WO2023162507A1 - Steel sheet and method for producing same - Google Patents

Steel sheet and method for producing same Download PDF

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Publication number
WO2023162507A1
WO2023162507A1 PCT/JP2023/001049 JP2023001049W WO2023162507A1 WO 2023162507 A1 WO2023162507 A1 WO 2023162507A1 JP 2023001049 W JP2023001049 W JP 2023001049W WO 2023162507 A1 WO2023162507 A1 WO 2023162507A1
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steel sheet
content
steel
cooling
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PCT/JP2023/001049
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French (fr)
Japanese (ja)
Inventor
恭野 安田
和彦 塩谷
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Ｊｆｅスチール株式会社
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Priority to JP2023528191A priority Critical patent/JPWO2023162507A1/ja
Publication of WO2023162507A1 publication Critical patent/WO2023162507A1/en

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention provides high-strength steel sheets with excellent toughness and corrosion resistance, particularly high-strength steel sheets with excellent low-temperature toughness and liquid ammonia stress corrosion cracking resistance suitable for structural members such as tanks used in a low-temperature, liquid-ammonia environment.
  • the present invention relates to a steel plate and its manufacturing method.
  • tanks may carry liquid ammonia as well as LPG.
  • ammonia SCC Stress Corrosion Cracking
  • Patent Documents 1 and 2 disclose techniques for satisfying the low-temperature toughness and strength range required for liquefied gas storage tanks as described above.
  • high low-temperature toughness and predetermined strength properties are obtained by heat-treating a thick steel plate cooled after hot rolling several times, or heat-treating a thick steel plate water-cooled after hot rolling several times. Realized.
  • Patent Literatures 1 and 2 above had the economic problem of requiring multiple heat treatments, which required high equipment and energy costs.
  • the present invention solves the above problems and provides a high-strength steel sheet with excellent ammonia SCC resistance and low-temperature toughness, which is used for storage tanks used for storing liquefied gas in energy transport ships, and a method for producing the same. for the purpose.
  • the present inventors used the TMCP process to extensively study various factors affecting the low temperature toughness and strength characteristics of steel sheets.
  • elements such as C, Si, Mn, and N are added to the steel sheet in a predetermined amount or more, and the total volume ratio of the ferrite structure and the bainite structure at the position of 1/2 of the plate thickness of the steel plate is 60% or more. It has been found that controlling the metallographic structure (microstructure) of the steel sheet to achieve the desired low temperature toughness and strength properties can be effectively achieved.
  • elements such as Cu, Cr, Sb, and Sn are added in a predetermined amount or more, and the hardness at a position 1.0 mm deep from the surface of the steel sheet is controlled to Hv 300 or less. It has been found that SCC resistance can be obtained and the costly heat treatment of the prior art can be omitted.
  • the present invention has been made based on the above findings, that is, the gist of the present invention is as follows. 1. in % by mass, C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn: 0.50-2.50%, Al: 0.060% or less, N: 0.0010 to 0.0100%, P: 0.020% or less, S: 0.0100% or less and O: 0.0100% or less, and Cu: 0.01-0.50%, Cr: 0.01 to 1.00%, Sb: 0.01-0.50% and Sn: 0.01-0.50%
  • a steel sheet having a chemical composition containing one or two or more of a hardness characteristic in which the hardness at a position 1.0 mm deep from the surface of the steel plate is Hv300 or less;
  • the component composition further, in mass %, Ni: 0.01 to 2.00%, Mo: 0.01-0.50% and W: 0.01-1.00% 2.
  • the component composition further, in mass %, V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0100%, Ca: 0.0005 to 0.0200%, Mg: 0.0005-0.0200% and REM: 0.0005-0.0200% 3.
  • V 0.01 to 1.00%
  • Ti 0.005 to 0.100%
  • Co 0.01 to 1.00%
  • Nb 0.005 to 0.100%
  • B 0.0001 to 0.0100%
  • Ca 0.0005 to 0.0200%
  • Mg 0.0005-0.0200%
  • REM 0.0005-0.0200% 3.
  • a method for manufacturing a steel sheet wherein hot rolling is performed at a temperature of the Ar 3 transformation point or higher, and then cooling is performed from a cooling start temperature of the Ar 3 transformation point or higher to a cooling stop temperature of 600 ° C. or lower, In the cooling, the cooling rate at a position 1.0 mm deep from the surface of the steel sheet is 150 ° C./s or less, and the cooling rate at a position 1/2 of the thickness of the steel plate is 10 ° C./s or more. manufacturing method.
  • CR 2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn] Expression (1)
  • [X] indicates the content (mass%) of the X element in the steel.
  • the chemical composition of the steel material is further, in mass%, Ni: 0.01 to 2.00%, Mo: 0.01-0.50% and W: 0.01-1.00% 4.
  • the chemical composition of the steel material is further, in mass%, V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0100%, Ca: 0.0005 to 0.0200%, Mg: 0.0005-0.0200% and REM: 0.0005-0.0200% 6.
  • V 0.01 to 1.00%
  • Ti 0.005 to 0.100%
  • Co 0.01 to 1.00%
  • Nb 0.005 to 0.100%
  • B 0.0001 to 0.0100%
  • Ca 0.0005 to 0.0200%
  • Mg 0.0005-0.0200%
  • REM 0.0005-0.0200% 6.
  • a steel sheet having excellent low-temperature toughness that is, impact resistance at low temperatures, and ammonia SCC resistance, and having high strength suitable for structural members such as tanks used in a low-temperature and liquid ammonia environment can be obtained at a low cost. It can be provided in a simple process.
  • % representing the content of the following components (elements) means “% by mass” unless otherwise specified.
  • C 0.010-0.200% C is the most effective element for increasing the strength of steel sheets produced by cooling according to the present invention.
  • the C content is specified to be 0.010% or more.
  • the C content is preferably 0.013% or more from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost.
  • the C content is specified at 0.200% or less.
  • the C content is preferably 0.170% or less from the viewpoint of toughness and weldability.
  • Si 0.01-0.50% Si is added for deoxidation.
  • the Si content is specified to be 0.01% or more. Furthermore, it is preferable to make it 0.03% or more.
  • the Si content is specified to be 0.50% or less. Furthermore, the Si content is preferably 0.40% or less from the viewpoint of toughness and weldability.
  • Mn 0.50-2.50%
  • Mn is an element that has the effect of increasing the hardenability of steel, and is one of the important elements that need to be added in order to achieve high strength as in the present invention.
  • the Mn content is specified to be 0.50% or more.
  • the content of Mn is preferably 0.70% or more from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost.
  • the Mn content is specified at 2.50% or less.
  • the Mn content is preferably 2.30% or less from the viewpoint of further suppressing deterioration of toughness and weldability.
  • Al 0.060% or less
  • Al is an element that acts as a deoxidizing agent and has the effect of refining crystal grains.
  • the Al content is preferably 0.001% or more.
  • the Al content is specified at 0.060% or less.
  • the Al content is preferably 0.050% or less from the viewpoint of further preventing toughness deterioration.
  • N 0.0010 to 0.0100% N contributes to the refinement of the structure and improves the toughness of the steel sheet.
  • the N content is specified to be 0.0010% or more. Preferably, it is 0.0020% or more.
  • the N content is specified at 0.0100% or less.
  • the N content is preferably 0.0080% or less from the viewpoint of further suppressing deterioration of toughness and weldability.
  • Ti when Ti is present, N can bond with Ti and precipitate as TiN.
  • P 0.020% or less
  • P has an adverse effect, such as lowering toughness and weldability, by segregating at grain boundaries. Therefore, it is desirable to make the P content as low as possible, but a P content of 0.020% or less is acceptable.
  • the lower limit of the P content is not particularly limited, and may be 0%, but excessive reduction causes a rise in refining costs, so from the viewpoint of cost, the P content should be 0.0005% or more. is preferred.
  • S 0.0100% or less S is present in steel as sulfide-based inclusions such as MnS, and is an element that exerts adverse effects, such as deteriorating the toughness of the steel sheet by becoming the origin of fracture. Therefore, it is desirable that the S content be as low as possible, but a content of 0.0100% or less is permissible.
  • the lower limit of the S content is not particularly limited, and may be 0%, but excessive reduction causes a rise in refining costs, so from the viewpoint of cost, the S content should be 0.0005% or more. is preferred.
