WO2023162522A1 - Tôle en acier, et procédé de fabrication de celle-ci - Google Patents

Tôle en acier, et procédé de fabrication de celle-ci Download PDF

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Publication number
WO2023162522A1
WO2023162522A1 PCT/JP2023/001392 JP2023001392W WO2023162522A1 WO 2023162522 A1 WO2023162522 A1 WO 2023162522A1 JP 2023001392 W JP2023001392 W JP 2023001392W WO 2023162522 A1 WO2023162522 A1 WO 2023162522A1
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steel sheet
content
cooling
steel
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PCT/JP2023/001392
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English (en)
Japanese (ja)
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恭野 安田
直樹 ▲高▼山
和彦 塩谷
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Ｊｆｅスチール株式会社
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Priority to JP2023520276A priority Critical patent/JP7323088B1/ja
Publication of WO2023162522A1 publication Critical patent/WO2023162522A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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
    • C22C38/06—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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention provides a high-strength steel sheet with excellent toughness and corrosion resistance, particularly a high-strength steel sheet with excellent low-temperature toughness and liquid ammonia stress corrosion cracking resistance suitable for structural members such as tanks used in a low-temperature and liquid ammonia environment. and its manufacturing method.
  • tanks may carry liquid ammonia as well as LPG.
  • ammonia SCC Stress Corrosion Cracking
  • Patent Documents 1 and 2 disclose techniques for satisfying the low-temperature toughness and strength range required for liquefied gas storage tanks as described above.
  • high low-temperature toughness and predetermined strength properties are achieved by heat-treating a steel plate that has been cooled after hot-rolling, or by heat-treating a steel plate that has been water-cooled after hot-rolling several times. is realized.
  • Patent Literatures 1 and 2 above had the economic problem of requiring multiple heat treatments, which required high equipment and energy costs.
  • the present invention solves the above problems, and provides a high-strength steel sheet excellent in ammonia SCC resistance and low-temperature toughness, which is used for storage tanks used for storing liquefied gas in energy transport ships, and a method for producing the same. With the goal.
  • the present inventors used the TMCP process to extensively study various factors affecting the low temperature toughness and strength characteristics of steel sheets.
  • elements such as C, Si, Mn, and N are added to the steel sheet in a predetermined amount or more, and the total volume ratio of the ferrite structure and the bainite structure at the position of 1/2 of the plate thickness of the steel plate is 60% or more. It has been found that controlling the metallographic structure (microstructure) of the steel sheet to achieve the desired low temperature toughness and strength properties can be effectively achieved.
  • the microstructure is controlled so that the volume ratio of the bainite structure at a depth of 0.5 mm from the surface of the steel plate is 90% or more, and the average hardness at a depth of 0.5 mm from the surface of the steel plate is Hv210 or less, and by controlling the average hardness variation to Hv50 or less, SCC resistance in a liquid ammonia environment can be obtained, and costly heat treatment such as the conventional technology can be omitted. did.
  • the present invention was made based on the above findings, and the gist of the present invention is as follows. 1. in % by mass, C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn: 0.50-2.50%, Al: 0.060% or less, N: 0.0010 to 0.0100%, P: 0.020% or less, A steel sheet having a chemical composition containing S: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and inevitable impurities, Hardness characteristics in which the average hardness is Hv210 or less and the variation in the average hardness is Hv50 or less at a position 0.5 mm deep from the surface of the steel plate; A bainite structure has a volume fraction of 90% or more at a position 0.5 mm deep from the surface of the steel plate, and a bainite structure has a volume fraction of 20% or more and a ferrite structure at a half thickness position of the steel plate. and a metal structure in which the total volume fraction of the bainit
  • the component composition further, in mass %, Cu: 0.01-0.50%, Ni: 0.01 to 2.00%, Cr: 0.01 to 1.00%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.50%, Mo: 0.01-0.50% and W: 0.01-1.00% 2.
  • the component composition further, in mass %, V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0100%, Ca: 0.0005 to 0.0200%, Mg: 0.0005-0.0200% and REM: 0.0005-0.0200% 3.
