion is accelerated and non-uniform corrosion is induced. Therefore, the area ratio of martensite is set to 2.0% or less. In the present invention, martensite does not include island martensite.
  • the area ratio of ferrite is preferably 10% or more.
  • the area ratio of ferrite is preferably 75% or less.
  • the balance other than the above is pearlite and island martensite.
  • Perlite deteriorates corrosion resistance. Therefore, the area ratio of pearlite is preferably 5% or less. Also, the area ratio of the island-shaped martensite is preferably 5% or less.
  • the metallographic structure is measured by the following method. In the L section of the steel material, the metallographic structure is observed at a depth of 1 mm, 1/4t, and 1/2t from the surface. After mirror-polishing the observed surface and corroding it with nital, evaluation is made from a photograph taken with an optical microscope at a magnification of 200 times. In the photographed photograph, the portions excluding monochromatic white granular ferrite and black granular perlite are filled with arbitrary colors as bainite or acicular ferrite, and image analysis is performed. Also, martensite is formed in a region surrounded by ferrite, acicular ferrite, bainite, and/or pearlite structures, and island martensite is a structure formed between laths of the bainite structure.
  • the ratio of the portion filled with an arbitrary color to the whole is calculated as the total area ratio of bainite and/or acicular ferrite. Also, the ratio of ferrite to the whole is calculated as the area ratio of ferrite, and the ratio of martensite to the whole is calculated as the area ratio of martensite.
  • the total area ratio of bainite and/or acicular ferrite of 20 to 95% means that bainite and/or It means that the total area ratio of acicular ferrite is 20 to 95%.
  • the martensite area ratio of 2.0% or less means that the martensite area ratio is 2.0 at all of the 1 mm depth position, the 1/4t position, and the 1/2t position from the surface. % or less.
  • the shape of the steel material for crude oil tanks according to the present invention is not particularly limited, it is typically a thick steel plate with a thickness of 10 to 40 mm.
  • the steel material for crude oil tanks according to the present invention has a Vickers hardness of 210HV10 or less. When the Vickers hardness exceeds 210HV10, the corrosion resistance deteriorates although the detailed mechanism has not been elucidated. Also, the Vickers hardness is preferably 160HV10 or more. "HV10" means a "hardness symbol” when a Vickers hardness test is performed with a test force of 98 N (10 kgf) (see JIS Z 2244-1:2020).
  • the Vickers hardness is measured at 5 points at intervals of 1 mm in the direction parallel to the rolling direction at 1 mm depth position, 1/4t position, and 1/2t position from the surface of the steel material in the L cross section of the steel material. Then, the average value of Vickers hardness at each depth position is calculated. Vickers hardness is measured with a test force of 98 N (10 kgf).
  • the Vickers hardness of 210 HV10 or less means that the average value of the Vickers hardness is 210 HV10 or less at all of the 1 mm depth position, the 1/4t position, and the 1/2t position from the surface. .
  • (D) Production Method A preferred production method for the steel material for crude oil tanks of the present invention will be described.
  • Molten steel having the above composition is melted in a known furnace such as a converter or an electric furnace, and made into a steel material such as a slab or billet by a known method such as continuous casting or ingot casting.
  • vacuum degassing refining or the like may be performed at the time of smelting.
  • the composition adjustment method of molten steel should just follow a well-known steel smelting method.
  • the above steel material is hot-rolled into desired dimensions and shapes. It is preferable to heat the steel material to a temperature of 1020° C. or more and hold it for 20 minutes or more, and then perform hot rolling.
  • the heating temperature should be 1020°C or higher.
  • the heating temperature is preferably 1030° C. or higher, more preferably 1040° C. or higher.
  • the heating temperature is preferably 1350° C. or lower, more preferably 1300° C. or lower.
  • the soaking time is set to 20 to 120 minutes.
  • the soaking time is preferably 50 minutes or longer.
  • “soaking time” means the time for isothermal holding after the temperature of the slab reaches the above heating temperature.