  • O 0.0100% or less
  • O is an element that forms an oxide, becomes a starting point of fracture, and has an adverse effect such as lowering the toughness of the steel sheet.
  • the O content is preferably 0.0050% or less, more preferably 0.0030% or less.
  • the lower limit of the O content is not particularly limited, and may be 0%. is preferred.
  • CR value obtained by the formula (1) is 0.70 or more Cu, Cr, Sb and Sn are particularly important elements in the present invention for improving ammonia SCC resistance. Therefore, in the present invention, one or more of them must be contained in the above amount, and the CR value obtained by the following formula (1) must be 0.70 or more.
  • CR 2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn] Expression (1)
  • [X] indicates the content (mass%) of the X element in the steel.
  • Cu, Cr, Sb and Sn quickly form protective corrosion products in a liquid ammonia environment to suppress stress corrosion cracking.
  • the Cu content is set to 0.01% or more
  • the Cr content is set to 0.01% or more
  • Sb is added, Sb
  • the content must be limited to 0.01% or more
  • the Sn content must be limited to 0.01% or more.
  • the formula for calculating the CR value is a formula devised for estimating the ammonia SCC resistance from the content of each element, and the higher the CR value, the better the ammonia SCC resistance.
  • the Cu content is limited to 0.50% or less, the Cr content to 1.00% or less, the Sb content to 0.50% or less, and the Sn content to 0.50% or less. do.
  • the Cu content is 0.40% or less
  • the Cr content is 0.80% or less
  • the Sb content is 0.40% or less
  • the Sn content is 0.40% or less.
  • the upper limit of the CR value is not particularly limited, but when the CR value exceeds 7.00, the effect is saturated, and excessive addition of the above elements causes a rise in price. preferable.
  • the balance other than the above components is Fe and unavoidable impurities.
  • the above component composition can contain the elements described below, if necessary.
  • Ni, Mo, and W are ammonia-resistant It is an element that further improves the SCC property, and one or more of these elements can be contained.
  • the Ni content is 0.01% or more
  • Mo content is 0.01% or more
  • W is contained.
  • an excessive Ni content results in deterioration of weldability and an increase in alloy cost.
  • excessive addition of Mo and W degrades weldability and toughness, which is disadvantageous from the viewpoint of alloy cost.
  • the Ni content it is preferable to adjust the Ni content to 2.00% or less, the Mo content to 0.50% or less, and the W content to 1.00% or less. More preferably, the Ni content is adjusted to 1.50% or less, the Mo content to 0.40% or less, and the W content to 0.80% or less.
  • V 0.01-1.00%
  • V is an element that has the effect of improving the strength of the steel sheet, and can be optionally added.
  • the V content is preferably 0.01% or more.
  • the V content is preferably 1.00% or less. More preferably, the lower limit of V content is 0.05% and the upper limit is 0.50%.
  • Ti 0.005-0.100%
  • Ti is an element that has a strong tendency to form nitrides and has the action of fixing N and reducing solid solution N, and can be added arbitrarily.
  • Ti can improve the toughness of the base material and the weld zone.
  • the Ti content is preferably 0.005% or more. Furthermore, it is more preferable to make it 0.007% or more.
  • the Ti content exceeds 0.100%, the toughness rather decreases. Therefore, when adding Ti, the Ti content is preferably 0.100% or less. Furthermore, the Ti content is more preferably 0.090% or less.
  • Co 0.01-1.00%
  • Co is an element that has the effect of improving the strength of the steel sheet, and can be optionally added.
  • the Co content is preferably 0.01% or more.
  • the Co content is preferably 1.00% or less. More preferably, the Co content has a lower limit of 0.05% and an upper limit of 0.50%.
  • Nb 0.005-0.100%
  • Nb is an element that has the effect of reducing the grain size of prior austenite and improving the toughness by precipitating as a carbonitride.
  • the Nb content is made 0.005% or more. Furthermore, it is preferable to make it 0.007% or more.
  • the Nb content exceeds 0.100%, a large amount of NbC precipitates, resulting in a decrease in toughness. Therefore, when Nb is added, the Nb content is preferably 0.100% or less. Furthermore, it is more preferable to make it 0.060% or less.
  • B 0.0001 to 0.0100%
  • B is an element that has the effect of significantly improving hardenability even when added in a very small amount. That is, the strength of the steel sheet can be improved.
  • the B content is preferably 0.0001% or more.
  • the B content exceeds 0.0100%, the weldability deteriorates. Therefore, when B is added, the B content is preferably 0.0100% or less. More preferably, the B content has a lower limit of 0.0010% and an upper limit of 0.0030%.
  • Ca 0.0005-0.0200%
  • Ca is an element that binds to S and has the effect of suppressing the formation of MnS or the like elongated in the rolling direction. That is, by adding Ca, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like.
  • the Ca content is preferably 0.0005% or more.
  • the Ca content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Ca is added, the Ca content is preferably 0.0200% or less. More preferably, the Ca content has a lower limit of 0.0020% and an upper limit of 0.0100%.
  • Mg: 0.0005-0.0200% Mg, like Ca is an element that binds to S and has the effect of suppressing the formation of MnS or the like elongated in the rolling direction. That is, by adding Mg, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like. In order to obtain such an effect, when Mg is added, the Mg content is preferably 0.0005% or more. On the other hand, when the Mg content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less. More preferably, the Mg content has a lower limit of 0.0020% and an upper limit of 0.0100%.
  • REM 0.0005-0.0200%
  • REM rare earth metal
  • the REM content is preferably 0.0005% or more.
  • the REM content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when REM is added, the REM content is preferably 0.0200% or less. More preferably, the REM content has a lower limit of 0.0020% and an upper limit of 0.0100%.
  • the steel sheet of the present invention has the above chemical composition, and also has a hardness at a depth of 1.0 mm from the surface of the steel sheet (also referred to as a 1.0 mm position in the present invention). It has a hardness characteristic of Hv300 or less.
  • the steel plate of the present invention refers to the 1/2 position of the plate thickness of the steel plate (in the present invention, it means the position of the depth of 1/2 of the plate thickness. Hereinafter, it is simply referred to as the 1/2 position or the plate thickness center part.
  • the volume fraction of the bainite structure (hereinafter also simply referred to as bainite) is 20% or more, and the total volume fraction of the ferrite structure (hereinafter simply referred to as ferrite) and bainite is 60% or more. .
  • the hardness at the 1.0 mm position shall be Hv300 or less. If a high-hardness region exists in the extreme surface layer of the steel sheet, specifically, at a position of 1.0 mm from the surface of the steel sheet, stress corrosion cracking in a liquid ammonia environment is promoted. Therefore, in the steel sheet of the present invention, excellent ammonia SCC resistance can be ensured by adjusting the hardness characteristics so that the hardness at the 1.0 mm position is Hv300 or less.
  • the lower limit of the hardness at the 1.0 mm position is not particularly limited, it is preferably about Hv130.
  • the hardness can be calculated by measuring Vickers hardness at a plurality of points (for example, 100 points) at a position of 0.5 mm.
  • the volume fraction of bainite is 20% or more, and the total volume fraction of ferrite and bainite is 60% or more
  • the structure at the 1/2 position must have a bainite volume fraction of 20% or more and a total volume fraction of ferrite and bainite of 60% or more. Excessive generation of ferrite leads to a decrease in strength or toughness. Further, when the total volume fraction of ferrite and bainite is less than 60%, the volume fractions of structures other than this, namely, island-shaped martensite structure, martensite structure, pearlite structure and austenite structure, will increase, which is sufficient. sufficient strength or toughness cannot be obtained, and the mechanical properties cannot be satisfied.
  • the total volume fraction of ferrite and bainite may be 100%.
  • the ferrite means ferrite generated in the cooling process before tempering
  • the bainite means bainite generated in the cooling process before tempering.
  • the reason why the microstructure at the center of thickness is defined is that the microstructure at the center of thickness affects the strength characteristics of the center of thickness. This is because the strength properties affect the strength of the steel plate as a whole.
  • the remaining structure occupying 40% or less in volume fraction may include martensite structure in addition to pearlite structure and austenite structure.
  • the fraction of each structure in the remaining structure is not particularly limited, but the remaining structure is preferably a pearlite structure.
  • the volume ratio of various microstructures can be measured by the method described in Examples below.