  • V 0.01 to 1.00%
  • Ti 0.005 to 0.100%
  • Co 0.01 to 1.00%
  • Nb 0.005 to 0.100%
  • B 0.0001 to 0.0100%
  • Ca 0.0005 to 0.0200%
  • Mg 0.0005-0.0200%
  • REM 0.0005-0.0200% 3.
  • a steel material having a chemical composition containing S: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and inevitable impurities, is subjected to hot rolling at a rolling end temperature of Ar 3 transformation point or higher.
  • a method for manufacturing a steel sheet in which primary cooling is performed by cooling from a cooling start temperature equal to or higher than the Ar 3 transformation point, then surface heating is performed by reheating, and then secondary cooling is performed, In the primary cooling, the cooling rate from 600 ° C.
  • the steel sheet is manufactured at a cooling rate of 10° C./s or more to a cooling stop temperature of 600° C. or less at a half thickness position of the steel plate.
  • the chemical composition of the steel material is further, in mass%, Cu: 0.01-0.50%, Ni: 0.01 to 2.00%, Cr: 0.01 to 1.00%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.50%, Mo: 0.01-0.50% and W: 0.01-1.00% 4.
  • the chemical composition of the steel material is further, in mass%, V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0100%, Ca: 0.0005 to 0.0200%, Mg: 0.0005-0.0200% and REM: 0.0005-0.0200% 6.
  • V 0.01 to 1.00%
  • Ti 0.005 to 0.100%
  • Co 0.01 to 1.00%
  • Nb 0.005 to 0.100%
  • B 0.0001 to 0.0100%
  • Ca 0.0005 to 0.0200%
  • Mg 0.0005-0.0200%
  • REM 0.0005-0.0200% 6.
  • a steel sheet having excellent low-temperature toughness that is, excellent low-temperature impact resistance and ammonia SCC resistance, and having high strength suitable for structural members such as tanks used in a low-temperature and liquid ammonia environment can be produced at a low cost. It can be provided in a simple process.
  • % representing the content of the following components (elements) means “% by mass” unless otherwise specified.
  • C 0.010-0.200% C is the most effective element for increasing the strength of steel sheets produced by cooling according to the present invention.
  • the C content is specified to be 0.010% or more.
  • the C content is preferably 0.013% or more from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost.
  • the C content is specified at 0.200% or less.
  • the C content is preferably 0.170% or less from the viewpoint of toughness and weldability.
  • Si 0.01-0.50% Si is added for deoxidation.
  • the Si content is specified to be 0.01% or more. Furthermore, it is preferable to make it 0.03% or more.
  • the Si content is specified to be 0.50% or less. Furthermore, the Si content is preferably 0.40% or less from the viewpoint of toughness and weldability.
  • Mn 0.50-2.50%
  • Mn is an element that has the effect of increasing the hardenability of steel, and is one of the important elements that need to be added in order to achieve high strength as in the present invention.
  • the Mn content is specified to be 0.50% or more.
  • the content of Mn is preferably 0.70% or more from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost.
  • the Mn content is specified at 2.50% or less.
  • the Mn content is preferably 2.30% or less from the viewpoint of further suppressing deterioration of toughness and weldability.
  • Al 0.060% or less
  • Al is an element that acts as a deoxidizing agent and has the effect of refining crystal grains.
  • the Al content is preferably 0.001% or more.
  • the Al content is specified at 0.060% or less.
  • the Al content is preferably 0.050% or less from the viewpoint of further preventing toughness deterioration.
  • N 0.0010 to 0.0100% N contributes to the refinement of the structure and improves the toughness of the steel sheet.
  • the N content is specified to be 0.0010% or more. Preferably, it is 0.0020% or more.
  • the N content is specified at 0.0100% or less.
  • the N content is preferably 0.0080% or less from the viewpoint of further suppressing deterioration of toughness and weldability.
  • Ti when Ti is present, N can bond with Ti and precipitate as TiN.
  • P 0.020% or less
  • P has an adverse effect, such as lowering toughness and weldability, by segregating at grain boundaries. Therefore, it is desirable to make the P content as low as possible, but a P content of 0.020% or less is acceptable.
