WO2021106368A1 - Steel sheet and method for producing same - Google Patents
Steel sheet and method for producing same Download PDFInfo
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- 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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- 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
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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
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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
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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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
- 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
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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
-
- 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
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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
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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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
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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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
- C22C38/14—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
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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
- C22C38/18—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
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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
- C22C38/18—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
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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
- C22C38/18—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
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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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
- 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.
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Abstract
Description
C:0.05%以上0.15%以下、
Si:0.50%以下、
Mn:0.50%以上2.00%以下、
Al:0.060%以下、
N:0.0010%以上0.0100%以下、
Ti:0.005%以上0.100%以下、
P:0.020%以下、
S:0.010%以下および
O:0.0100%以下
を含み、残部Feおよび不可避的不純物の成分組成を有し、
鋼板の表面から1mmの深さにおける焼戻マルテンサイトおよび焼戻ベイナイトの合計体積率が90%以上であり、前記鋼板の板厚の1/2におけるフェライトおよびベイナイトの合計体積率が60%以上90%以下、かつ島状マルテンサイトの体積率が10%以下である、ミクロ組織を有する、鋼板。 1. 1. By mass%
C: 0.05% or more and 0.15% or less,
Si: 0.50% or less,
Mn: 0.50% or more and 2.00% or less,
Al: 0.060% or less,
N: 0.0010% or more and 0.0100% or less,
Ti: 0.005% or more and 0.100% or less,
P: 0.020% or less,
It contains S: 0.010% or less and O: 0.0100% or less, and has a component composition of the balance Fe and unavoidable impurities.
The total volume fraction of tempered martensite and bainite at a depth of 1 mm from the surface of the steel plate is 90% or more, and the total volume fraction of ferrite and bainite at 1/2 of the plate thickness of the steel plate is 60% or more 90. % Or less, and the volume fraction of island-shaped martensite is 10% or less, and the steel plate has a microstructure.
Cu:2.00%以下、
Ni:2.00%以下、
Cr:1.00%以下、
Mo:1.00%以下、
V:1.00%以下、
W:1.00%以下、
Co:1.00%以下、
Nb:0.100%以下、
B:0.0100%以下、
Ca:0.0200%以下、
Mg:0.0200%以下および
REM:0.0200%以下
のうちから選ばれる1種以上を含有する、前記1に記載の鋼板。 2. The composition of the components is further increased by mass%.
Cu: 2.00% or less,
Ni: 2.00% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
V: 1.00% 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.
C:0.05%以上0.15%以下、
Si:0.50%以下、
Mn:0.50%以上2.00%以下、
Al:0.060%以下、
N:0.0010%以上0.0100%以下、
Ti:0.005%以上0.100%以下、
P:0.020%以下、
S:0.010%以下および
O:0.0100%以下
を含み、残部Feおよび不可避的不純物の成分組成を有する鋼素材に、終了温度がAr3点以上の熱間圧延を施し、その後Ar3点以上の温度から冷却を開始し、鋼板表面から1mmの深さにおける温度が600℃以下になるまで平均冷却速度:10℃/s以上で冷却し、一旦冷却を停止して該冷却を10秒から600秒の間で中断し、次いで鋼板の板厚の1/2における平均冷却速度が5~50℃/sの冷却を行って、該冷却を板厚中心部における温度が200℃以上450℃以下の温度域にて終了する、鋼板の製造方法。 3. 3. By mass%
C: 0.05% or more and 0.15% or less,
Si: 0.50% or less,
Mn: 0.50% or more and 2.00% or less,
Al: 0.060% or less,
N: 0.0010% or more and 0.0100% or less,
Ti: 0.005% or more and 0.100% or less,
P: 0.020% or less,
A steel material containing S: 0.010% or less and O: 0.0100% or less and having a component composition of the balance Fe and unavoidable impurities is hot-rolled at an end temperature of Ar 3 points or more, and then Ar 3 Start cooling from a temperature above the point, cool at an average cooling rate of 10 ° C / s or more until the temperature at a depth of 1 mm from the steel sheet surface becomes 600 ° C or less, stop cooling once, and cool the steel sheet for 10 seconds. It is interrupted for 600 seconds, and then cooling is performed at an average cooling rate of 5 to 50 ° C./s at 1/2 of the thickness of the steel sheet, and the cooling is performed at a temperature of 200 ° C. or higher and 450 ° C. at the center of the sheet thickness. A method for manufacturing a steel sheet, which is completed in the following temperature range.
Cu:2.00%以下、
Ni:2.00%以下、
Cr:1.00%以下、
Mo:1.00%以下、
V:1.00%以下、
W:1.00%以下、
Co:1.00%以下、
Nb:0.100%以下、
B:0.0100%以下、
Ca:0.0200%以下、
Mg:0.0200%以下および
REM:0.0200%以下
のうちから選ばれる1種以上を含有する、前記3に記載の鋼板の製造方法。 4. The composition of the components is further increased by mass%.
