WO2021106368A1 - Tôle d'acier et son procédé de production - Google Patents

Tôle d'acier et son procédé de production Download PDF

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Publication number
WO2021106368A1
WO2021106368A1 PCT/JP2020/037483 JP2020037483W WO2021106368A1 WO 2021106368 A1 WO2021106368 A1 WO 2021106368A1 JP 2020037483 W JP2020037483 W JP 2020037483W WO 2021106368 A1 WO2021106368 A1 WO 2021106368A1
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steel sheet
content
temperature
steel
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PCT/JP2020/037483
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English (en)
Japanese (ja)
Inventor
直樹 ▲高▼山
亮 荒尾
植田　圭治
進典 秋吉
直一 山村
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Ｊｆｅスチール株式会社
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Application filed by Ｊｆｅスチール株式会社 filed Critical Ｊｆｅスチール株式会社
Priority to JP2021507721A priority Critical patent/JP7147960B2/ja
Priority to CN202080082601.5A priority patent/CN114761595B/zh
Priority to KR1020227003947A priority patent/KR20220030288A/ko
Publication of WO2021106368A1 publication Critical patent/WO2021106368A1/fr

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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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • 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
    • C22C38/02—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
    • C22C38/06—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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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 provides a steel sheet having excellent toughness and corrosion resistance, particularly a steel sheet having excellent low-temperature toughness and ammonia stress corrosion cracking resistance, which is used in a multipurpose tank in which liquefied petroleum gas (hereinafter referred to as LPG) and liquid ammonia are mixed, and a method for producing the same. It is about.
  • LPG liquefied petroleum gas
  • liquid ammonia may be transported to the tank.
  • TS tensile strength
  • YS yield strength
  • Patent Documents 1 and 2 it is necessary to carry out heat treatment a plurality of times, and there is an economic problem in that the cost for equipment and energy for that purpose is large. Further, when the first quenching temperature is high, the improvement margin of toughness at 1 / 4t is small, so that the toughness at the surface layer of the steel sheet exposed to high temperature for a long time becomes unstable. There was a risk.
  • the present invention solves the above problems and provides, for example, a steel sheet having excellent ammonia stress corrosion cracking resistance and low temperature toughness and a method for producing the same, which is used for a storage tank used for accommodating liquefied gas in an energy transport ship.
  • a steel sheet having excellent ammonia stress corrosion cracking resistance and low temperature toughness and a method for producing the same, which is used for a storage tank used for accommodating liquefied gas in an energy transport ship. The purpose.
  • the present inventors have diligently studied various factors for the low temperature toughness and strength characteristics of the steel sheet by using an online heating / cooling device.
  • elements such as C, Si, Mn, and Ti were added in a predetermined amount or more, and the total volume fraction of tempered martensite and tempered bainite at a depth of 1 mm from the surface of the steel plate was 90% or more.
  • the microstructure is controlled so that the total volume fraction of ferrite and bainite at a depth of 1/2 of the plate thickness from the surface of the steel plate is 60 to 90% and the volume fraction of island-shaped martensite is 10% or less. It was found that the low temperature toughness and strength characteristics of bainite can be exhibited and costly heat treatment can be omitted.
  • the present invention has been completed by further studying based on such findings. That is, the gist of the present invention is as follows.
  • composition of the components is further increased by mass%.
  • Cu 2.00% or less
  • W 1.00% or less
  • Co 1.00% or less
  • Nb 0.100% or less
  • B 0.0100% or less
  • Ca 0.0200% or less
  • the steel sheet according to 1 above which contains at least one selected from Mg: 0.0200% or less and REM: 0.0200% or less.
  • composition of the components is further increased by mass%.
  • Cu 2.00% or less
  • W 1.00% or less
  • Co 1.00% or less
  • Nb 0.100% or less
  • B 0.0100% or less
  • Ca 0.0200% or less
  • the steel sheet of the present invention will be specifically described.
  • the steel sheet and the steel material used for producing the steel sheet have the above-mentioned composition. Therefore, first, the reason for limiting the component composition of steel in the present invention as described above will be described.
  • “%” regarding a component composition shall mean “mass%” unless otherwise specified.