  • the lower limit of the P content is not particularly limited, and may be 0%, but excessive reduction causes a rise in refining costs, so from the viewpoint of cost, the P content should be 0.0005% or more. is preferred.
  • S 0.0100% or less S is present in steel as sulfide-based inclusions such as MnS, and is an element that exerts adverse effects, such as deteriorating the toughness of the steel sheet by becoming the origin of fracture. Therefore, it is desirable that the S content be as low as possible, but a content of 0.0100% or less is permissible.
  • the lower limit of the S content is not particularly limited, and may be 0%, but excessive reduction causes a rise in refining costs, so from the viewpoint of cost, the S content should be 0.0005% or more. is preferred.
  • O 0.0100% or less
  • O is an element that forms an oxide, becomes a starting point of fracture, and has an adverse effect such as lowering the toughness of the steel sheet.
  • the O content is preferably 0.0050% or less, more preferably 0.0030% or less.
  • the lower limit of the O content is not particularly limited, and may be 0%. is preferred.
  • the balance other than the above components is Fe and unavoidable impurities.
  • the above component composition can contain the elements described below, if necessary.
  • Cu 0.01-0.50%, Ni: 0.01-2.00%, Cr: 0.01-1.00%, Sn: 0.01-0.50%, Sb: 0.01- 0.50%, Mo: 0.01 to 0.50%, and W: one or more selected from 0.01 to 1.00% Cu, Ni, Cr, Sn, Sb, Mo and W are It is an element that improves strength and ammonia SCC resistance, and one or more of these elements can be contained.
  • the Cu content is 0.01% or more, when Ni is contained, the Ni content is 0.01% or more, and when Cr is contained, When the Cr content is 0.01% or more, the Sn content is 0.01% or more when Sn is contained, and the Sb content is 0.01% or more when Sb is contained.
  • Mo is contained, the Mo content is preferably adjusted to 0.01% or more, and when W is contained, the W content is preferably adjusted to 0.01% or more.
  • an excessive Ni content causes deterioration of weldability and an increase in alloy cost.
  • the Cu content is 0.50% or less
  • the Ni content is 2.00% or less
  • the Cr content is 1.00% or less
  • the Sn content is 0.50% or less
  • the Sb content is It is preferable to adjust the Mo content to 0.50% or less
  • the W content is 0.50% or less
  • the W content is 1.00% or less.
  • the Cu content is 0.40% or less
  • the Ni content is 1.50% or less
  • the Cr content is 0.80% or less
  • the Sn content is 0.40% or less
  • the Sb content is The amount is adjusted to 0.40% or less
  • the Mo content to 0.40% or less
  • the W content to 0.80% or less.
  • V 0.01-1.00%
  • V is an element that has the effect of improving the strength of the steel sheet, and can be optionally added.
  • the V content is preferably 0.01% or more.
  • the V content is preferably 1.00% or less. More preferably, the lower limit of V content is 0.05% and the upper limit is 0.50%.
  • Ti 0.005-0.100%
  • Ti is an element that has a strong tendency to form nitrides and has the action of fixing N and reducing solid solution N, and can be added arbitrarily.
  • Ti can improve the toughness of the base material and the weld zone.
  • the Ti content is preferably 0.005% or more. Furthermore, it is more preferable to make it 0.007% or more.
  • the Ti content exceeds 0.100%, the toughness rather decreases. Therefore, when adding Ti, the Ti content is preferably 0.100% or less. Furthermore, the Ti content is more preferably 0.090% or less.
  • Co 0.01-1.00%
  • Co is an element that has the effect of improving the strength of the steel sheet, and can be optionally added.
  • the Co content is preferably 0.01% or more.
  • the Co content is preferably 1.00% or less. More preferably, the Co content has a lower limit of 0.05% and an upper limit of 0.50%.
  • Nb 0.005-0.100%
  • Nb is an element that has the effect of reducing the grain size of prior austenite and improving the toughness by precipitating as a carbonitride.
  • the Nb content is preferably 0.005% or more. Furthermore, it is more preferable to make it 0.007% or more.