Cu: 2.00% or less,
Ni: 2.00% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
V: 1.00% 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 method for producing a steel sheet according to 3 above, which contains at least one selected from Mg: 0.0200% or less and REM: 0.0200% or less.
C:0.05%以上0.15%以下
Cは、鋼の焼入れ性を増加させる作用を有する元素であり、高強度を達成するためには添加が必要になる、重要な元素の1つである。前記効果を得るためには、C含有量を0.05%以上とする。さらに、他の合金元素の含有量を少なくし、より低コストで製造するという観点からは、C含有量は0.07%以上とすることが好ましい。一方、C含有量が0.15%を超えると、強度が過剰に高くなることに加えて、靭性や溶接性が低下する。そのため、C含有量は0.15%以下とする。さらに、靱性や溶接性の低下を抑制する観点からは、C含有量を0.13%以下とすることが好ましい。 [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. In order to obtain the above effect, the C content is set to 0.05% or more. Further, from the viewpoint of reducing the content of other alloying elements and producing at a lower cost, the C content is preferably 0.07% or more. On the other hand, when the C content exceeds 0.15%, the strength becomes excessively high and the toughness and weldability decrease. Therefore, the C content is set to 0.15% or less. Further, from the viewpoint of suppressing deterioration of toughness and weldability, the C content is preferably 0.13% or less.
Siは、脱酸剤として作用する元素であるが、一方で靭性や溶接性の低下を招く元素である。そのため、できる限り含有量を低くすることが望ましいが、0.50%以下であれば許容できる。なお、鋼の脱酸はAlやTiなどでも十分可能であることから、Si含有量の下限は特に限定されず、0%であってよい。靭性や溶接性の観点からは、0.40%以下とすることが好ましく、0.30%以下とすることがより好ましい。 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は、鋼の焼入れ性を増加させる作用を有する元素であり、高強度を満足するためには添加が必要になる、重要な元素の1つである。前記効果を得るためには、Mn含有量を0.50%以上とする。さらに、他の合金元素の含有量を少なくし、より低コストで製造するという観点からは、Mn含有量は0.70%以上とすることが好ましく、0.90%以上とすることがより好ましい。一方、Mn含有量が2.00%を超えると、強度が過剰に高くなることおよび靭性や溶接性が低下することに加えて、合金コストが過度に高くなってしまう。そのため、Mn含有量は2.00%以下とする。さらに、靭性および溶接性の低下を抑制する観点からは、Mn含有量を1.80%以下とすることが好ましく、1.60%以下とすることがより好ましい。 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. In order to obtain the above effect, the Mn content is set to 0.50% or more. Further, from the viewpoint of reducing the content of other alloying elements and producing at a lower cost, the Mn content is preferably 0.70% or more, more preferably 0.90% or more. .. On the other hand, if the Mn content exceeds 2.00%, the strength becomes excessively high, the toughness and weldability are lowered, and the alloy cost becomes excessively high. Therefore, the Mn content is set to 2.00% or less. Further, from the viewpoint of suppressing deterioration of toughness and weldability, the Mn content is preferably 1.80% or less, and more preferably 1.60% or less.
Alは、脱酸剤として作用するとともに、結晶粒を微細化する作用を有する元素である。これらの効果を得るためには、Al含有量を0.010%以上とすることが好ましい。一方、Al含有量が0.060%を超えると、酸化物系介在物が増加して清浄度が低下することで靭性が低下する。そのため、Al含有量は0.060%以下とする。なお、Al含有量は0.050%以下とすることが好ましく、0.040%以下とすることがより好ましい。 Al: 0.060% or less Al is an element that acts as a deoxidizing agent and also has an action of refining crystal grains. In order to obtain these effects, the Al content is preferably 0.010% or more. On the other hand, when the Al content exceeds 0.060%, oxide-based inclusions increase and the cleanliness decreases, resulting in a decrease in toughness. Therefore, 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は、Tiと結合してTiNとして析出し、組織の微細化に寄与し、靭性を向上させる。この効果を得るためには、N含有量を0.0010%以上とする。好ましくは、0.0020%以上である。一方、N含有量が0.0100%を超えると、かえって靭性の低下を招く。したがって、靭性や溶接性の低下を抑制する観点からは、0.0100%以下とする。N含有量は、0.0080%以下とすることが好ましく、0.0060%以下とすることがより好ましい。 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. In order to obtain this effect, the N content is set to 0.0010% or more. Preferably, it is 0.0020% or more. On the other hand, if the N content exceeds 0.0100%, 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は、窒化物の形成傾向が強く、Nを固定して固溶Nを低減する作用を有する元素である。そのため、Tiの添加により、母材および溶接部の靭性を向上させることができる。この効果を得るためには、Ti含有量を0.005%以上とする。Ti含有量は、0.012%以上とすることが好ましい。一方、Ti含有量が0.100%を超えると、かえって靭性が低下する。そのため、Ti含有量は0.100%とする。Ti含有量は、0.090%以下とすることが好ましく、0.080%以下とするのがさらに好ましい。 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. In order to obtain this effect, the Ti content is set to 0.005% or more. The Ti content is preferably 0.012% or more. On the other hand, if the Ti content exceeds 0.100%, the toughness is rather lowered. Therefore, the Ti content is set to 0.100%. The Ti content is preferably 0.090% or less, more preferably 0.080% or less.