  • the manufacturing method in the present invention is C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, Al: 0.50%. 060% or less, N: 0.0010 to 0.0100%, P: 0.020% or less, S: 0.0100% or less and O: 0.0100% or less, and Cu: 0.01 to 0.50%, Cr: 0.01 to 1.00%, Sb: 0.01 to 0.50% and Sn: 0.01 to 0.50% containing one or more, and
  • the CR value obtained by the above formula (1) is set to 0.70 or more, and in addition, if necessary, Ni: 0.01 to 2.00%, Mo: 0.01 to 0.50% and W: 0.5%.
  • a steel material having a chemical composition containing one or more selected from 0.0200% with the balance being Fe and inevitable impurities is heated and hot-rolled, and then subjected to predetermined cooling according to the present invention. It is something to do. Reasons for limiting the manufacturing conditions of the steel sheet will be described below. First, the manufacturing conditions of the steel material need not be particularly limited. It is preferable to use a steel material such as a slab of predetermined dimensions in the method. It should be noted that there is no problem in making a steel material such as a slab having a predetermined size by the ingot casting-decomposition rolling method.
  • the steel material thus obtained is directly hot-rolled without cooling or hot-rolled after reheating.
  • Such hot rolling is performed at a rolling end temperature equal to or higher than the Ar 3 transformation point (hereinafter simply referred to as the Ar 3 transformation point).
  • cooling is performed under predetermined conditions from a cooling start temperature above the Ar 3 transformation point to a cooling stop temperature below 600°C.
  • the heating temperature of the steel material (the temperature at which it is subjected to hot rolling) is not particularly limited, but if the heating temperature is too low, the deformation resistance increases, the load on the hot rolling mill increases, and hot rolling becomes difficult. may become On the other hand, if the temperature exceeds 1300° C., the oxidation becomes significant, the oxidation loss increases, and the yield increases. For these reasons, the heating temperature is preferably 950° C. or higher and 1300° C. or lower.
  • hot rolling [Rolling end temperature: Ar 3 transformation point or higher]
  • the rolling end temperature in hot rolling is a temperature of Ar 3 transformation point +10°C or higher.
  • the rolling end temperature is preferably 950°C or less.
  • Ar 3 transformation point can be obtained by the following formula.
  • Ar 3 (° C.) 910-310 ⁇ C-80 ⁇ Mn-20 ⁇ Cu-15 ⁇ Cr-55 ⁇ Ni-80 ⁇ Mo
  • each element indicates the content of the element in steel (% by mass).
  • cooling start temperature Ar 3 transformation point or higher
  • the hot-rolled steel sheet is cooled from a cooling start temperature equal to or higher than the Ar 3 transformation point. If the cooling start temperature is lower than the Ar 3 transformation point, excessive ferrite is formed, resulting in insufficient strength. Therefore, the cooling start temperature should be the Ar 3 transformation point or higher.
  • cooling stop temperature 600°C or less
  • the cooling stop temperature is specified at 600° C. or less.
  • the lower limit of the cooling stop temperature is not particularly limited, but if the cooling stop temperature is excessively low, the volume fraction of the island-shaped martensite structure becomes too large, resulting in a decrease in toughness. Therefore, the cooling stop temperature is preferably 200° C. or higher.
  • the cooling stop temperature is the temperature at the 1/2 position of the steel plate.
  • the cooling rate at the 1.0 mm position is specified at 150° C./s or less.
  • the lower limit of the cooling rate is not particularly limited, but if the cooling rate is excessively low, excessive generation of ferrite structure and pearlite structure may lead to insufficient strength and deterioration of toughness. Therefore, from the viewpoint of preventing this more reliably, the cooling rate is preferably 50° C./s or higher.
  • the cooling rate can be controlled by controlled cooling through intermittent cooling including a cooling stop period. Also, it is difficult to physically and directly measure the temperature at the 1.0 mm position. However, based on the surface temperature at the start of cooling measured by a radiation thermometer and the target surface temperature at the end of cooling, for example, by using a process computer to calculate the difference, the temperature in the thickness cross section The distribution, especially the temperature at the 1.0 mm position, can be obtained in real time.
  • Cooling performed at a cooling rate of 10°C/s or more at the 1/2 position is an essential process for obtaining a high-strength and high-toughness steel sheet, and cooling at a high cooling rate has the effect of increasing strength due to transformation strengthening. can get.
  • the cooling rate at the 1/2 position during cooling according to the present invention is specified to be 10° C./s or more. If the cooling rate is less than 10°C/s, ferrite and pearlite are excessively formed, and sufficient strength cannot be obtained. Therefore, the cooling rate at the plate thickness 1/2 position is specified to be 10° C. or higher.
  • the upper limit of the cooling rate is not particularly limited, but if the cooling rate is excessively high, the volume fraction of island-shaped martensite becomes too large, which may lead to deterioration of toughness. Therefore, the cooling rate at the 1/2 position is preferably 80° C./s or less.
  • the cooling rate can be controlled by controlled cooling through intermittent cooling including a cooling stop period.
  • the temperature at the 1/2 position is physically difficult to measure directly. However, based on the surface temperature at the start of cooling measured by a radiation thermometer and the target surface temperature at the end of cooling, for example, by using a process computer to calculate the difference, the temperature in the thickness cross section
  • the distribution, in particular the temperature at the 1/2 position can be determined in real time.
  • the cooling rate at the 1.0 mm position and the cooling rate at the 1/2 position can each be changed by, for example, adjusting the cooling start temperature, the amount of water, etc. in a complex manner.
  • the steel sheet thus obtained will have excellent strength properties and toughness.
  • the excellent strength characteristics are yield strength YS (yield point YP when there is a yield point, 0.2% yield strength ⁇ 0.2 when there is no yield point): 360 MPa or more and tensile strength (TS): 490 MPa or more is.
  • excellent toughness means that vTrs conforming to JIS Z 2241 is -30°C or less.
  • any item not described in this specification can be used by a conventional method.
  • Slabs were made from steels (steel grades A to AH, the balance being Fe and unavoidable impurities) having the chemical compositions shown in Table 1, and used to make thick steel plates (No. 1 to 44) with a thickness of 30 mm. Then, hot rolling and cooling were sequentially performed under the conditions shown in Table 2 to obtain steel sheets. The obtained steel plate was subjected to measurement of the metal structure fraction at the position of 1/2 of the plate thickness, measurement of hardness at a position of 1.0 mm from the steel plate surface, evaluation of strength characteristics and toughness, and evaluation of ammonia SCC resistance. implemented each. Each test method is as follows. These results are also shown in Table 2.
  • the determination when obtaining the fraction of the metal structure of the sample was performed as follows. That is, in the photographed image described above, the polygonal ferrite is discriminated as ferrite (F in Table 2), and it has elongated lath-shaped ferrite and contains carbide with an equivalent circle diameter of 0.05 ⁇ m or more. The texture was identified as bainite (B in Table 2).
  • Ammonia SCC resistance was evaluated by an accelerated test in which a four-point bending test was performed using a test solution and constant potential anodic electrolysis was performed to promote corrosion. Specifically, we performed the following steps: A test piece with a thickness of 5 mm x 15 mm x 115 mm was taken from the surface of the steel plate, subjected to ultrasonic degreasing in acetone for 5 minutes, and stress of 100% YS of the actual yield strength of each steel plate was applied by four-point bending. .
  • the invention examples (No. 1 to 26) all have a yield strength YS of 360 MPa or more and a tensile strength TS of 490 MPa or more, and vTrs is -30 ° C. or less at low temperatures.
  • YS yield strength
  • TS tensile strength
  • TS tensile strength
  • the chemical compositions of the steels are outside the range of the present invention, so they are inferior in any of yield strength YS, tensile strength TS, low temperature toughness, or ammonia SCC resistance.
  • the chemical composition of the steel may be considered as the chemical composition of the steel sheet.

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  • Filling Or Discharging Of Gas Storage Vessels (AREA)

Abstract

Provided is a high-strength steel sheet having excellent low-temperature toughness and resistance to ammonia SCC, the steel sheet being provided to a storage tank, or similar, that is used to accommodate a liquefied gas in an energy transport vessel. This steel sheet has a specific component composition which contains in particular one or more of Cu: 0.01-05%, Cr: 0.01-1.0%, Sb: 0.01-0.50% and Sn: 0.01-0.50%, the content of Cu, Cr, Sb and Sn satisfying a specific relationship. Said steel sheet has: a hardness property wherein the hardness at a position at a depth of 1.0 mm from the surface of the steel sheet is Hv300 or less; and a metal structure in which, at a position equal to half the sheet thickness of the steel sheet, the volume fraction of bainite structure is 20% or more, and the total volume fraction of ferrite structure and bainite structure is 60% or more.