  • [Ingredient composition] C 0.05% or more and 0.15% or less
  • C is an element having an action of increasing the hardenability of steel, and is one of the important elements that need to be added in order to achieve high strength. is there.
  • the C content is set to 0.05% or more.
  • the C content is preferably 0.07% or more.
  • the C content is set to 0.15% or less.
  • the C content is preferably 0.13% or less.
  • Si 0.50% or less Si is an element that acts as an antacid, but on the other hand, it is an element that causes deterioration of toughness and weldability. Therefore, it is desirable to reduce the content as much as possible, but 0.50% or less is acceptable. Since deoxidation of steel is sufficiently possible with Al, Ti, etc., the lower limit of the Si content is not particularly limited and may be 0%. From the viewpoint of toughness and weldability, it is preferably 0.40% or less, and more preferably 0.30% or less.
  • Mn 0.50% or more and 2.00% or less
  • Mn is an element having an action of increasing the hardenability of steel, and is one of the important elements that need to be added in order to satisfy high strength. is there.
  • the Mn content is set to 0.50% or more.
  • the Mn content is preferably 0.70% or more, more preferably 0.90% or more. ..
  • the Mn content is set to 2.00% or less.
  • the Mn content is preferably 1.80% or less, and more preferably 1.60% or less.
  • Al 0.060% or less
  • Al is an element that acts as a deoxidizing agent and also has an action of refining crystal grains.
  • the Al content is preferably 0.010% or more.
  • the Al content is set to 0.060% or less.
  • the Al content is preferably 0.050% or less, and more preferably 0.040% or less.
  • N 0.0010% or more and 0.0100% or less N combines with Ti and precipitates as TiN, which contributes to the miniaturization of the structure and improves the toughness.
  • the N content is set to 0.0010% or more. Preferably, it is 0.0020% or more.
  • the toughness is rather lowered. Therefore, from the viewpoint of suppressing deterioration of toughness and weldability, it is set to 0.0100% or less.
  • the N content is preferably 0.0080% or less, and more preferably 0.0060% or less.
  • Ti 0.005% or more and 0.100% or less
  • Ti is an element that has a strong tendency to form a nitride and has an effect of fixing N and reducing solid solution N. Therefore, the toughness of the base metal and the welded portion can be improved by adding Ti.
  • the Ti content is set to 0.005% or more.
  • the Ti content is preferably 0.012% or more.
  • the Ti content is set to 0.100%.
  • the Ti content is preferably 0.090% or less, more preferably 0.080% or less.
  • P 0.020% or less
  • P is an element contained as an unavoidable impurity, and has an adverse effect such as lowering toughness and weldability by segregating at grain boundaries. Therefore, it is desirable to reduce the P content as much as possible, but 0.020% or less is acceptable.
  • the lower limit of the P content is not particularly limited and may be 0%, but since P is an element unavoidably contained in steel as an impurity, it is industrially more than 0%. It's okay. Further, since excessive reduction causes an increase in refining cost, the P content is preferably 0.0005% or more.
  • S 0.010% or less
  • S is an element contained as an unavoidable impurity, which is present in steel as a sulfide-based inclusion such as MnS and has an adverse effect such as being a starting point of fracture. .. Therefore, it is desirable to reduce the S content as much as possible, but 0.010% or less is acceptable.
  • the lower limit of the S content is not particularly limited and may be 0%.
  • S is an element that is unavoidably contained in steel as an impurity, and therefore may be industrially more than 0%. That is, since excessive reduction causes an increase in refining cost, it is preferable to set the S content to 0.0005% or more from the viewpoint of cost.
  • O 0.0100% or less
  • O is an element contained as an unavoidable impurity and has an adverse effect such as forming an oxide and becoming a starting point of fracture. Therefore, it should be 0.0100% or less.
  • 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%, but since O is an element unavoidably contained in steel as an impurity, it is industrially more than 0%. It's okay. That is, since excessive reduction causes an increase in refining cost, it is preferable to set the O content to 0.0020% or more from the viewpoint of cost.
  • the component composition of the balance Fe and the unavoidable impurities is the basic component composition in the present invention.