Pは、不可避的不純物として含有される元素であり、粒界に偏析することによって靱性や溶接性を低下させるなど、悪影響を及ぼす。そのため、できる限りP含有量を低くすることが望ましいが、0.020%以下であれば許容できる。なお、P含有量の下限は特に限定されず、0%であってよいが、通常、Pは不純物として鋼中に不可避的に含有される元素であるため、工業的には0%超であってよい。また、過剰の低減は精錬コストの高騰を招くため、P含有量は0.0005%以上とすることが好ましい。 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は、不可避的不純物として含有される元素であり、MnS等の硫化物系介在物として鋼中に存在し、破壊の発生起点となるなど、悪影響を及ぼす元素である。そのため、できる限りS含有量を低くすることが望ましいが、0.010%以下であれば許容できる。なお、S含有量の下限は特に限定されず、0%であってよい。通常、Sは不純物として鋼中に不可避的に含有される元素であるため、工業的には0%超であってもよい。すなわち、過剰の低減は精錬コストの高騰を招くため、コストの観点からはS含有量を0.0005%以上とすることが好ましい。 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%. Normally, 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は、不可避的不純物として含有される元素であり、酸化物を形成し、破壊の発生起点となるなど、悪影響を及ぼす元素であることから、0.0100%以下に制限する。O含有量は、0.0050%以下とすることが好ましく、0.0030%以下とすることがより好ましい。一方、O含有量の下限は特に限定されず、0%であってよいが、通常、Oは不純物として鋼中に不可避的に含有される元素であるため、工業的には0%超であってよい。すなわち、過剰の低減は精錬コストの高騰を招くため、コストの観点からはO含有量を0.0020%以上とすることが好ましい。 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. Restrict. The O content is preferably 0.0050% or less, more preferably 0.0030% or less. On the other hand, 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.
Cuは、鋼の焼入れ性を増加させて鋼板の強度を向上させる作用を有する元素であり、任意に添加することができる。Cuを添加する場合、前記効果を得るためにCu含有量を0.01%以上とすることが好ましい。より好ましくは、0.20%以上である。一方、Cu含有量が2.00%を超えると、靭性の劣化や合金コストの上昇を招く。そのため、Cuを添加する場合、Cu含有量を2.00%以下とする。より好ましくは、1.00%以下である。 Cu: 2.00% 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. When Cu is 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. On the other hand, if 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は、Cuと同様に鋼板の強度を向上させる作用を有する元素であり、任意に添加することができる。Niを添加する場合、前記効果を得るためにNi含有量を0.01%以上とすることが好ましい。より好ましくは、0.20%以上である。一方、Ni含有量が2.00%を超えると、溶接性の劣化や合金コストの上昇を招く。そのため、Niを添加する場合、Ni含有量を2.00%以下とする。より好ましくは、1.00%以下である。 Ni: 2.00% or less Ni is an element having an action of improving the strength of a steel sheet like Cu, and can be arbitrarily added. When Ni is 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. On the other hand, if the Ni content exceeds 2.00%, the weldability deteriorates and the alloy cost increases. Therefore, when Ni is added, the Ni content is set to 2.00% or less. More preferably, it is 1.00% or less.
Crは、Cuと同様に鋼板の強度を向上させる作用を有する元素であり、任意に添加することができる。前記効果を得るためにCr含有量を0.01%以上とすることが好ましい。より好ましくは、0.05%以上である。一方、Cr含有量が1.00%を超えると、溶接性の劣化や合金コストの上昇を招く。そのため、Crを添加する場合、Cr含有量を1.00%以下とする。より好ましくは、0.50%以下である。 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. In order to obtain the above effect, the Cr content is preferably 0.01% or more. More preferably, it is 0.05% or more. On the other hand, if the Cr content exceeds 1.00%, the weldability deteriorates and the alloy cost increases. Therefore, when Cr is added, the Cr content is set to 1.00% or less. More preferably, it is 0.50% or less.