Description

鋼板およびその製造方法Steel plate and its manufacturing method
　本発明は、靭性および耐食性に優れた高強度鋼板、特に低温かつ液体アンモニア環境下で使用されるタンクなどの構造用部材に好適な、低温靱性および耐液体アンモニア応力腐食割れ性に優れた高強度鋼板およびその製造方法に関するものである。 The present invention provides high-strength steel sheets with excellent toughness and corrosion resistance, particularly high-strength steel sheets with excellent low-temperature toughness and liquid ammonia stress corrosion cracking resistance suitable for structural members such as tanks used in a low-temperature, liquid-ammonia environment. The present invention relates to a steel plate and its manufacturing method.
　近年のエネルギー需要の増加に伴い、エネルギー輸送船による液化ガスの輸送が盛んに行われている。エネルギー輸送船の効率的な運用のため、タンクにはＬＰＧだけでなく液体アンモニアが共に運搬される場合がある。 With the recent increase in energy demand, liquefied gas is being actively transported by energy transport ships. For efficient operation of energy carriers, tanks may carry liquid ammonia as well as LPG.
　ここで、液化アンモニアを取り扱う炭素鋼製の配管、貯槽、タンク車、ラインパイプなどにおいては、液体アンモニアによる応力腐食割れ（以下、アンモニアＳＣＣ（Ｓｔｒｅｓｓ　Ｃｏｒｒｏｓｉｏｎ　Ｃｒａｃｋｉｎｇ）を引き起こすことが知られている。このため、液体アンモニア環境下で使用される鋼材に対しては、アンモニアＳＣＣ感受性の低い鋼材の適用や、アンモニアＳＣＣを抑制するエンジニアリング措置が講じられてきた。 Here, in carbon steel pipes, storage tanks, tank cars, line pipes, etc. that handle liquefied ammonia, stress corrosion cracking (hereinafter referred to as ammonia SCC (Stress Corrosion Cracking)) due to liquid ammonia is known to occur. Therefore, for steel materials used in a liquid ammonia environment, application of steel materials with low ammonia SCC susceptibility and engineering measures to suppress ammonia SCC have been taken.
　例えば、アンモニアＳＣＣの発生については、材料の強度と相関があることが知られており、炭素鋼の使用にあたっては、４４０ＭＰa以下の降伏強度（ＹＳ）に制御することで、アンモニアによる応力腐食割れの回避が図られている。その一方で、近年のタンク大型化、鋼材使用量の削減の観点から、鋼板の高強度化の要求が高まっている。 For example, it is known that the occurrence of ammonia SCC is correlated with the strength of the material. Avoidance is being attempted. On the other hand, from the standpoint of increasing the size of tanks and reducing the amount of steel used in recent years, there is an increasing demand for higher strength steel sheets.
　また、ＬＰＧや液体アンモニアといった液化ガスは低温で輸送されるため、これらの液化ガスの貯蔵用タンクに使用される鋼板は、優れた低温靱性が要求される。 In addition, since liquefied gases such as LPG and liquid ammonia are transported at low temperatures, the steel sheets used in storage tanks for these liquefied gases are required to have excellent low temperature toughness.
　前述したような、液化ガス貯蔵用タンクに必要な、低温靱性と強度範囲とを満たすための技術が、特許文献１および２に開示されている。これらの文献に記載の技術では、熱間圧延後冷却した厚鋼板を数回熱処理する、あるいは熱間圧延後水冷した厚鋼板を数回熱処理という方法にて、高い低温靱性および所定の強度特性を実現している。 Patent Documents 1 and 2 disclose techniques for satisfying the low-temperature toughness and strength range required for liquefied gas storage tanks as described above. In the techniques described in these documents, high low-temperature toughness and predetermined strength properties are obtained by heat-treating a thick steel plate cooled after hot rolling several times, or heat-treating a thick steel plate water-cooled after hot rolling several times. Realized.
特開平１０－１４０２３５号公報JP-A-10-140235 特開平１０－１６８５１６号公報JP-A-10-168516
　しかしながら、上記の特許文献１および２に記載された方法では、複数回の熱処理を行う必要があり、そのための設備やエネルギーにかかるコストが大きいという経済的な問題があった。 However, the methods described in Patent Literatures 1 and 2 above had the economic problem of requiring multiple heat treatments, which required high equipment and energy costs.
　本発明は、上記の問題を解決し、エネルギー輸送船において液化ガスの収容に使用される貯蔵用タンク等に供する、耐アンモニアＳＣＣ性および低温靭性に優れる高強度の鋼板並びにその製造方法を提供することを目的とする。 The present invention solves the above problems and provides a high-strength steel sheet with excellent ammonia SCC resistance and low-temperature toughness, which is used for storage tanks used for storing liquefied gas in energy transport ships, and a method for producing the same. for the purpose.
　本発明者らは、上記目的を達成するために、ＴＭＣＰプロセスを用いて、鋼板の低温靱性、強度特性に対する各種要因について、鋭意検討を重ねた。その結果、鋼板に対し、Ｃ、Ｓｉ、Ｍｎ、Ｎ等の元素を所定量以上で添加し、前記鋼板の板厚の１／２位置におけるフェライト組織およびベイナイト組織の合計体積率が６０％以上となるように鋼板の金属組織（ミクロ組織）を制御すれば、所望の低温靱性および強度特性の達成に有効に寄与し得ることを見出した。 In order to achieve the above objectives, the present inventors used the TMCP process to extensively study various factors affecting the low temperature toughness and strength characteristics of steel sheets. As a result, elements such as C, Si, Mn, and N are added to the steel sheet in a predetermined amount or more, and the total volume ratio of the ferrite structure and the bainite structure at the position of 1/2 of the plate thickness of the steel plate is 60% or more. It has been found that controlling the metallographic structure (microstructure) of the steel sheet to achieve the desired low temperature toughness and strength properties can be effectively achieved.
　さらに、Ｃｕ、Ｃｒ、Ｓｂ、Ｓｎ等の元素を所定量以上で添加し、前記鋼板の表面から１．０ｍｍ深さの位置における硬さをＨｖ３００以下に制御することで、液体アンモニア環境下での耐ＳＣＣ性が得られ、従来技術のようなコストがかかる熱処理を省略できることを知見した。 Furthermore, elements such as Cu, Cr, Sb, and Sn are added in a predetermined amount or more, and the hardness at a position 1.0 mm deep from the surface of the steel sheet is controlled to Hv 300 or less. It has been found that SCC resistance can be obtained and the costly heat treatment of the prior art can be omitted.
　本発明は、上記の知見に基づきなされたもので、すなわち、本発明の要旨は次のとおりである。
　１．質量％で、
　Ｃ：０．０１０～０．２００％、
　Ｓｉ：０．０１～０．５０％、
　Ｍｎ：０．５０～２．５０％、
　Ａｌ：０．０６０％以下、
　Ｎ：０．００１０～０．０１００％、
　Ｐ：０．０２０％以下、
　Ｓ：０．０１００％以下および
　Ｏ：０．０１００％以下
を含有し、さらに、
　Ｃｕ：０．０１～０．５０％、
　Ｃｒ：０．０１～１．００％、
　Ｓｂ：０．０１～０．５０％および
　Ｓｎ：０．０１～０．５０％
のうち１種または２種以上を含有し、以下の式（１）によって求められるＣＲ値が０．７０以上であり、残部がＦｅおよび不可避的不純物である成分組成を有する鋼板であって、
　前記鋼板の表面から１．０ｍｍ深さの位置における硬さがＨｖ３００以下である硬さ特性と、
　前記鋼板の板厚の１／２位置において、ベイナイト組織の体積率が２０％以上で、かつフェライト組織およびベイナイト組織の合計体積率が６０％以上である金属組織と、を有する、鋼板。
　ＣＲ＝２．３[Ｃｕ]＋２．８[Ｃｒ]＋７．３[Ｓｂ]＋３．６[Ｓｎ]・・・式（１）
ただし、[Ｘ]はＸ元素の鋼中含有量（質量％）を示す。 The present invention has been made based on the above findings, that is, the gist of the present invention is as follows.