  • Hot rolling includes rough rolling and finish rolling, and the time from the end of rough rolling to the start of finish rolling (hereinafter also referred to as "finish rolling waiting time") is preferably 40 seconds or less.
  • finish rolling waiting time is preferably 40 seconds or less.
  • the surface layer of the steel material cools faster than the inside, but by shortening the waiting time for finish rolling, the temperature difference between the surface layer and the inside of the steel material can be reduced.
  • variations in the metallographic structure at the 1 mm depth position, the 1/4t position, and the 1/2t position from the surface layer are suppressed, and bainite And/or it is possible to obtain a metal structure in which the total area ratio of acicular ferrite is 20 to 95% and the area ratio of martensite is 2.0% or less.
  • the finish rolling end temperature is 830 to 930°C. If the finishing temperature of finish rolling is less than 830°C, austenite contains a large amount of strain that becomes the nucleus of ferrite transformation. As a result, the transformation from austenite to ferrite is promoted, and the ferrite area ratio becomes excessive. On the other hand, when the finishing temperature of finish rolling is higher than 930° C., the strain in austenite is small, so the transformation to ferrite, bainite and ascular ferrite is suppressed and the area ratio of martensite becomes excessive.
  • water cooling is used to cool the steel material after hot rolling.
  • water cooling after hot rolling it is possible to obtain a metal structure containing bainite and/or acicular ferrite at a total area ratio of 20% or more.
  • the water cooling start temperature is 900-800°C.
  • the water cooling start temperature is less than 800° C., water cooling is started after the transformation to ferrite has started, so the total area ratio of bainite and/or acicular ferrite is reduced.
  • a steel ingot having a chemical composition shown in Table 2 and a thickness of 120 mm was melted. Furthermore, it was heated and held at 1150° C. for 90 minutes, and hot-rolled under the conditions shown in Table 3 to a thickness of 30 mm. After that, as shown in Table 3, water cooling or air cooling was performed to obtain a hot rolled steel sheet.
  • the metal structure was observed at a depth of 1 mm, 1/4t, and 1/2t from the surface.
  • the observation surface was mirror-polished, corroded with nital, and evaluated from a photograph taken with an optical microscope at a magnification of 200 times.
  • the portions excluding monochromatic white granular ferrite and black granular perlite were colored as bainite or acicular ferrite, and image analysis was performed.
  • martensite is generated in a region surrounded by ferrite, acicular ferrite, bainite, and/or pearlite structures, and island martensite is a structure generated between laths of the bainite structure.
  • the ratio of the portion filled with an arbitrary color to the whole was calculated as the total area ratio of bainite and/or acicular ferrite.
  • the ratio of ferrite to the whole was calculated as the area ratio of ferrite, and the ratio of martensite to the whole was calculated as the area ratio of martensite.
  • the Vickers hardness is measured at 5 points at intervals of 1 mm in the direction parallel to the rolling direction at 1 mm depth position, 1/4t position, and 1/2t position from the surface of the steel material in the L cross section of the steel material. The average value of Vickers hardness at the depth position of was calculated. Vickers hardness was measured with a test force of 98 N (10 kgf).
  • the corrosion resistance test is a corrosion test for COT bottom plate specified by IMO (SOLAS Chapter II-I, Part A-1, Reg. 3-11, as amended by resolution MSC. 291 (87), APPENDIX, Test Procedures for Qualification of Corrosion Resistant Steel for Cargo Tanks in Crude Oil Tankers.).
  • IMO SOLAS Chapter II-I, Part A-1, Reg. 3-11, as amended by resolution MSC. 291 (87), APPENDIX, Test Procedures for Qualification of Corrosion Resistant Steel for Cargo Tanks in Crude Oil Tankers.
  • the pH of the solution was changed from 0.85 to 0.5 in order to make it a stricter condition.
  • a test piece of 25 mm x 60 mm x 5 mm (thickness) was taken from the 1 mm depth position, the 1/4t position, and the 1/2t position from the surface.
  • the sampling method is that the surface 1 mm is sampled so that one side of the test piece becomes the surface 1 mm part, and for the 1/4 t position and 1/2 t position, the center of the plate thickness of the test piece is the 1/4 t position and 1/2 t position, respectively. It was taken so that it would be in position.