Moは、Cuと同様に鋼板の強度を向上させる作用を有する元素であり、任意に添加することができる。前記効果を得るためにMo含有量を0.01%以上とすることが好ましい。より好ましくは、0.05%以上である。一方、Mo含有量が1.00%を超えると、溶接性の劣化や合金コストの上昇を招く。そのため、Moを添加する場合、Mo含有量を1.00%以下とする。より好ましくは、0.50%以下である。 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. In order to obtain the above effect, the Mo content is preferably 0.01% or more. More preferably, it is 0.05% or more. On the other hand, if the Mo content exceeds 1.00%, the weldability deteriorates and the alloy cost increases. Therefore, when Mo is added, the Mo content is set to 1.00% or less. More preferably, it is 0.50% or less.
Vは、Cuと同様に鋼板の強度を向上させる作用を有する元素であり、任意に添加することができる。前記効果を得るためにV含有量を0.01%以上とすることが好ましい。より好ましくは、0.05%以上である。一方、V含有量が1.00%を超えると、溶接性の劣化や合金コストの上昇を招く。そのため、Vを添加する場合、V含有量を1.00%以下とする。より好ましくは、0.50%以下である。 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. In order to obtain the above effect, the V content is preferably 0.01% or more. More preferably, it is 0.05% or more. On the other hand, if the V content exceeds 1.00%, the weldability deteriorates and the alloy cost increases. Therefore, when V is added, the V content is set to 1.00% or less. More preferably, it is 0.50% or less.
Wは、Cuと同様に鋼板の強度を向上させる作用を有する元素であり、任意に添加することができる。前記効果を得るためにW含有量を0.01%以上とすることが好ましい。より好ましくは、0.05%以上である。一方、W含有量が1.00%を超えると、溶接性の劣化や合金コストの上昇を招く。そのため、Wを添加する場合、Mo含有量を1.00%以下とする。より好ましくは、0.50%以下である。 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. In order to obtain the above effect, the W content is preferably 0.01% or more. More preferably, it is 0.05% or more. On the other hand, if the W content exceeds 1.00%, the weldability deteriorates and the alloy cost increases. Therefore, when W is added, the Mo content is set to 1.00% or less. More preferably, it is 0.50% or less.
Coは、Cuと同様に鋼板の強度を向上させる作用を有する元素であり、任意に添加することができる。前記効果を得るためにCo含有量を0.01%以上とすることが好ましい。より好ましくは、0.05%以上である。一方、Co含有量が1.00%を超えると、溶接性の劣化や合金コストの上昇を招く。そのため、Coを添加する場合、Co含有量を1.00%以下とする。より好ましくは、0.50%以下である。 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. In order to obtain the above effect, the Co content is preferably 0.01% or more. More preferably, it is 0.05% or more. On the other hand, if the Co content exceeds 1.00%, the weldability deteriorates and the alloy cost increases. Therefore, when Co is added, the Co content is set to 1.00% or less. More preferably, it is 0.50% or less.
Nbは、炭窒化物として析出することで旧オーステナイト粒径を小さくし、靭性を向上させる効果を有する元素である。Nbを添加する場合、前記効果を得るためにNb含有量を0.005%以上とすることが好ましい。さらに、Nb含有量は0.007%以上とすることがより好ましい。一方、Nb含有量が0.100%を超えるとNbCが多量に析出し、靭性が低下する。そのため、Nbを添加する場合、Nb含有量を0.100%以下とする。Nb含有量は、0.080%以下とすることが好ましく、0.060%以下とするのがさらに好ましく、0.045%以下とするのがさらに好ましい。 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. When Nb is added, 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. On the other hand, when 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は、微量の添加でも焼入れ性を著しく向上させる作用を有する元素である。したがって、鋼板の強度を向上させることができる。前記効果を得るために、Bを添加する場合、B含有量を0.0001%以上とすることが好ましい。B含有量は、0.0005%以上とすることがより好ましく、0.0010%以上とすることがさらに好ましい。一方、B含有量が0.0100%を超えると溶接性が低下する。そのため、Bを添加する場合、B含有量を0.0100%以下とする。B含有量は0.0050%以下とすることが好ましく、0.0030%以下とすることがさらに好ましい。 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. When B is added in order to obtain the above effect, 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. On the other hand, if the B content exceeds 0.0100%, the weldability deteriorates. Therefore, when B is added, 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は、Sと結合し、圧延方向に長く伸びるMnS等の形成を抑制する作用を有する元素である。したがって、Caを添加することにより、硫化物系介在物が球状を呈するように形態制御し、溶接部等の靭性を向上させることができる。前記効果を得るために、Caを添加する場合、Ca含有量を0.0005%以上とすることが好ましい。より好ましくは、0.0020%以上である。一方、Ca含有量が0.0050%を超えると、鋼の清浄度が低下する。清浄度の低下は、表面疵の増加による表面性状の劣化と、曲げ加工性の低下を招く。そのため、Caを添加する場合、Ca含有量を0.0050%以下とする。より好ましくは、0.0100%以下である。 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. When Ca is added in order to obtain the above effect, the Ca content is preferably 0.0005% or more. More preferably, it is 0.0020% or more. On the other hand, when 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は、Caと同様、Sと結合し、圧延方向に長く伸びるMnS等の形成を抑制する作用を有する元素である。したがって、Mgを添加することにより、硫化物系介在物が球状を呈するように形態制御し、溶接部等の靭性を向上させることができる。前記効果を得るために、Mgを添加する場合、Mg含有量を0.0005%以上とすることが好ましい。より好ましくは、0.0020%以上である。一方、Mg含有量が0.0050%を超えると、鋼の清状度が低下する。清浄度の低下は、表面疵の増加による表面性状の劣化と、曲げ加工性の低下を招く。そのため、Mgを添加する場合、Mg含有量を0.0050%以下とする。より好ましくは、0.0100%以下である。 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. When Mg is added in order to obtain the above effect, 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. 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 Mg is added, the Mg content is set to 0.0050% or less. More preferably, it is 0.0100% or less.