  • the basic component composition is optionally Cu: 2.00% or less, Ni: 2.00% or less, Cr: 1.00% or less, Mo: 1.00% or less for the purpose of further improving the strength characteristics or toughness.
  • B 0.0100% or less
  • Ca 0.0200% or less
  • Mg It can further contain 1 or more selected from the group consisting of: 0.0200% or less and REM: 0.0200% or less.
  • Cu is an element having an action of increasing the hardenability of steel and improving the strength of a steel sheet, and can be arbitrarily added.
  • the Cu content is preferably 0.01% or more in order to obtain the above effect. More preferably, it is 0.20% or more.
  • the Cu content exceeds 2.00%, the toughness deteriorates and the alloy cost increases. Therefore, when Cu is added, the Cu content is set to 2.00% or less. More preferably, it is 1.00% or less.
  • Ni is an element having an action of improving the strength of a steel sheet like Cu, and can be arbitrarily added.
  • the Ni content is preferably 0.01% or more in order to obtain the above effect. More preferably, it is 0.20% or more.
  • the Ni content is set to 2.00% or less. More preferably, it is 1.00% or less.
  • Cr 1.00% or less
  • Cr is an element having an action of improving the strength of the steel sheet like Cu, and can be arbitrarily added.
  • the Cr content is preferably 0.01% or more. More preferably, it is 0.05% or more.
  • the Cr content is set to 1.00% or less. More preferably, it is 0.50% or less.
  • Mo 1.00% or less
  • Mo is an element having an action of improving the strength of the steel sheet like Cu, and can be arbitrarily added.
  • the Mo content is preferably 0.01% or more. More preferably, it is 0.05% or more.
  • the Mo content is set to 1.00% or less. More preferably, it is 0.50% or less.
  • V 1.00% or less
  • V is an element having an action of improving the strength of the steel sheet like Cu, and can be arbitrarily added.
  • the V content is preferably 0.01% or more. More preferably, it is 0.05% or more.
  • the V content is set to 1.00% or less. More preferably, it is 0.50% or less.
  • W 1.00% or less W is an element having an action of improving the strength of the steel sheet like Cu, and can be arbitrarily added.
  • the W content is preferably 0.01% or more. More preferably, it is 0.05% or more.
  • the Mo content is set to 1.00% or less. More preferably, it is 0.50% or less.
  • Co 1.00% or less
  • Co is an element having an action of improving the strength of the steel sheet like Cu, and can be arbitrarily added.
  • the Co content is preferably 0.01% or more. More preferably, it is 0.05% or more.
  • the Co content is set to 1.00% or less. More preferably, it is 0.50% or less.
  • Nb 0.100% or less
  • Nb is an element having the effect of reducing the particle size of the old austenite and improving the toughness by precipitating as a carbonitride.
  • the Nb content is preferably 0.005% or more in order to obtain the above effect. Further, the Nb content is more preferably 0.007% or more.
  • the Nb content exceeds 0.100%, a large amount of NbC is precipitated and the toughness is lowered. Therefore, when Nb is added, the Nb content is set to 0.100% or less.
  • the Nb content is preferably 0.080% or less, more preferably 0.060% or less, and further preferably 0.045% or less.
  • B 0.0100% or less
  • B is an element that has the effect of significantly improving hardenability even when added in a small amount. Therefore, the strength of the steel sheet can be improved.
  • the B content is preferably 0.0001% or more.
  • the B content is more preferably 0.0005% or more, and further preferably 0.0010% or more.
  • the B content is set to 0.0100% or less.
  • the B content is preferably 0.0050% or less, and more preferably 0.0030% or less.
  • Ca 0.0200% or less
  • Ca is an element that binds to S and has an effect of suppressing the formation of MnS or the like that extends long in the rolling direction. Therefore, by adding Ca, the morphology of the sulfide-based inclusions can be controlled so as to have a spherical shape, and the toughness of the welded portion or the like can be improved.
  • the Ca content is preferably 0.0005% or more. More preferably, it is 0.0020% or more.
  • the Ca content exceeds 0.0050%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to deterioration of surface properties due to an increase in surface defects and a decrease in bending workability. Therefore, when Ca is added, the Ca content is set to 0.0050% or less. More preferably, it is 0.0100% or less.