  • the Nb content exceeds 0.100%, a large amount of NbC precipitates, resulting in a decrease in toughness. Therefore, when Nb is added, the Nb content is preferably 0.100% or less. Furthermore, it is more preferable to make it 0.060% or less.
  • B 0.0001 to 0.0100%
  • B is an element that has the effect of significantly improving hardenability even when added in a very small amount. That is, the strength of the steel sheet can be improved.
  • the B content is preferably 0.0001% or more.
  • the B content exceeds 0.0100%, the weldability deteriorates. Therefore, when B is added, the B content is preferably 0.0100% or less. More preferably, the B content has a lower limit of 0.0010% and an upper limit of 0.0030%.
  • Ca 0.0005-0.0200%
  • Ca is an element that binds to S and has the effect of suppressing the formation of MnS or the like elongated in the rolling direction. That is, by adding Ca, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like.
  • the Ca content is preferably 0.0005% or more.
  • the Ca content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Ca is added, the Ca content is preferably 0.0200% or less. More preferably, the Ca content has a lower limit of 0.0020% and an upper limit of 0.0100%.
  • Mg: 0.0005-0.0200% Mg, like Ca is an element that binds to S and has the effect of suppressing the formation of MnS or the like elongated in the rolling direction. That is, by adding Mg, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like. In order to obtain such an effect, when Mg is added, the Mg content is preferably 0.0005% or more. On the other hand, when the Mg content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less. More preferably, the Mg content has a lower limit of 0.0020% and an upper limit of 0.0100%.
  • REM 0.0005-0.0200%
  • REM rare earth metal
  • the REM content is preferably 0.0005% or more.
  • the REM content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when REM is added, the REM content is preferably 0.0200% or less. More preferably, the REM content has a lower limit of 0.0020% and an upper limit of 0.0100%.
  • the steel sheet of the present invention has the above chemical composition, and in addition, the average position at a depth of 0.5 mm from the surface of the steel sheet (also referred to as the 0.5 mm position in the present invention)
  • the hardness is Hv210 or less and the average hardness variation is Hv50 or less.
  • the steel sheet of the present invention has a bainite structure (hereinafter also simply referred to as bainite) volume ratio of 90% or more at the 0.5 mm position, and the 1/2 position of the plate thickness of the steel plate (1 /2 depth position (hereinafter also simply referred to as 1/2 position or plate thickness center), the volume fraction of bainite is 20% or more, and the ferrite structure (hereinafter simply referred to as ferrite) and It has a metal structure in which the total volume fraction of bainite is 60% or more.
  • the average hardness is Hv210 or less, and its variation is Hv50 or less
  • the average hardness at the 0.5 mm position is Hv210 or less, and its variation is Hv50 or less. If a high-hardness region exists in the extreme surface layer of the steel sheet, specifically, at a position of 0.5 mm from the surface of the steel sheet, stress corrosion cracking in a liquid ammonia environment is promoted. Moreover, when a local high-hardness region exists, stress concentration occurs when stress is applied to the steel sheet, and stress corrosion cracking is promoted.
  • the steel sheet of the present invention excellent ammonia SCC resistance can be secured by adjusting the hardness characteristics so that the average hardness at the 0.5 mm position is Hv210 or less and the variation is Hv50 or less.
  • the lower limit of the average hardness at the 0.5 mm position is not particularly limited, it is preferably about Hv130.
  • the lower limit of the variation in average hardness may be Hv0, but it is industrially about Hv10.
  • the average hardness can be calculated by measuring Vickers hardness at a plurality of points (for example, 100 points) at a position of 0.5 mm.
  • the variation in average hardness means the standard deviation of the Vickers hardness measured to obtain the average hardness.
  • the structure at the 0.5 mm position must have a volume fraction of bainite of 90% or more.
  • a hard phase such as a martensite structure or an island-shaped martensite (MA) structure
  • the surface layer hardness increases, and the variation in hardness within the steel plate increases, impeding material uniformity.
  • the volume fraction of bainite is less than 90%
  • the volume fractions of other structures that is, ferrite, island-shaped martensite, martensite, pearlite, and austenite, increase. Strength and/or ammonia SCC resistance is not obtained.