REM(希土類金属)は、CaやMgと同様、Sと結合し、圧延方向に長く伸びるMnS等の形成を抑制する作用を有する元素である。したがって、REMを添加することにより、硫化物系介在物が球状を呈するように形態制御し、溶接部等の靭性を向上させることができる。前記効果を得るために、REMを添加する場合、REM含有量を0.0005%以上とすることが好ましい。より好ましくは、0.0020%以上である。一方、REM含有量が0.0050%を超えると、鋼の清状度が低下する。清浄度の低下は、表面疵の増加による表面性状の劣化と、曲げ加工性の低下を招く。そのため、REMを添加する場合、REM含有量を0.0080%以下とする。より好ましくは、0.0100%以下である。 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. When REM is added in order to obtain the above effect, 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. 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 REM is added, the REM content is set to 0.0080% or less. More preferably, it is 0.0100% or less.
本発明の鋼板のミクロ組織について説明する。
[鋼板の表面から1mmの深さにおける焼戻マルテンサイトおよび焼戻ベイナイトの合計体積率が90%以上]
通常、熱間圧延後に引き続き冷却を行った鋼板において、最も冷却速度の速い表面の組織はマルテンサイトあるいはベイナイトとなる。本発明では、鋼板の製造条件を後述するように、熱間圧延後の冷却を一旦中断して鋼板表層部のみを意図的に焼戻すことによって、鋼板表面の過度な硬化を防ぎ、所定の強度特性を満足させ、かつ低温での靭性を向上させている。従って、鋼板の表面から1mmの深さ(以下、表層部ともいう)における組織は、焼戻マルテンサイトおよび焼戻ベイナイトの合計体積率が90%以上である。焼戻マルテンサイトあるいは焼戻ベイナイト以外の残部組織が10%以上になると、焼戻マルテンサイトあるいは焼戻ベイナイトと残部組織との間の強度差が大きくなり強度特性が満足されなくなり、あるいは低温での靭性が低下することになるため、焼戻マルテンサイトおよび焼戻ベイナイトの合計体積率を90%以上とする。焼戻マルテンサイトおよび焼戻ベイナイトの体積率は高いほどよいため、該体積率の上限は特に限定されず、100%であってよい。なお、焼戻マルテンサイトおよび焼戻ベイナイトの各比率は特に限定する必要はないが、焼戻マルテンサイトが80%以上であることが好ましい。 [Micro tissue]
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]
Usually, in a steel sheet that has been continuously cooled after hot rolling, the surface structure having the fastest cooling rate is martensite or bainite. In the present invention, as will be described later, 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. Therefore, in the structure at a depth of 1 mm from the surface of the steel sheet (hereinafter, also referred to as a surface layer portion), the total volume fraction of tempered martensite and tempered bainite is 90% or more. When 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. Since 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.
鋼板の板厚の1/2(以下、板厚中心部ともいう)における組織は、フェライトおよびベイナイトの合計体積率が60%以上90%以下であり、残部に含まれる島状マルテンサイトの体積率が10%以下である。すなわち、フェライトおよびベイナイトの合計体積率が60%未満であると、これ以外のマルテンサイト、パーライト、オーステナイトの体積分率が増加することになり、十分な強度および/または靭性が得られず、機械特性を満足することができない。一方で、前記組織の合計体積率が90%を超えると、マルテンサイト、パーライト、オーステナイトなどの体積分率が低くなりすぎるため、強度特性が満足されない。 [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]
In the structure at 1/2 of the plate thickness of the steel sheet (hereinafter, also referred to as the central portion of the plate thickness), 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.