  • Mg 0.0200% or less Mg, like Ca, is an element that binds to S and suppresses the formation of MnS and the like that extend long in the rolling direction. Therefore, by adding Mg, the morphology of the sulfide-based inclusions can be controlled so as to have a spherical shape, and the toughness of the welded portion or the like can be improved.
  • the Mg content is preferably 0.0005% or more. More preferably, it is 0.0020% or more. On the other hand, when the Mg content exceeds 0.0050%, the cleanliness of the steel is lowered.
  • the Mg content is set to 0.0050% or less. More preferably, it is 0.0100% or less.
  • REM 0.0200% or less REM (rare earth metal) is an element that binds to S and suppresses the formation of MnS and the like that extend long in the rolling direction, like Ca and Mg. Therefore, by adding REM, the morphology of the sulfide-based inclusions can be controlled so as to have a spherical shape, and the toughness of the welded portion or the like can be improved.
  • the REM content is preferably 0.0005% or more. More preferably, it is 0.0020% or more. On the other hand, when the REM content exceeds 0.0050%, the cleanliness of the steel is lowered.
  • the REM content is set to 0.0080% or less. More preferably, it is 0.0100% or less.
  • the steel plate of the present invention has a total volume fraction of tempered martensite and tempered bainite at a depth of 1 mm from the surface of the steel plate of 90% or more, and is a plate from the surface of the steel plate. It has a microstructure in which the total volume fraction of ferrite and bainite at a depth of 1/2 of the thickness is 60-90% and the volume fraction of island martensite is 10% or less. The reason for limiting the microstructure of steel as described above will be described below.
  • the microstructure of the steel sheet of the present invention will be described.
  • the total volume fraction of tempered martensite and tempered bainite at a depth of 1 mm from the surface of the steel sheet is 90% or more
  • the surface structure having the fastest cooling rate is martensite or bainite.
  • the manufacturing conditions of the steel sheet are described later, and by temporarily suspending the cooling after hot rolling and intentionally tempering only the surface layer portion of the steel sheet, excessive hardening of the surface of the steel sheet is prevented and a predetermined strength is obtained. It satisfies the characteristics and improves the toughness at low temperature.
  • the total volume fraction of tempered martensite and tempered bainite is 90% or more.
  • the residual structure other than tempered martensite or tempered bainite is 10% or more, the strength difference between the tempered martensite or tempered bainite and the residual structure becomes large and the strength characteristics are not satisfied, or at low temperature.
  • the toughness will decrease, the total volume ratio of tempered martensite and tempered bainite should be 90% or more. Since the higher the volume fraction of tempered martensite and tempered bainite is, the upper limit of the volume fraction is not particularly limited and may be 100%.
  • the ratios of tempered martensite and tempered bainite are not particularly limited, but it is preferable that the tempered martensite is 80% or more.
  • the type of the residual structure is not particularly limited, but structures such as ferrite, pearlite, austenite, bainite, and martensite may be mixed, but the total volume fraction thereof is less than 10%.
  • the fraction of each tissue in the residual structure is not particularly limited, but from the viewpoint of toughness, it is preferable that the hardness difference from tempered martensite or tempered bainite is small, so that the residual structure is preferably bainite.
  • the volume fractions of the tempered martensite and the tempered bainite are values at a depth of 1 mm from the surface of the steel sheet. This is to improve the toughness of the surface layer portion.
  • the volume fraction of various microstructures can be measured by the method described in Examples described later.
  • the total volume fraction of ferrite and bainite at 1/2 of the thickness of the steel sheet is 60% or more and 90% or less, and the volume fraction of island-shaped martensite is 10% or less
  • the total volume fraction of ferrite and bainite is 60% or more and 90% or less, and the volume fraction of island-shaped martensite contained in the rest. Is 10% or less. That is, if the total volume fraction of ferrite and bainite is less than 60%, the volume fractions of other martensite, pearlite, and austenite will increase, and sufficient strength and / or toughness cannot be obtained. The characteristics cannot be satisfied. On the other hand, if the total volume fraction of the tissue exceeds 90%, the volume fraction of martensite, pearlite, austenite and the like becomes too low, and the strength characteristics are not satisfied.