  • bainite includes a structure called bainitic ferrite or granular ferrite that transforms during or after cooling, which contributes to transformation strengthening, and a structure obtained by tempering them.
  • the remaining structure occupying 10% or less in volume fraction may include a martensite structure in addition to the ferrite, pearlite, and austenite structures.
  • the fraction of each structure in the remaining structure is not particularly limited, but the remaining structure is preferably a pearlite structure.
  • the volume fraction of bainite is 20% or more, and the total volume fraction of ferrite and bainite is 60% or more
  • the structure at the 1/2 position must have a bainite volume fraction of 20% or more and a total volume fraction of ferrite and bainite of 60% or more. Excessive generation of ferrite leads to a decrease in strength or toughness. Further, when the total volume fraction of ferrite and bainite is less than 60%, the volume fractions of structures other than this, namely, island-shaped martensite structure, martensite structure, pearlite structure and austenite structure, will increase, which is sufficient. sufficient strength or toughness cannot be obtained, and the mechanical properties cannot be satisfied.
  • the total volume fraction of ferrite and bainite may be 100%.
  • the ferrite means ferrite generated in the cooling process before tempering
  • the bainite means bainite generated in the cooling process before tempering.
  • the reason why the microstructure at the center of thickness is defined is that the microstructure at the center of thickness affects the strength characteristics of the center of thickness. This is because the strength properties affect the strength of the steel plate as a whole.
  • the remaining structure occupying 40% or less in volume fraction may include martensite structure in addition to pearlite structure and austenite structure.
  • the fraction of each structure in the remaining structure is not particularly limited, but the remaining structure is preferably a pearlite structure.
  • the volume ratio of various microstructures can be measured by the method described in Examples below.
  • the manufacturing method in the present invention is C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, Al: 0.50%. 060% or less, N: 0.0010 to 0.0100%, P: 0.020% or less, S: 0.0100% or less and O: 0.0100% or less, and if necessary, Cu: 0.01-0.50%, Ni: 0.01-2.00%, Cr: 0.01-1.00%, Sn: 0.01-0.50%, Sb: 0.01%-0 .50%, one or more selected from Mo: 0.01 to 0.50% and W: 0.01 to 1.00% and/or V: 0.01 to 1.00%, Ti: 0 .005-0.100%, Co: 0.01-1.00%, Nb: 0.005-0.100%, B: 0.0001-0.0100%, Ca: 0.0005-0.0200 %, Mg: 0.0005 to 0.0200%, and REM: 0.0005 to 0.0200%, with the balance being Fe and
  • the manufacturing conditions of the steel material need not be particularly limited. It is preferable to use a steel material such as a slab of predetermined dimensions in the method. It should be noted that there is no problem in making a steel material such as a slab having a predetermined size by the ingot casting-decomposition rolling method.
  • the steel material thus obtained is directly hot-rolled without cooling or hot-rolled after reheating.
  • Such hot rolling is performed at a rolling end temperature equal to or higher than the Ar 3 transformation point (hereinafter simply referred to as the Ar 3 transformation point).
  • primary cooling is performed under predetermined conditions from a cooling start temperature equal to or higher than the Ar 3 transformation point, followed by surface heating by reheating under predetermined conditions, and then secondary cooling under predetermined conditions.
  • the heating temperature of the steel material (the temperature at which it is subjected to hot rolling) is not particularly limited, but if the heating temperature is too low, the deformation resistance increases, the load on the hot rolling mill increases, and hot rolling becomes difficult. may become On the other hand, if the temperature exceeds 1300° C., the oxidation becomes significant, the oxidation loss increases, and the yield increases. For these reasons, the heating temperature is preferably 950° C. or higher and 1300° C. or lower.
  • hot rolling [Rolling end temperature: Ar 3 transformation point or higher]
  • the rolling end temperature in hot rolling should be the Ar 3 transformation point or higher. More preferably, the rolling end temperature in hot rolling is a temperature of Ar 3 transformation point +10°C or higher.