1. in % by mass,
C: 0.010 to 0.200%,
Si: 0.01 to 0.50%,
Mn: 0.50-2.50%,
Al: 0.060% or less,
N: 0.0010 to 0.0100%,
P: 0.020% or less,
S: 0.0100% or less and O: 0.0100% or less, and
Cu: 0.01-0.50%,
Cr: 0.01 to 1.00%,
Sb: 0.01-0.50% and Sn: 0.01-0.50%
A steel sheet having a chemical composition containing one or two or more of
a hardness characteristic in which the hardness at a position 1.0 mm deep from the surface of the steel plate is Hv300 or less;
A steel sheet having a metal structure in which a bainite structure has a volume fraction of 20% or more and a total volume fraction of a ferrite structure and a bainite structure is 60% or more at a position of 1/2 of the plate thickness of the steel plate.
CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn] Expression (1)
However, [X] indicates the content (mass%) of the X element in the steel.
　２．前記成分組成が、さらに、質量％で、
　Ｎｉ：０．０１～２．００％、
　Ｍｏ：０．０１～０．５０％および
　Ｗ：０．０１～１．００％
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 component composition further, in mass %,
Ni: 0.01 to 2.00%,
Mo: 0.01-0.50% and W: 0.01-1.00%
2. The steel sheet according to 1 above, containing one or more selected from.
　３．前記成分組成が、さらに、質量％で、
　Ｖ：０．０１～１．００％、
　Ｔｉ：０．００５～０．１００％、
　Ｃｏ：０．０１～１．００％、
　Ｎｂ：０．００５～０．１００％、
　Ｂ：０．０００１～０．０１００％、
　Ｃａ：０．０００５～０．０２００％、
　Ｍｇ：０．０００５～０．０２００％および
　ＲＥＭ：０．０００５～０．０２００％
のうちから選ばれる１種以上を含有する、前記１または２に記載の鋼板。 3. The component composition further, in mass %,
V: 0.01 to 1.00%,
Ti: 0.005 to 0.100%,
Co: 0.01 to 1.00%,
Nb: 0.005 to 0.100%,
B: 0.0001 to 0.0100%,
Ca: 0.0005 to 0.0200%,
Mg: 0.0005-0.0200% and REM: 0.0005-0.0200%
3. The steel sheet according to 1 or 2 above, containing one or more selected from.
　質量％で、
　Ｃ：０．０１０～０．２００％、
　Ｓｉ：０．０１～０．５０％、
　Ｍｎ：０．５０～２．５０％、
　Ａｌ：０．０６０％以下、
　Ｎ：０．００１０～０．０１００％、
　Ｐ：０．０２０％以下、
　Ｓ：０．０１００％以下および
　Ｏ：０．０１００％以下
を含有し、さらに、
　Ｃｕ：０．０１～０．５０％、
　Ｃｒ：０．０１～１．００％、
　Ｓｂ：０．０１～０．５０％および、
　Ｓｎ：０．０１～０．５０％
のうち１種または２種以上を含有し、以下の式（１）によって求められるＣＲ値が０．７０以上であり、残部がＦｅおよび不可避的不純物である成分組成を有する鋼素材について、圧延終了温度をＡｒ３変態点以上として熱間圧延を行い、次いで、Ａｒ３変態点以上の冷却開始温度から６００℃以下の冷却停止温度までの冷却を行う、鋼板の製造方法であって、
　前記冷却では、鋼板の表面から１．０ｍｍ深さの位置における冷却速度を１５０℃／ｓ以下とし、かつ、鋼板の板厚の１／２位置における冷却速度を１０℃／ｓ以上とする、鋼板の製造方法。
　ＣＲ＝２．３[Ｃｕ]＋２．８[Ｃｒ]＋７．３[Ｓｂ]＋３．６[Ｓｎ]・・・式（１）
ただし、[Ｘ]はＸ元素の鋼中含有量（質量％）を示す。 in % by mass,
C: 0.010 to 0.200%,
Si: 0.01 to 0.50%,
Mn: 0.50-2.50%,
Al: 0.060% or less,
N: 0.0010 to 0.0100%,
P: 0.020% or less,
S: 0.0100% or less and O: 0.0100% or less, and
Cu: 0.01-0.50%,
Cr: 0.01 to 1.00%,
Sb: 0.01 to 0.50% and
Sn: 0.01-0.50%
of which the CR value obtained by the following formula (1) is 0.70 or more, and the balance is Fe and unavoidable impurities. A method for manufacturing a steel sheet, wherein hot rolling is performed at a temperature of the Ar 3 transformation point or higher, and then cooling is performed from a cooling start temperature of the Ar 3 transformation point or higher to a cooling stop temperature of 600 ° C. or lower,
In the cooling, the cooling rate at a position 1.0 mm deep from the surface of the steel sheet is 150 ° C./s or less, and the cooling rate at a position 1/2 of the thickness of the steel plate is 10 ° C./s or more. manufacturing method.
CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn] Expression (1)
However, [X] indicates the content (mass%) of the X element in the steel.
　５．前記鋼素材の成分組成が、さらに、質量％で、
　Ｎｉ：０．０１～２．００％、
　Ｍｏ：０．０１～０．５０％および
　Ｗ：０．０１～１．００％
のうちから選ばれる１種以上を含有する、前記４に記載の鋼板の製造方法。 5. The chemical composition of the steel material is further, in mass%,
Ni: 0.01 to 2.00%,
Mo: 0.01-0.50% and W: 0.01-1.00%
4. The method for producing a steel sheet according to 4 above, containing one or more selected from.
　６．前記鋼素材の成分組成が、さらに、質量％で、
　Ｖ：０．０１～１．００％、
　Ｔｉ：０．００５～０．１００％、
　Ｃｏ：０．０１～１．００％、
　Ｎｂ：０．００５～０．１００％、
　Ｂ：０．０００１～０．０１００％、
　Ｃａ：０．０００５～０．０２００％、
　Ｍｇ：０．０００５～０．０２００％および
　ＲＥＭ：０．０００５～０．０２００％
のうちから選ばれる１種以上を含有する、前記４または５に記載の鋼板の製造方法。 6. The chemical composition of the steel material is further, in mass%,
V: 0.01 to 1.00%,
Ti: 0.005 to 0.100%,
Co: 0.01 to 1.00%,
Nb: 0.005 to 0.100%,
B: 0.0001 to 0.0100%,
Ca: 0.0005 to 0.0200%,
Mg: 0.0005-0.0200% and REM: 0.0005-0.0200%
6. The method for producing a steel sheet according to 4 or 5 above, containing one or more selected from.
　本発明によれば、低温靭性すなわち低温での耐衝撃特性、および耐アンモニアＳＣＣ性に優れ、低温かつ液体アンモニア環境下で使用されるタンクなどの構造用部材に好適な高い強度を有する鋼板を安価な工程で提供することができる。 According to the present invention, a steel sheet having excellent low-temperature toughness, that is, impact resistance at low temperatures, and ammonia SCC resistance, and having high strength suitable for structural members such as tanks used in a low-temperature and liquid ammonia environment can be obtained at a low cost. It can be provided in a simple process.
　以下に、本発明の実施形態を説明する。なお、以下の成分（元素）の含有量を表す「％」は、特に断らない限り「質量％」を意味する。 An embodiment of the present invention will be described below. In addition, "%" representing the content of the following components (elements) means "% by mass" unless otherwise specified.
（１）成分組成について
　以下、鋼板の成分組成（化学成分）について説明する。 (1) Regarding chemical composition The chemical composition (chemical composition) of the steel sheet will be described below.
Ｃ：０．０１０～０．２００％
　Ｃは、本発明に従う冷却によって製造される鋼板の強度を高めるために最も有効な元素である。かかる効果を得るため、Ｃ含有量を０．０１０％以上に規定する。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０．０１３％以上とすることが好ましい。一方、Ｃ含有量が０．２００％を超えると鋼板の靭性および溶接性の劣化を招く。従って、Ｃ含有量を０．２００％以下に規定する。さらに、Ｃ含有量は、靭性および溶接性の観点から、０．１７０％以下とすることが好ましい。 C: 0.010-0.200%
C is the most effective element for increasing the strength of steel sheets produced by cooling according to the present invention. In order to obtain such effects, the C content is specified to be 0.010% or more. Furthermore, the C content is preferably 0.013% or more from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost. On the other hand, if the C content exceeds 0.200%, the toughness and weldability of the steel sheet deteriorate. Therefore, the C content is specified at 0.200% or less. Furthermore, the C content is preferably 0.170% or less from the viewpoint of toughness and weldability.