  • the ferrite is a ferrite produced in a cooling process that has not undergone tempering or the like
  • the bainite is a bainite produced in a cooling process that has not undergone tempering.
  • the microstructure at the center of the plate thickness is defined because it affects the strength characteristics of 1/2 of the plate thickness.
  • the ratios of ferrite and bainite are not particularly limited, but from the viewpoint of further improving the strength characteristics, it is desirable to contain a plurality of structures having different strengths, and the ferrite content is 10% or more. preferable.
  • the balance other than ferrite and bainite may have a microstructure such as pearlite or austenite, but if the balance contains island-like martensite in an amount of more than 10%, the toughness is significantly reduced, so that island-like martensite
  • the volume ratio of the site shall be 10% or less. It is preferably 5% or less, and of course 0%. That is, since the island-shaped martensite in the remaining tissue has higher strength and lower toughness than normal martensite, it becomes a starting point of fracture, and thus defines the volume fraction of the tissue.
  • the residual structure occupying 10% or more and 40% or less in volume fraction may contain martensite in addition to pearlite and austenite.
  • the fraction of each tissue in the residual tissue is not particularly limited, but the residual tissue is preferably pearlite.
  • the volume fraction of various microstructures can be measured by the method described in Examples described later.
  • a steel material having the above-mentioned composition is heated and hot-rolled to obtain a hot-rolled steel sheet, and cooled so that the starting temperature is at or above the Ar 3 transformation point to obtain a steel sheet.
  • each manufacturing condition will be described in detail.
  • the production conditions of the steel material are not particularly limited, but the molten steel having the above-mentioned composition is melted by a known melting method such as a converter to be used as a known casting method such as a continuous casting method. Therefore, it is preferable to use a steel material such as a slab having a predetermined size. It should be noted that there is no problem even if it is a steel material such as a slab having a predetermined size by the ingot-decomposition rolling method.
  • the obtained steel material is hot-rolled directly without cooling, or is heated once and then hot-rolled.
  • Hot rolling was performed at Ar 3 point or higher, the average cooling rate from then Ar 3 point or more temperature to start the cooling, the temperature at a depth of 1mm from the hot rolled sheet surface is at a temperature of 600 ° C. or less : Cool at 10 ° C / s or higher (first cooling), and when the above temperature reaches 600 ° C or lower, stop cooling once and leave it for 10 to 600 seconds, then continue to the temperature at the center of the plate thickness. Cooling at an average cooling rate of 5 to 50 ° C./s (second cooling) is performed, and the cooling is completed in a temperature range in which the temperature at the center of the plate thickness is 200 ° C.
  • the temperature at the center of the plate thickness at the first cooling stop is less than 600 ° C, the surface layer may not be tempered without double heating. Therefore, it is desirable that the temperature at the center of the plate thickness is 600 ° C or higher, and 650 ° C or higher is further increased. desirable.
  • Heating temperature of steel material 950 ° C. or higher and 1250 ° C. or lower
  • the heating temperature of the steel material is not particularly limited, but if the heating temperature is less than 950 ° C., the heating temperature is too low and the deformation resistance becomes high, so that the hot rolling mill There is a risk that the load on the steel will increase and hot rolling will become difficult.
  • the heating temperature is preferably 950 ° C. or higher and 1250 ° C. or lower. More preferably, it is 1000 ° C. or higher and 1150 ° C. or lower.
  • Hot rolling temperature Ar 3 transformation point or higher After heating to the above temperature, hot rolling is started and rolling is completed at a temperature equal to or higher than the Ar 3 transformation point. That is, when the rolling temperature is lower than the Ar 3 transformation point, ferrite is generated, and the produced ferrite is affected by processing, so that the toughness deteriorates. Further, the load on the hot rolling mill becomes large. Therefore, the hot rolling temperature is set to be equal to or higher than the Ar 3 transformation point.
  • the Ar 3 transformation point is + 20 ° C. or higher.
  • the temperature is preferably 950 ° C. or lower. More preferably, it is 930 ° C. or lower.
  • Ar 3 (° C.) 910-273 x C-74 x Mn-57 x Ni-16 x Cr-9 x Mo-5 x Cu
  • each element indicates the content (mass%) of the element.