  • the rolling end temperature exceeds 950°C, the structure may coarsen and the toughness may deteriorate, so the rolling end temperature is preferably 950°C or less.
  • each element indicates the content of the element in steel (% by mass).
  • the hot-rolled steel sheet is subjected to primary cooling from a cooling start temperature equal to or higher than the Ar 3 transformation point. If the cooling start temperature in the primary cooling is lower than the Ar 3 transformation point, ferrite will be excessively formed, the strength will be insufficient, and the ammonia SCC will deteriorate. Therefore, the cooling start temperature should be the Ar 3 transformation point or higher.
  • the primary cooling rate is specified at 30 to 100° C./s.
  • the primary cooling rate can be controlled by controlled cooling by intermittent cooling including a cooling stop period.
  • the temperature in the thickness cross section The distribution, especially the temperature at the 0.5 mm position, can be determined in real time.
  • the temperature reached (reheating temperature) at the 0.5 mm position is less than 500°C, the effect of tempering is insufficient, so the hardness of the surface layer is high, and the uniformity of the material cannot be obtained, resulting in deterioration of the ammonia SCC resistance.
  • the upper limit of the temperature reached at the 0.5 mm position is not particularly limited, but can be, for example, 700° C. or less.
  • Secondary cooling [Cooling stop temperature at 1/2 position: 600 ° C. or less]
  • secondary cooling is performed. This secondary cooling is performed until the temperature at the 1/2 position becomes 600° C. or lower.
  • secondary cooling is performed under predetermined conditions to an arbitrarily set cooling stop temperature of 600 ° C. or less, so that the ferrite and bainite structures are reduced to a predetermined volume fraction at the center of the plate thickness.
  • the cooling stop temperature is specified at 600° C. or less.
  • the lower limit of the cooling stop temperature is not particularly limited, but if the cooling stop temperature is excessively low, the volume fraction of island-shaped martensite becomes too large, resulting in a decrease in toughness. Therefore, the cooling stop temperature is preferably 200° C. or higher.
  • the cooling rate to the cooling stop temperature of 600 ° C. or less at 1/2 position 10 ° C./s or more
  • the cooling rate to the cooling stop temperature of 600 ° C. or less at the 1/2 position so that the ferrite or bainite has a predetermined volume ratio (also referred to as the secondary cooling rate) ) is set to 10° C./s or more. If the secondary cooling rate is less than 10° C./s, excessive ferrite and pearlite may be generated, resulting in insufficient strength.
  • the upper limit of the secondary cooling rate is not particularly limited, but can be, for example, 65° C./s or less.
  • the cooling start temperature (cooling start temperature at the 1/2 position) in the secondary cooling can usually be the temperature at the 1/2 position immediately after the surface is heated by recuperation.
  • the secondary cooling rate can be controlled by controlled cooling through intermittent cooling including a cooling stop period.
  • the temperature at the 1/2 position is physically difficult to measure directly.
  • the temperature in the thickness cross section The distribution, in particular the temperature at the 1/2 position, can be determined in real time.
  • the steel sheet thus obtained will have excellent strength properties and toughness.
  • the excellent strength characteristics are yield strength YS (yield point YP when there is a yield point, 0.2% yield strength ⁇ 0.2 when there is no yield point): 360 MPa or more and tensile strength (TS): 490 MPa or more is.
  • excellent toughness means that vTrs conforming to JIS Z 2241 is -30°C or less.
  • any item not described in this specification can be used by a conventional method.
  • Slabs were made by continuous casting from steels having the chemical compositions shown in Table 1 (steel grades A to AH, the balance being Fe and unavoidable impurities), and used to make thick steel plates (No. 1 to 50) with a thickness of 25 mm. Then, under the conditions shown in Table 2, hot rolling, primary cooling, surface heating by reheating, and secondary cooling were sequentially performed to obtain a steel sheet. For the obtained steel sheet, measurement of the composition fraction of the metal structure at the 0.5 mm position from the steel plate surface and the 1/2 position of the plate thickness, evaluation of hardness characteristics at the 0.5 mm position from the steel plate surface, strength characteristics and toughness Evaluation and evaluation of ammonia SCC resistance were carried out. Each test method is as follows. These results are also shown in Table 2.