なお、各種ミクロ組織の体積率は、後述の実施例に記載した方法で測定することができる。 On the other hand, 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.
上記した成分組成を有する鋼素材を加熱し、熱間圧延を施して熱延鋼板とし、開始温度がAr3変態点以上である冷却を行って鋼板とする。以下、製造条件毎に詳しく説明する。 Next, the method for manufacturing the steel sheet of the present invention will be described.
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. Hereinafter, each manufacturing condition will be described in detail.
鋼素材の加熱温度は特に限定されないが、加熱温度が950℃未満では、加熱温度が低すぎて変形抵抗が高くなり、熱間圧延機への負荷が増大し、熱間圧延が困難になる、おそれがある。一方、1250℃を超える高温になると、酸化が著しくなり、酸化ロスが増大し歩留りが低下する、おそれがある。このようなことから、加熱温度は950℃以上1250℃以下にすることが好ましい。なお、より好ましくは1000℃以上1150℃以下である。 (A) 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. On the other hand, when the temperature is higher than 1250 ° C., oxidation becomes remarkable, oxidation loss increases, and the yield may decrease. For this reason, 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.
上記温度に加熱後、熱間圧延を開始して、Ar3変態点以上の温度で圧延を終了する。すなわち、圧延温度がAr3変態点未満となると、フェライトが生成し、生成したフェライトが加工の影響を受けるため、靭性が悪化することになる。さらには、熱間圧延機への負荷が大きくなる。したがって、熱間圧延温度は、Ar3変態点以上とする。好ましくは、Ar3変態点+20℃以上である。
一方、圧延温度が950℃を超えると、組織が粗大化し靭性が劣化する、おそれがあるため、950℃以下とすることが好ましい。より好ましくは、930℃以下である。 (B) 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. Preferably, the Ar 3 transformation point is + 20 ° C. or higher.
On the other hand, if the rolling temperature exceeds 950 ° C., the structure may become coarse and the toughness may deteriorate. Therefore, the temperature is preferably 950 ° C. or lower. More preferably, it is 930 ° C. or lower.
Ar3(℃)=910-273×C-74×Mn-57×Ni-16×Cr-9×Mo-5×Cu
ただし、各元素は当該元素の含有量(質量%)を示す。 Here, the Ar 3 transformation point can be obtained by, for example, the following equation.
Ar 3 (° C.) = 910-273 x C-74 x Mn-57 x Ni-16 x Cr-9 x Mo-5 x Cu
However, each element indicates the content (mass%) of the element.
次に、熱間圧延後の鋼板に、Ar3変態点以上から冷却を行う。冷却開始温度がAr3変態点未満では、鋼板表層部にフェライトが生成し、強度差が大きいマルテンサイト組織あるいはベイナイト組織と共存することになる結果、靭性が低下する。そのため、冷却開始温度はAr3変態点以上とする。 (C) Cooling start temperature: Ar 3 transformation point or higher Next, 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.
1回目冷却における速度は、10℃/s以上とする。なぜなら、焼戻しベイナイトあるいは焼戻しベイナイトと硬度差が大きいフェライトが生成することにより低温靭性が確保されない。好ましくは、10℃/s以上である。冷却速度の上限は特に限定されないが、過度の冷却を行うと冷却コストが増加するため、200℃/s以下とすることが好ましい。 (D) Speed in the first cooling: 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.
1回目の冷却の停止温度は、表層部の組織を合計90%以上のマルテンサイトおよび/あるいはベイナイトとするため、600℃以下とする。冷却停止温度が600℃を超えると、フェライトが多く生成し靭性が低下する。従って、冷却停止温度は600℃以下とする。一方、冷却停止温度の下限は限定しないが、実質的には、冷却水の温度以下にはならないため5℃以上である。しかし、表層部の冷却停止温度が低すぎると、続く板厚中心部も冷却されすぎてしまうため、好ましくは100℃以上、より好ましくは200℃以上である。 (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. When 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. On the other hand, although 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. However, if 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.
上記の1回目の冷却後、一時的に冷却を10秒以上600秒以下の間にわたり停止する。冷却停止によって、表層部に生成したマルテンサイトあるいはベイナイトの組織を板厚中心部側からの復熱によって焼戻す。停止時間が10秒未満では、焼戻しの効果は不十分となり、靭性が低下するとともに、強度が過剰に高くなる。一方、600秒を超えると、板厚中心部での変態が開始しフェライト組織が多く生成してしまい、さらに組織が粗大になるために、強度さらには靭性が低下してしまう。 (F) Cooling stop time: 10 seconds or more and 600 seconds or less After the first cooling described above, cooling is temporarily stopped for 10 seconds or more and 600 seconds or less. By stopping cooling, 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.