Ｓｉ：０．０１～０．５０％
　Ｓｉは、脱酸のため添加する。かかる効果を得るため、Ｓｉ含有量を０．０１％以上に規定する。さらに、０．０３％以上とすることが好ましい。一方、Ｓｉ含有量が０．５０％を超えると鋼板の靭性や溶接性の劣化を招く。従って、Ｓｉ含有量を０．５０％以下に規定する。さらに、Ｓｉ含有量は、靭性および溶接性の観点から、０．４０％以下とすることが好ましい。 Si: 0.01-0.50%
Si is added for deoxidation. In order to obtain such effects, the Si content is specified to be 0.01% or more. Furthermore, it is preferable to make it 0.03% or more. On the other hand, if the Si content exceeds 0.50%, the toughness and weldability of the steel sheet are deteriorated. Therefore, the Si content is specified to be 0.50% or less. Furthermore, the Si content is preferably 0.40% or less from the viewpoint of toughness and weldability.
Ｍｎ：０．５０～２．５０％
　Ｍｎは、鋼の焼入れ性を増加させる作用を有する元素であり、本発明のように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。かかる効果を得るため、Ｍｎ含有量を０．５０％以上に規定する。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い。一方、Ｍｎ含有量が２．５０％を超えると、鋼板の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。従って、Ｍｎ含有量を２．５０％以下に規定する。さらに、Ｍｎ含有量は、靭性および溶接性の低下をより一層抑制する観点から、２．３０％以下とすることが好ましい。 Mn: 0.50-2.50%
Mn is an element that has the effect of increasing the hardenability of steel, and is one of the important elements that need to be added in order to achieve high strength as in the present invention. In order to obtain such effects, the Mn content is specified to be 0.50% or more. Furthermore, the content of Mn is preferably 0.70% or more from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost. On the other hand, if the Mn content exceeds 2.50%, the toughness and weldability of the steel sheet deteriorate, and the alloy cost becomes excessively high. Therefore, the Mn content is specified at 2.50% or less. Furthermore, the Mn content is preferably 2.30% or less from the viewpoint of further suppressing deterioration of toughness and weldability.
Ａｌ：０．０６０％以下
　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。かかる効果を得るためには、Ａｌ含有量を０．００１％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と共に、靭性の劣化が懸念される。従って、Ａｌ含有量を０．０６０％以下に規定する。さらに、Ａｌ含有量は、靭性劣化をより一層防止する観点から、０．０５０％以下とすることが好ましい。 Al: 0.060% or less Al is an element that acts as a deoxidizing agent and has the effect of refining crystal grains. In order to obtain such effects, the Al content is preferably 0.001% or more. On the other hand, if the Al content exceeds 0.060%, oxide-based inclusions increase and cleanliness deteriorates, and there is concern about deterioration of toughness. Therefore, the Al content is specified at 0.060% or less. Furthermore, the Al content is preferably 0.050% or less from the viewpoint of further preventing toughness deterioration.
Ｎ：０．００１０～０．０１００％
　Ｎは、組織の微細化に寄与し、鋼板の靭性を向上させる。かかる効果を得るため、Ｎ含有量を０．００１０％以上に規定する。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。従って、Ｎ含有量を０．０１００％以下に規定する。さらに、Ｎ含有量は、靭性や溶接性の低下をより一層抑制する観点から、０．００８０％以下とすることが好ましい。なお、Ｎは、Ｔｉが存在する場合には、そのＴｉと結合して、ＴｉＮとして析出し得る。 N: 0.0010 to 0.0100%
N contributes to the refinement of the structure and improves the toughness of the steel sheet. In order to obtain such effects, the N content is specified to be 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, the N content is specified at 0.0100% or less. Furthermore, the N content is preferably 0.0080% or less from the viewpoint of further suppressing deterioration of toughness and weldability. Incidentally, when Ti is present, N can bond with Ti and precipitate as TiN.
Ｐ：０．０２０％以下
　Ｐは、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、Ｐ含有量は、できる限り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。一方、Ｐ含有量の下限は特に限定されず、０％であってよいが、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Ｐ含有量を０．０００５％以上とすることが好ましい。 P: 0.020% or less P has an adverse effect, such as lowering toughness and weldability, by segregating at grain boundaries. Therefore, it is desirable to make the P content as low as possible, but a P content of 0.020% or less is acceptable. On the other hand, the lower limit of the P content is not particularly limited, and may be 0%, but excessive reduction causes a rise in refining costs, so from the viewpoint of cost, the P content should be 0.0005% or more. is preferred.
Ｓ：０．０１００％以下
　Ｓは、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って鋼板の靭性を低下させるなど、悪影響を及ぼす元素である。そのため、Ｓ含有量は、できる限り低くすることが望ましいが、０．０１００％以下であれば許容できる。一方、Ｓ含有量の下限は特に限定されず、０％であってよいが、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: 0.0100% or less S is present in steel as sulfide-based inclusions such as MnS, and is an element that exerts adverse effects, such as deteriorating the toughness of the steel sheet by becoming the origin of fracture. Therefore, it is desirable that the S content be as low as possible, but a content of 0.0100% or less is permissible. On the other hand, the lower limit of the S content is not particularly limited, and may be 0%, but excessive reduction causes a rise in refining costs, so from the viewpoint of cost, the S content should be 0.0005% or more. is preferred.
Ｏ：０．０１００％以下
　Ｏは、酸化物を形成し、破壊の発生起点となり、鋼板の靭性を低下させるなど、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、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０．００１０％以上とすることが好ましい。 O: 0.0100% or less O is an element that forms an oxide, becomes a starting point of fracture, and has an adverse effect such as lowering the toughness of the steel sheet. The O content is preferably 0.0050% or less, more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited, and may be 0%. is preferred.
Ｃｕ：０．０１～０．５０％、Ｃｒ：０．０１～１．００％、Ｓｂ：０．０１～０．５０％およびＳｎ：０．０１～０．５０％のうち１種または２種以上、かつ式（１）によって求められるＣＲ値が０．７０以上
　Ｃｕ、Ｃｒ、ＳｂおよびＳｎは、耐アンモニアＳＣＣ性向上のために本発明では特に重要な元素である。そのため、本発明では、このうちの１種または２種以上を上記の量で含有し、かつ以下の式（１）によって求められるＣＲ値が０．７０以上である必要がある。
　ＣＲ＝２．３[Ｃｕ]＋２．８[Ｃｒ]＋７．３[Ｓｂ]＋３．６[Ｓｎ]・・・式（１）
ただし、[Ｘ]はＸ元素の鋼中含有量（質量％）を示す。
　すなわち、Ｃｕ、Ｃｒ、ＳｂおよびＳｎは、液体アンモニア環境中において、速やかに保護性のある腐食生成物を形成し、応力腐食割れが抑制される。かかる効果を得るため、Ｃｕを添加する場合にはＣｕ含有量を０．０１％以上に、Ｃｒを添加する場合にはＣｒ含有量を０．０１％以上に、Ｓｂを添加する場合にはＳｂ含有量を０．０１％以上に、また、Ｓｎを添加する場合には、Ｓｎ含有量を０．０１％以上に、それぞれ限定する必要がある。
　また、上記ＣＲ値の算出式は、各元素の含有量から、耐アンモニアＳＣＣ性を推定するために考案された式であり、上記ＣＲ値が高いほど耐アンモニアＳＣＣが向上する。そして、上記ＣＲ値を０．７０以上とすることで、液体アンモニア環境中において応力腐食割れを抑制することが可能となる。
　一方、Ｃｕ、Ｃｒ、ＳｂおよびＳｎを過剰に添加すると、溶接性や靱性が劣化し、また合金コストの観点からも不利になる。従って、Ｃｕ含有量を０．５０％以下に、Ｃｒ含有量を１．００％以下に、Ｓｂ含有量を０．５０％以下に、また、Ｓｎ含有量を０．５０％以下に、それぞれ限定する。好ましくは、Ｃｕ含有量は０．４０％以下、Ｃｒ含有量は０．８０％以下、Ｓｂ含有量は０．４０％以下、また、Ｓｎ含有量は０．４０％以下である。また、上記ＣＲ値の上限は、特に限定されないが、ＣＲ値が７．００を超えると、その効果が飽和するとともに、上記元素の過剰な添加は価格の高騰を招くため、７．００程度が好ましい。 One or two of Cu: 0.01 to 0.50%, Cr: 0.01 to 1.00%, Sb: 0.01 to 0.50% and Sn: 0.01 to 0.50% Above and the CR value obtained by the formula (1) is 0.70 or more Cu, Cr, Sb and Sn are particularly important elements in the present invention for improving ammonia SCC resistance. Therefore, in the present invention, one or more of them must be contained in the above amount, and the CR value obtained by the following formula (1) must be 0.70 or more.
CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn] Expression (1)
However, [X] indicates the content (mass%) of the X element in the steel.
That is, Cu, Cr, Sb and Sn quickly form protective corrosion products in a liquid ammonia environment to suppress stress corrosion cracking. In order to obtain such an effect, when Cu is added, the Cu content is set to 0.01% or more, when Cr is added, the Cr content is set to 0.01% or more, and when Sb is added, Sb The content must be limited to 0.01% or more, and when Sn is added, the Sn content must be limited to 0.01% or more.
The formula for calculating the CR value is a formula devised for estimating the ammonia SCC resistance from the content of each element, and the higher the CR value, the better the ammonia SCC resistance. By setting the CR value to 0.70 or more, stress corrosion cracking can be suppressed in a liquid ammonia environment.
On the other hand, excessive addition of Cu, Cr, Sb and Sn deteriorates weldability and toughness, and is disadvantageous from the viewpoint of alloy cost. Therefore, the Cu content is limited to 0.50% or less, the Cr content to 1.00% or less, the Sb content to 0.50% or less, and the Sn content to 0.50% or less. do. Preferably, the Cu content is 0.40% or less, the Cr content is 0.80% or less, the Sb content is 0.40% or less, and the Sn content is 0.40% or less. In addition, the upper limit of the CR value is not particularly limited, but when the CR value exceeds 7.00, the effect is saturated, and excessive addition of the above elements causes a rise in price. preferable.
　本発明の鋼板の成分組成において、上記成分以外の残部は、Ｆｅおよび不可避的不純物である。ただし、上記成分組成は、必要に応じて、以下に記載する元素を含有することができる。 In the chemical composition of the steel sheet of the present invention, the balance other than the above components is Fe and unavoidable impurities. However, the above component composition can contain the elements described below, if necessary.
Ｎｉ：０．０１～２．００％、Ｍｏ：０．０１～０．５０％、およびＷ：０．０１～１．００％のうちから選ばれる１種以上
　Ｎｉ、ＭｏおよびＷは、耐アンモニアＳＣＣ性を一層向上させる元素であり、これらのうちの１種以上を含有させることができる。かかる効果を得るため、Ｎｉを含有させる場合には、Ｎｉ含有量を０．０１％以上に、Ｍｏを含有させる場合には、Ｍｏ含有量を０．０１％以上に、また、Ｗを含有させる場合には、Ｗ含有量を０．０１％以上に、それぞれ調整するのが好ましい。一方、Ｎｉ含有量を過剰に含有させると、溶接性の劣化や合金コストの上昇を招く。また、ＭｏおよびＷを過剰に添加すると、溶接性や靱性が劣化し、合金コストの観点からも不利になる。従って、Ｎｉ含有量を２．００％以下に、Ｍｏ含有量を０．５０％以下に、また、Ｗ含有量を１．００％以下に、それぞれ調整するのが好ましい。より好ましくは、Ｎｉ含有量を１．５０％以下に、Ｍｏ含有量を０．４０％以下に、また、Ｗ含有量を０．８０％以下に、それぞれ調整する。 One or more selected from Ni: 0.01 to 2.00%, Mo: 0.01 to 0.50%, and W: 0.01 to 1.00% Ni, Mo, and W are ammonia-resistant It is an element that further improves the SCC property, and one or more of these elements can be contained. In order to obtain such an effect, when Ni is contained, the Ni content is 0.01% or more, and when Mo is contained, the Mo content is 0.01% or more, and W is contained. In this case, it is preferable to adjust the W content to 0.01% or more. On the other hand, an excessive Ni content results in deterioration of weldability and an increase in alloy cost. Moreover, excessive addition of Mo and W degrades weldability and toughness, which is disadvantageous from the viewpoint of alloy cost. Therefore, it is preferable to adjust the Ni content to 2.00% or less, the Mo content to 0.50% or less, and the W content to 1.00% or less. More preferably, the Ni content is adjusted to 1.50% or less, the Mo content to 0.40% or less, and the W content to 0.80% or less.
Ｖ：０．０１～１．００％
　Ｖは、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。かかる効果を得るため、Ｖを添加する場合には、Ｖ含有量を０．０１％以上とするのが好ましい。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。従って、Ｖを添加する場合には、Ｖ含有量を１．００％以下とするのが好ましい。より好ましくは、Ｖ含有量の下限が０．０５％であり、上限が０．５０％である。 V: 0.01-1.00%
V is an element that has the effect of improving the strength of the steel sheet, and can be optionally added. In order to obtain such an effect, when V is added, the V content is preferably 0.01% 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 preferably 1.00% or less. More preferably, the lower limit of V content is 0.05% and the upper limit is 0.50%.
Ｔｉ：０．００５～０．１００％
　Ｔｉ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り、任意に添加することができる。また、Ｔｉは、母材および溶接部の靭性を向上させることができる。これらの効果を得るため、Ｔｉを添加する場合には、Ｔｉ含有量を０．００５％以上とするのが好ましい。さらに、０．００７％以上とすることがより好ましい。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。従って、Ｔｉを添加する場合には、Ｔｉ含有量を０．１００％以下とするのが好ましい。さらに、Ｔｉ含有量は、０．０９０％以下とすることがより好ましい。 Ti: 0.005-0.100%
Ti is an element that has a strong tendency to form nitrides and has the action of fixing N and reducing solid solution N, and can be added arbitrarily. In addition, Ti can improve the toughness of the base material and the weld zone. In order to obtain these effects, when adding Ti, the Ti content is preferably 0.005% or more. Furthermore, it is more preferable to make it 0.007% or more. On the other hand, when the Ti content exceeds 0.100%, the toughness rather decreases. Therefore, when adding Ti, the Ti content is preferably 0.100% or less. Furthermore, the Ti content is more preferably 0.090% or less.
Ｃｏ：０．０１～１．００％
　Ｃｏは、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。かかる効果を得るため、Ｃｏを添加する場合には、Ｃｏ含有量を０．０１％以上とするのが好ましい。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。従って、Ｃｏを添加する場合には、Ｃｏ含有量を１．００％以下とするのが好ましい。より好ましくは、Ｃｏ含有量の下限が０．０５％であり、上限が０．５０％である。 Co: 0.01-1.00%
Co is an element that has the effect of improving the strength of the steel sheet, and can be optionally added. In order to obtain such an effect, when Co is added, the Co content is preferably 0.01% 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 preferably 1.00% or less. More preferably, the Co content has a lower limit of 0.05% and an upper limit of 0.50%.
Ｎｂ：０．００５～０．１００％
　Ｎｂ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。かかる効果を得るため、Ｎｂを添加する場合には、Ｎｂ含有量を０．００５％以上とする。さらに、０．００７％以上とすることが好ましい。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。従って、Ｎｂを添加する場合には、Ｎｂ含有量を０．１００％以下とするのが好ましい。さらに、０．０６０％以下とすることがより好ましい。 Nb: 0.005-0.100%
Nb is an element that has the effect of reducing the grain size of prior austenite and improving the toughness by precipitating as a carbonitride. In order to obtain such an effect, when Nb is added, the Nb content is made 0.005% or more. Furthermore, it is preferable to make it 0.007% or more. On the other hand, when the Nb content exceeds 0.100%, a large amount of NbC precipitates, resulting in a decrease in toughness. Therefore, when Nb is added, the Nb content is preferably 0.100% or less. Furthermore, it is more preferable to make it 0.060% or less.