  • Cooling start temperature Ar 3 transformation point or higher
  • the steel sheet after hot rolling is cooled from the Ar 3 transformation point or higher. If the cooling start temperature is less than the Ar 3 transformation point, ferrite is formed on the surface layer of the steel sheet and coexists with the martensite structure or bainite structure having a large difference in strength, resulting in a decrease in toughness. Therefore, the cooling start temperature is set to be equal to or higher than the Ar 3 transformation point.
  • the cooling rate at a depth of 1 mm from the surface of the steel sheet is 10 ° C./s or more, and the speed in the first cooling is 10 ° C./s or more. This is because low temperature toughness cannot be ensured due to the formation of tempered bainite or ferrite having a large hardness difference from tempered bainite. Preferably, it is 10 ° C./s or higher.
  • the upper limit of the cooling rate is not particularly limited, but it is preferably 200 ° C./s or less because the cooling cost increases when excessive cooling is performed.
  • (E) First cooling stop temperature The temperature at a depth of 1 mm from the surface of the steel sheet is 600 ° C. or less.
  • the first cooling stop temperature is for martensite and / or bainite having a total surface structure of 90% or more.
  • the temperature is 600 ° C or lower.
  • the cooling shutdown temperature exceeds 600 ° C., a large amount of ferrite is generated and the toughness is lowered. Therefore, the cooling shutdown temperature is set to 600 ° C. or lower.
  • the lower limit of the cooling stop temperature is not limited, it is substantially 5 ° C. or higher because the temperature does not fall below the temperature of the cooling water.
  • the cooling stop temperature of the surface layer portion is too low, the subsequent plate thickness center portion is also cooled too much, so that the temperature is preferably 100 ° C. or higher, more preferably 200 ° C. or higher.
  • Cooling stop time 10 seconds or more and 600 seconds or less
  • the structure of martensite or bainite formed on the surface layer is reheated by reheating from the central part of the plate thickness. If the stop time is less than 10 seconds, the effect of tempering will be insufficient, the toughness will decrease, and the strength will become excessively high. On the other hand, if it exceeds 600 seconds, transformation at the center of the plate thickness starts, a large amount of ferrite structure is generated, and the structure becomes coarser, so that the strength and toughness are lowered.
  • (G) Second cooling rate The cooling rate at the center of the plate thickness is 5 ° C./s or more and 50 ° C./s or less. After the above cooling is stopped, cooling is restarted.
  • the cooling rate here is 5 ° C./s or more and 50 ° C./s or less so that ferrite or martensite has a predetermined volume fraction. That is, if the cooling rate is less than 5 ° C./s, the volume fraction of the ferrite or bainite structure becomes too large, and the strength characteristics are not satisfied. On the other hand, if the cooling rate exceeds 50 ° C./s, the volume fraction of martensite becomes too large and the toughness decreases.
  • (H) Second cooling end temperature The cooling end temperature at the center of the plate thickness is 200 ° C. or higher and 450 ° C. or lower. Therefore, the temperature is set to 200 ° C or higher and 450 ° C or lower.
  • the cooling end temperature exceeds 450 ° C.
  • the total volume fraction of ferrite and bainite at the center of the plate thickness exceeds 90%, and the strength characteristics are not satisfied.
  • the cooling end temperature is less than 200 ° C., the volume fraction of the island-shaped martensite becomes too large, the strength becomes excessively high, and the toughness decreases.
  • a steel sheet having the above-mentioned structure By manufacturing a steel material having the above-mentioned composition according to the above-mentioned manufacturing conditions, a steel sheet having the above-mentioned structure can be obtained.
  • the steel sheet thus obtained has excellent strength characteristics and toughness.
  • the excellent strength characteristics are the yield strength YS (yield point YP when there is a yield point, 0.2% proof stress ⁇ 0.2 when there is no yield point): 440 MPa or less and the tensile strength (TS): 490 MPa or more. Is.
  • the yield strength YS is closely related to ammonia stress corrosion cracking, and as a structural member of a liquefied gas bulk carrier, the risk of ammonia stress corrosion cracking is determined by the IMO gas code and ship class regulations of the International Maritime Organization.