  • the determination when obtaining the fraction of the metal structure of the sample was performed as follows. That is, in the photographed image described above, the polygonal ferrite is discriminated as ferrite (F in Table 2), and it has elongated lath-shaped ferrite and contains carbide with an equivalent circle diameter of 0.05 ⁇ m or more. The texture was identified as bainite (B in Table 2).
  • HV0.1 Vickers hardness
  • Ammonia SCC resistance was evaluated by an accelerated test in which a four-point bending test was performed using a test solution and constant potential anodic electrolysis was performed to promote corrosion. Specifically, we performed the following steps: A test piece with a thickness of 5 mm x 15 mm x 115 mm was taken from the surface of the steel plate, subjected to ultrasonic degreasing in acetone for 5 minutes, and stress of 100% YS of the actual yield strength of each steel plate was applied by four-point bending. .
  • the invention examples (No. 1 to 31) all have a yield strength YS of 360 MPa or more and a tensile strength TS of 490 MPa or more, and vTrs is -30 ° C. or less at low temperatures.
  • YS yield strength
  • TS tensile strength
  • TS tensile strength
  • the chemical composition of the steel is outside the range of the present invention, so the yield strength YS, tensile strength TS, toughness at low temperatures, or ammonia SCC resistance are inferior.
  • the chemical composition of the steel may be considered as the chemical composition of the steel sheet.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
  • Filling Or Discharging Of Gas Storage Vessels (AREA)

Abstract

L'invention fournit une tôle en acier hautement résistante qui à son tour fournit un réservoir de stockage, ou similaire, mis en œuvre dans le chargement d'un gaz liquéfié à bord d'un navire de transport d'énergie, et qui se révèle excellente en termes de résistance à la fissuration par corrosion sous contrainte provoquée par l'ammoniac et de résilience aux basses températures. La tôle en acier de l'invention présente une composition prédéfinie. Cette tôle en acier présente également des caractéristiques de dureté telles que la dureté moyenne en une position à 0,5mm de profondeur à partir de sa surface est inférieure ou égale à Ｈｖ２１０, et la variation de cette dureté moyenne est inférieure ou égale à Ｈｖ５０. Enfin, cette tôle en acier présente une structure métallique dans laquelle le rapport en volume d'une structure de bainite en une position à 0,5mm de profondeur à partir de sa surface est supérieur ou égal à 90%, le rapport en volume de la structure de bainite en une position à 1/2 d'épaisseur de tôle est supérieur ou égal à 20%, et le rapport en volume total d'une structure de ferrite et de la structure de bainite est supérieur ou égal à 60%.
PCT/JP2023/001392 2022-02-24 2023-01-18 Tôle en acier, et procédé de fabrication de celle-ci WO2023162522A1 (fr)

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2011105963A (ja) * 2009-11-12 2011-06-02 Nippon Steel Corp 低温靭性の優れた低降伏比高張力鋼板の製造方法
JP2011208222A (ja) * 2010-03-30 2011-10-20 Sumitomo Metal Ind Ltd Ｌｐｇ・アンモニア混載用鋼材の製造方法
WO2021106368A1 (fr) * 2019-11-27 2021-06-03 Ｊｆｅスチール株式会社 Tôle d'acier et son procédé de production
JP2021088753A (ja) * 2019-12-06 2021-06-10 日本製鉄株式会社 タンク用鋼板

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2011105963A (ja) * 2009-11-12 2011-06-02 Nippon Steel Corp 低温靭性の優れた低降伏比高張力鋼板の製造方法
JP2011208222A (ja) * 2010-03-30 2011-10-20 Sumitomo Metal Ind Ltd Ｌｐｇ・アンモニア混載用鋼材の製造方法
WO2021106368A1 (fr) * 2019-11-27 2021-06-03 Ｊｆｅスチール株式会社 Tôle d'acier et son procédé de production
JP2021088753A (ja) * 2019-12-06 2021-06-10 日本製鉄株式会社 タンク用鋼板

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