  • a 2 mm diameter hole was drilled near the longitudinal end for hanging immersion testing.
  • the surface of the test piece was polished with No. 600 emery polishing paper. Before the test, the test piece was degreased and the pre-test weight and dimensions of the test piece were measured.
  • test solution used was a 10% weight concentration NaCl aqueous solution adjusted to pH 0.5 with hydrochloric acid. 800 ml of this solution (with a specific liquid volume of 20 ml/cm 2 or more) is placed in a beaker, and the temperature of the test solution is maintained at 30°C. The specimen was then suspended using a nylon No. 4 fishing line and immersed in the beaker. The solution was replaced every 24 hours and tested for 3 days. After the test, it was washed to remove corrosion products, dried, and weighed.
  • the corrosion rate was calculated from the weight loss before the test and the surface area and density (here, 7.87 g/cm 3 was used) before the test. Five steel sheets were tested, and whether or not the average value was 0.5 mm or less was investigated. In addition, the evaluation standard for corrosion-resistant steel in the corrosion test for COT bottom plate specified by IMO is 1 mm or less.
  • Table 4 summarizes the results of the area ratio, Vickers hardness, and corrosion rate of bainite and/or acicular ferrite in the metal structure at the 1 mm depth position, the 1/4t position, and the 1/2t position from the surface.
  • ⁇ was given, and when even one site exceeded 0.5 mm/y, x was given.
  • Test No. that satisfies all the provisions of the present invention. 1 to 21 and 26 gave excellent results in all performances.
  • Test No. which is a comparative example. 22-25, 27, and 28 resulted in deterioration in at least one of Vickers hardness or corrosion resistance.

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JP2010196166A (ja) * 2009-01-30 2010-09-09 Jfe Steel Corp 原油タンク用耐食鋼材とその製造方法ならびに原油タンク
JP2010222665A (ja) * 2009-03-25 2010-10-07 Jfe Steel Corp 原油タンク用耐食形鋼材とその製造方法
JP2019131840A (ja) * 2018-01-29 2019-08-08 Ｊｆｅスチール株式会社 耐サワーラインパイプ用高強度鋼板の製造方法、及び耐サワーラインパイプ用高強度鋼板、並びに耐サワーラインパイプ用高強度鋼板を用いた高強度鋼管
WO2020184683A1 (ja) * 2019-03-14 2020-09-17 日本製鉄株式会社 鋼板およびその製造方法

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JP3996727B2 (ja) 2000-01-31 2007-10-24 新日本製鐵株式会社 ダブルハル型石油タンカー貯蔵庫用耐食鋼
JP3570376B2 (ja) 2000-12-04 2004-09-29 Ｊｆｅスチール株式会社 耐原油タンク腐食性に優れた鋼材およびその製造方法
JP2003105487A (ja) 2001-09-28 2003-04-09 Nkk Corp 溶接部の耐食性に優れた貨油タンク用耐食鋼板およびその溶接方法
KR102508129B1 (ko) * 2020-12-21 2023-03-09 주식회사 포스코 저온 충격인성이 우수한 극후물 강재 및 그 제조방법

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Publication number Priority date Publication date Assignee Title
JP2010196166A (ja) * 2009-01-30 2010-09-09 Jfe Steel Corp 原油タンク用耐食鋼材とその製造方法ならびに原油タンク
JP2010222665A (ja) * 2009-03-25 2010-10-07 Jfe Steel Corp 原油タンク用耐食形鋼材とその製造方法
JP2019131840A (ja) * 2018-01-29 2019-08-08 Ｊｆｅスチール株式会社 耐サワーラインパイプ用高強度鋼板の製造方法、及び耐サワーラインパイプ用高強度鋼板、並びに耐サワーラインパイプ用高強度鋼板を用いた高強度鋼管
WO2020184683A1 (ja) * 2019-03-14 2020-09-17 日本製鉄株式会社 鋼板およびその製造方法

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