  • the yield point is specified to be 440 MPa or less to minimize it. Therefore, if YS is 440 MPa or less, it can be said that it has excellent ammonia stress corrosion cracking property.
  • the yield strength YS yield point YP when there is a yield point, 0.2% proof stress ⁇ 0.2 when there is no yield point: 440 MPa or less cannot be compatible with each other to ensure the ammonia stress corrosion cracking property.
  • the tensile strength (TS) is preferably 620 MPa or less.
  • the tensile strength (TS) of the steel sheet obtained in the present invention is substantially 620 MPa or less.
  • the molten steel with the composition shown in Table 1 was melted and used as a steel material (slab). These steel materials (slabs) were hot-rolled and cooled under the conditions shown in Table 2.
  • test method For the obtained steel sheet, measure the microstructure fraction in the microstructure at a depth of 1 mm from the steel sheet surface (surface layer part) and 1/2 of the sheet thickness from the steel sheet surface (plate thickness center part), and evaluate the tensile properties and toughness. Carried out.
  • Each test method is as follows.
  • the microstructure at the center of the plate thickness was investigated by collecting samples from each of the obtained steel plates so that the center of the plate thickness became the observation surface. That is, the sample was mirror-polished, further corroded with nital, and then photographed in a range of 10 mm ⁇ 10 mm using a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • the surface fraction of the microstructure was determined by analyzing the captured image using an image analysis device. Since the volume fraction corresponds to the volume fraction when the anisotropy of the microstructure is small, the volume fraction is used as the volume fraction in this patent.
  • each tissue was discriminated as follows when determining the fraction of the microstructure.
  • the steel material was mirror-polished and night-tar-etched to reveal the structure, magnified 500 to 3000 times, and observed by SEM.
  • Ferrite has a structure that does not contain isotropically grown carbides and the inside of the grain looks black
  • pearlite has a structure in which ferrite (black) and carbides (white) look like stripes (stripes).
  • Bainite has an elongated lath-shaped ferrite structure, and has a structure containing carbides with a diameter equivalent to a circle of 0.05 ⁇ m or more, and is defined as tempered bainite when it contains 1.0 ⁇ 10 4 pieces / mm 2 or more of carbides. did.
  • the carbides are divided, and for example, the elongated carbides that appear between the bainite laths are a plurality of round carbides, so it is easy to distinguish them by looking at the carbides.
  • Martensite has an elongated lath-like ferrite structure similar to bainite, and has a structure containing carbides with a diameter equivalent to a circle of 0.05 ⁇ m or less, and further contains 1.0 ⁇ 10 4 pieces / mm 2 or more of carbides.
  • the carbides appear as white dots.
  • austenite was defined as a non-carbide structure having a diameter equivalent to a circle of 0.50 ⁇ m or more, which exists between bainite or martensite structures.
  • the yield strength YS is closely related to the ammonia stress corrosion cracking property, and as a structural member of the liquefied gas bulk carrier, the risk of ammonia stress corrosion cracking is minimized in the IMO gas code and ship class regulations.
  • the yield point is specified to be 440 MPa or less. Therefore, as described above, a steel sheet having a YS of 440 MPa or less was determined to be a steel sheet having excellent ammonia stress corrosion cracking resistance.
  • all of the invention examples have a yield strength YS of 440 MPa or less and a tensile strength TS of 490 MPa or more, a ductile brittle temperature of -60 ° C or less, toughness at low temperature and ammonia stress corrosion.
  • a steel plate having excellent crackability has been obtained.
  • the steel plate No. corresponding to the comparative example In 5, 7, 9, 11, 12, 14, 17, 18, 20, 21, 24, 25, 51, the microstructure at the surface layer and the microstructure at the center of the plate thickness are different from those of the invention examples, and the yield strength YS, tensile strength TS, or toughness at low temperature is inferior to the invention examples.
  • the steel plate No. corresponding to the comparative example In 38, the carbon content is low and the tensile strength TS is inferior to that of the invention example.
  • Steel plate No. In No. 39 the carbon content is high, the yield strength YS is high as compared with the invention example, the ammonia stress corrosion cracking property is inferior, and the toughness at low temperature is also inferior as compared with the invention example.