上記冷却停止後、冷却を再開する。ここでの冷却速度は、フェライトあるいはマルテンサイトが所定の体積率になるように、5℃/s以上50℃/s以下とする。すなわち、冷却速度が5℃/s未満であると、フェライトあるいはベイナイト組織の体積率多くなりすぎてしまい、強度特性を満足しなくなる。一方で、冷却速度が50℃/sを超えると、マルテンサイトの体積率が多くなりすぎてしまい、靭性が低下する。 (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.
2回目の冷却の終了温度は、板厚中心部にてフェライトおよびベイナイトの組織を所定の体積率得るために200℃以上450℃以下とする。冷却終了温度が450℃超では、板厚中心部のフェライトおよびベイナイトの合計体積率が90%を超え、強度特性を満足しなくなる。一方、冷却終了温度が200℃未満の温度では、島状マルテンサイトの体積率が多くなりすぎてしまい、強度が過剰に高くなることに加え靭性が低下する。 (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. When 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. On the other hand, when 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.
得られた各鋼板から、該鋼板表面から1mmの深さの位置が観察面となるように、サンプルを採取した。前記サンプルの表面を鏡面研磨し、さらにナイタール腐食した後、走査型電子顕微鏡(SEM)を用いて10mm×10mmの範囲を撮影した。撮影された像について画像解析装置を用いて解析することによってミクロ組織の分率を求め、その値を体積率とした。 [Measurement of microstructure fraction in the surface layer and the center of the plate thickness]
From each of the obtained steel sheets, a sample was taken so that the observation surface was located at a depth of 1 mm from the surface of the steel sheet. The surface of the sample was mirror-polished and further corroded with nital, and then an area of 10 mm × 10 mm was photographed using a scanning electron microscope (SEM). The captured image was analyzed using an image analyzer to obtain the fraction of the microstructure, and the value was taken as the volume fraction.
各鋼板の全厚から、圧延方向に直角の方向にJIS Z 2201の1B号試験片を採取して、JIS Z 2241の要領で引張試験を行い、降伏強さYS(降伏点があるときは降伏点YP、ないときは0.2%耐力σ0.2)および引張強さ(TS)を測定した。そして降伏強さ:440MPa以下のものを、アンモニア応力腐食割れ性に優れた鋼板とし、引張強さが490MPa以上のものを引張強度に優れた鋼板と評価した。なお、降伏強さYSは、アンモニア応力腐食割れ性と密接に関係し、液化ガスばら積み船の構造部材として、IMOガスコードや船級規則にて、アンモニア応力腐食割れの危険性を最小限にするため降伏点を440MPa以下と規定されている。従って、上記の通り、YS440MPa以下のものをアンモニア応力腐食割れ性に優れた鋼板と判定した。 [Strength characteristics]
From the total thickness of each steel plate, take a 1B test piece of JIS Z 2201 in the direction perpendicular to the rolling direction, perform a tensile test in the manner of JIS Z 2241, and perform a tensile test, yield strength YS (yield if there is a yield point). Point YP, 0.2% proof stress σ0.2) and tensile strength (TS) were measured. A steel sheet having a yield strength of 440 MPa or less was evaluated as a steel sheet having excellent ammonia stress corrosion cracking property, and a steel sheet having a tensile strength of 490 MPa or more was evaluated as a steel sheet having excellent tensile strength. 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.
また各鋼板の表面側から1mm削った部位から、圧延方向にJIS Z 2202のVノッチ試験片を採取して、JIS Z 2242の要領でシャルピー衝撃試験を行い、vTrsを測定した。そして、vTrsが-60℃以下のものを靭性に優れた鋼板と評価した。
かくして得られた評価結果を表2に併記する。 [Toughness]
Further, a V-notch test piece of JIS Z 2202 was collected from a portion cut by 1 mm from the surface side of each steel sheet, and a Charpy impact test was performed in the manner of JIS Z 2242 to measure vTrs. Then, those having vTrs of −60 ° C. or lower were evaluated as steel sheets having excellent toughness.
The evaluation results thus obtained are also shown in Table 2.
Claims (4)
- 質量%で、
C:0.05%以上0.15%以下、
Si:0.50%以下、
Mn:0.50%以上2.00%以下、
Al:0.060%以下、
N:0.0010%以上0.0100%以下、
Ti:0.005%以上0.100%以下、
P:0.020%以下、
S:0.010%以下および
O:0.0100%以下
を含み、残部Feおよび不可避的不純物の成分組成を有し、
鋼板の表面から1mmの深さにおける焼戻マルテンサイトおよび焼戻ベイナイトの合計体積率が90%以上であり、前記鋼板の板厚の1/2におけるフェライトおよびベイナイトの合計体積率が60%以上90%以下、かつ島状マルテンサイトの体積率が10%以下である、ミクロ組織を有する、鋼板。 By mass%
C: 0.05% or more and 0.15% or less,
Si: 0.50% or less,
Mn: 0.50% or more and 2.00% or less,
Al: 0.060% or less,
N: 0.0010% or more and 0.0100% or less,
Ti: 0.005% or more and 0.100% or less,
P: 0.020% or less,
It contains S: 0.010% or less and O: 0.0100% or less, and has a component composition of the balance Fe and unavoidable impurities.