  • Steel plate No. In 40, 43, 44, 45, 49, and 50 the amount of various elements added is larger than that of the invention example, and the toughness at low temperature is inferior to that of the invention example.
  • Steel plate No. In No. 41 the amount of manganese is low and the tensile strength TS is inferior to that of the invention example.
  • Steel plate No. In No. 42 the amount of manganese is high, the yield strength YS is high as compared with the invention example, the ammonia stress corrosion cracking property is inferior, and the toughness at low temperature is also inferior as compared with the invention example.
  • Steel plate No. 46 and 48 have a low amount of nitrogen or titanium, and their toughness at low temperature is inferior to that of the invention examples.

Abstract

L'invention concerne une tôle d'acier ayant des propriétés exceptionnelles de fissuration par corrosion par réaction à l'ammoniac et de ténacité à basse température, la tôle d'acier étant fournie à un réservoir de stockage qui est utilisé pour recevoir un gaz liquéfié dans un récipient de transport d'énergie.　Cette tôle d'acier a : une composition de composant comprenant 0,05-0,15 % (inclus) de C, 0,50 % ou moins de Si, 0,50-2,00 % (inclus) de Mn, 0,060 % ou moins d'Al, 0,0010-0,0100 % (inclus) de N, 0,005-0,100 % (inclus) de Ti, 0,020 % ou moins de P, 0,010 % ou moins de S, et 0,0100 % ou moins de O, le reste étant du Fe et des impuretés inévitables ; et une microstructure dans laquelle le rapport volumique total de martensite revenue et de bainite revenue à une profondeur de 1 mm depuis la surface de la tôle d'acier est de 90 % ou plus, le rapport volumique total de ferrite et de bainite à une profondeur de 1/2 de l'épaisseur de tôle de la tôle d'acier est de 60-90 % (inclus), et le rapport volumique de martensite en forme d'îlot est de 10 % ou moins.
PCT/JP2020/037483 2019-11-27 2020-10-01 Tôle d'acier et son procédé de production WO2021106368A1 (fr)

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WO2023002812A1 (fr) * 2021-07-20 2023-01-26 株式会社神戸製鋼所 Tôle épaisse d'acier et procédé pour sa fabrication
JP7323088B1 (ja) 2022-02-24 2023-08-08 Ｊｆｅスチール株式会社 鋼板およびその製造方法
WO2023162522A1 (fr) * 2022-02-24 2023-08-31 Ｊｆｅスチール株式会社 Tôle en acier, et procédé de fabrication de celle-ci
WO2023162507A1 (fr) * 2022-02-24 2023-08-31 Ｊｆｅスチール株式会社 Tôle en acier, et procédé de fabrication de celle-ci
JP7364137B1 (ja) 2022-04-20 2023-10-18 Ｊｆｅスチール株式会社 鋼板およびその製造方法
WO2023204109A1 (fr) * 2022-04-20 2023-10-26 Ｊｆｅスチール株式会社 Tôle en acier, et procédé de fabrication de celle-ci
JP7482231B2 (ja) 2019-12-31 2024-05-13 宝山鋼鉄股▲分▼有限公司 低炭素低コスト超高強度多相鋼板／鋼帯およびその製造方法

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JP7482231B2 (ja) 2019-12-31 2024-05-13 宝山鋼鉄股▲分▼有限公司 低炭素低コスト超高強度多相鋼板／鋼帯およびその製造方法
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WO2023162522A1 (fr) * 2022-02-24 2023-08-31 Ｊｆｅスチール株式会社 Tôle en acier, et procédé de fabrication de celle-ci
WO2023162507A1 (fr) * 2022-02-24 2023-08-31 Ｊｆｅスチール株式会社 Tôle en acier, et procédé de fabrication de celle-ci
JP7364137B1 (ja) 2022-04-20 2023-10-18 Ｊｆｅスチール株式会社 鋼板およびその製造方法
WO2023204109A1 (fr) * 2022-04-20 2023-10-26 Ｊｆｅスチール株式会社 Tôle en acier, et procédé de fabrication de celle-ci
WO2023203702A1 (fr) * 2022-04-20 2023-10-26 Ｊｆｅスチール株式会社 Tôle en acier, et procédé de fabrication de celle-ci

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