The total volume fraction of tempered martensite and bainite at a depth of 1 mm from the surface of the steel plate is 90% or more, and the total volume fraction of ferrite and bainite at 1/2 of the plate thickness of the steel plate is 60% or more 90. % Or less, and the volume fraction of island-shaped martensite is 10% or less, and the steel plate has a microstructure. - 前記成分組成はさらに、質量%で、
Cu:2.00%以下、
Ni:2.00%以下、
Cr:1.00%以下、
Mo:1.00%以下、
V:1.00%以下、
W:1.00%以下、
Co:1.00%以下、
Nb:0.100%以下、
B:0.0100%以下、
Ca:0.0200%以下、
Mg:0.0200%以下および
REM:0.0200%以下
のうちから選ばれる1種以上を含有する、請求項1に記載の鋼板。 The composition of the components is further increased by mass%.
Cu: 2.00% or less,
Ni: 2.00% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
V: 1.00% 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 claim 1, which contains at least one selected from Mg: 0.0200% or less and REM: 0.0200% or less. - 質量%で、
C:0.05%以上0.15%以下、
Si:0.50%以下、
Mn:0.50%以上2.00%以下、
Al:0.060%以下、
N:0.0010%以上0.0100%以下、
Ti:0.005%以上0.100%以下、
P:0.020%以下、
S:0.010%以下および
O:0.0100%以下
を含み、残部Feおよび不可避的不純物の成分組成を有する鋼素材に、終了温度がAr3点以上の熱間圧延を施し、その後Ar3点以上の温度から冷却を開始し、鋼板表面から1mmの深さにおける温度が600℃以下になるまで平均冷却速度:10℃/s以上で冷却し、一旦冷却を停止して該冷却を10秒から600秒の間で中断し、次いで鋼板の板厚の1/2における平均冷却速度が5~50℃/sの冷却を行って、該冷却を板厚中心部における温度が200℃以上450℃以下の温度域にて終了する、鋼板の製造方法。 By mass%
C: 0.05% or more and 0.15% or less,
Si: 0.50% or less,
Mn: 0.50% or more and 2.00% or less,
Al: 0.060% or less,
N: 0.0010% or more and 0.0100% or less,
Ti: 0.005% or more and 0.100% or less,
P: 0.020% or less,
A steel material containing S: 0.010% or less and O: 0.0100% or less and having a component composition of the balance Fe and unavoidable impurities is hot-rolled at an end temperature of Ar 3 points or more, and then Ar 3 Start cooling from a temperature above the point, cool at an average cooling rate of 10 ° C / s or more until the temperature at a depth of 1 mm from the steel sheet surface becomes 600 ° C or less, stop cooling once, and cool the steel sheet for 10 seconds. It is interrupted for 600 seconds, and then cooling is performed at an average cooling rate of 5 to 50 ° C./s at 1/2 of the thickness of the steel sheet, and the cooling is performed at a temperature of 200 ° C. or higher and 450 ° C. at the center of the sheet thickness. A method for manufacturing a steel sheet, which is completed in the following temperature range. - 前記成分組成はさらに、質量%で、
Cu:2.00%以下、
Ni:2.00%以下、
Cr:1.00%以下、
Mo:1.00%以下、
V:1.00%以下、
W:1.00%以下、
Co:1.00%以下、
Nb:0.100%以下、
B:0.0100%以下、
Ca:0.0200%以下、
Mg:0.0200%以下および
REM:0.0200%以下のうちから選ばれる1種以上を含有する、請求項3に記載の鋼板の製造方法。 The composition of the components is further increased by mass%.
Cu: 2.00% or less,
Ni: 2.00% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
V: 1.00% 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 method for producing a steel sheet according to claim 3, which contains at least one selected from Mg: 0.0200% or less and REM: 0.0200% or less.
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JP7323088B1 (en) | 2022-02-24 | 2023-08-08 | Jfeスチール株式会社 | Steel plate and its manufacturing method |
WO2023162507A1 (en) * | 2022-02-24 | 2023-08-31 | Jfeスチール株式会社 | Steel sheet and method for producing same |
WO2023162522A1 (en) * | 2022-02-24 | 2023-08-31 | Jfeスチール株式会社 | Steel sheet and method for producing same |
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