Ｂ：０．０００１～０．０１００％
　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。すなわち、鋼板の強度を向上させることができる。かかる効果を得るため、Ｂを添加する場合には、Ｂ含有量を０．０００１％以上とするのが好ましい。一方、Ｂ含有量が０．０１００％を超えると溶接性が低下する。従って、Ｂを添加する場合には、Ｂ含有量を０．０１００％以下とするのが好ましい。より好ましくは、Ｂ含有量の下限が０．００１０％であり、上限が０．００３０％である。 B: 0.0001 to 0.0100%
B is an element that has the effect of significantly improving hardenability even when added in a very small amount. That is, the strength of the steel sheet can be improved. In order to obtain such an effect, when B is added, the B content is preferably 0.0001% or more. On the other hand, when the B content exceeds 0.0100%, the weldability deteriorates. Therefore, when B is added, the B content is preferably 0.0100% or less. More preferably, the B content has a lower limit of 0.0010% and an upper limit of 0.0030%.
Ｃａ：０．０００５～０．０２００％
　Ｃａ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。すなわち、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。かかる効果を得るために、Ｃａを添加する場合には、Ｃａ含有量を０．０００５％以上とするのが好ましい。一方、Ｃａ含有量が０．０２００％を超えると、鋼の清浄度が低下する。清浄度の低下は、靭性の低下を招く。従って、Ｃａを添加する場合、Ｃａ含有量を０．０２００％以下とするのが好ましい。より好ましくは、Ｃａ含有量の下限が０．００２０％であり、上限が０．０１００％である。 Ca: 0.0005-0.0200%
Ca is an element that binds to S and has the effect of suppressing the formation of MnS or the like elongated in the rolling direction. That is, by adding Ca, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like. In order to obtain such effects, when Ca is added, the Ca content is preferably 0.0005% or more. On the other hand, when the Ca content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Ca is added, the Ca content is preferably 0.0200% or less. More preferably, the Ca content has a lower limit of 0.0020% and an upper limit of 0.0100%.
Ｍｇ：０．０００５～０．０２００％
　Ｍｇ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。すなわち、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。かかる効果を得るために、Ｍｇを添加する場合には、Ｍｇ含有量を０．０００５％以上とするのが好ましい。一方、Ｍｇ含有量が０．０２００％を超えると、鋼の清状度が低下する。清浄度の低下は、靭性の低下を招く。従って、Ｍｇを添加する場合には、Ｍｇ含有量を０．０２００％以下とするのが好ましい。より好ましくは、Ｍｇ含有量の下限が０．００２０％であり、上限が０．０１００％である。 Mg: 0.0005-0.0200%
Mg, like Ca, is an element that binds to S and has the effect of suppressing the formation of MnS or the like elongated in the rolling direction. That is, by adding Mg, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like. In order to obtain such an effect, when Mg is added, the Mg content is preferably 0.0005% or more. On the other hand, when the Mg content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less. More preferably, the Mg content has a lower limit of 0.0020% and an upper limit of 0.0100%.
ＲＥＭ：０．０００５～０．０２００％
　ＲＥＭ（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。すなわち、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。かかる効果を得るために、ＲＥＭを添加する場合には、ＲＥＭ含有量は０．０００５％以上が好ましい。一方、ＲＥＭ含有量が０．０２００％を超えると、鋼の清状度が低下する。清浄度の低下は、靭性の低下を招く。従って、ＲＥＭを添加する場合、ＲＥＭ含有量は０．０２００％以下が好ましい。より好ましくは、ＲＥＭ含有量の下限が０．００２０％であり、上限が０．０１００％である。 REM: 0.0005-0.0200%
Like Ca and Mg, REM (rare earth metal) is an element that binds to S and suppresses the formation of MnS or the like elongated in the rolling direction. That is, by adding REM, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like. In order to obtain such an effect, when REM is added, the REM content is preferably 0.0005% or more. On the other hand, when the REM content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when REM is added, the REM content is preferably 0.0200% or less. More preferably, the REM content has a lower limit of 0.0020% and an upper limit of 0.0100%.
（２）硬さ特性および金属組織について
　本発明の鋼板は、上記成分組成を有することに加えて、鋼板の表面から１．０ｍｍ深さの位置（本発明において１．０ｍｍ位置ともいう）の硬さがＨｖ３００以下である硬さ特性を有する。
　さらに、本発明の鋼板は、前記鋼板の板厚の１／２位置（本発明において板厚の１／２の深さの位置を意味する。以下、単に１／２位置または板厚中心部ともいう）において、ベイナイト組織（以下、単にベイナイトともいう）の体積率が２０％以上で、かつフェライト組織（以下、単にフェライトともいう）およびベイナイトの合計体積率が６０％以上である金属組織を有する。
　鋼板の硬さ特性および金属組織を上記のように限定する理由を、以下に説明する。 (2) Hardness properties and metallographic structure The steel sheet of the present invention has the above chemical composition, and also has a hardness at a depth of 1.0 mm from the surface of the steel sheet (also referred to as a 1.0 mm position in the present invention). It has a hardness characteristic of Hv300 or less.
Furthermore, the steel plate of the present invention refers to the 1/2 position of the plate thickness of the steel plate (in the present invention, it means the position of the depth of 1/2 of the plate thickness. Hereinafter, it is simply referred to as the 1/2 position or the plate thickness center part. ), the volume fraction of the bainite structure (hereinafter also simply referred to as bainite) is 20% or more, and the total volume fraction of the ferrite structure (hereinafter simply referred to as ferrite) and bainite is 60% or more. .
The reasons for limiting the hardness properties and metallographic structure of the steel sheet as described above will be explained below.
［１．０ｍｍ位置における硬さがＨｖ３００以下］
　１．０ｍｍ位置における硬さは、Ｈｖ３００以下とする。鋼板の極表層、具体的には鋼板の表面から１．０ｍｍ位置に高硬度領域が存在すると、液体アンモニア環境中での応力腐食割れが助長されてしまう。そこで、本発明の鋼板では、１．０ｍｍ位置における硬さをＨｖ３００以下として硬さ特性を調整することで、優れた耐アンモニアＳＣＣ性を確保することができる。なお、１．０ｍｍ位置における硬さの下限は、特に限定されないが、Ｈｖ１３０程度が好ましい。
　ここで、上記硬さは、０．５ｍｍ位置におけるビッカース硬さを複数箇所（例えば、１００点）測定して算出することができる。 [Hardness at 1.0 mm position is Hv300 or less]
The hardness at the 1.0 mm position shall be Hv300 or less. If a high-hardness region exists in the extreme surface layer of the steel sheet, specifically, at a position of 1.0 mm from the surface of the steel sheet, stress corrosion cracking in a liquid ammonia environment is promoted. Therefore, in the steel sheet of the present invention, excellent ammonia SCC resistance can be ensured by adjusting the hardness characteristics so that the hardness at the 1.0 mm position is Hv300 or less. Although the lower limit of the hardness at the 1.0 mm position is not particularly limited, it is preferably about Hv130.
Here, the hardness can be calculated by measuring Vickers hardness at a plurality of points (for example, 100 points) at a position of 0.5 mm.
［１／２位置において、ベイナイトの体積率が２０％以上、かつフェライトおよびベイナイトの合計体積率が６０％以上］
　１／２位置における組織は、ベイナイトの体積率が２０％以上、かつフェライトおよびベイナイトの合計体積率が６０％以上である必要がある。フェライトが過剰に生成した場合、強度あるいは靭性の低下を招く。また、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、これ以外の組織、すなわち島状マルテンサイト組織、マルテンサイト組織、パーライト組織およびオーステナイト組織の体積分率が増加することになり、十分な強度あるいは靭性が得られずに、機械特性を満足することができない。なお、フェライトおよびベイナイトの合計体積率は１００％であって良い。 [At the 1/2 position, the volume fraction of bainite is 20% or more, and the total volume fraction of ferrite and bainite is 60% or more]
The structure at the 1/2 position must have a bainite volume fraction of 20% or more and a total volume fraction of ferrite and bainite of 60% or more. Excessive generation of ferrite leads to a decrease in strength or toughness. Further, when the total volume fraction of ferrite and bainite is less than 60%, the volume fractions of structures other than this, namely, island-shaped martensite structure, martensite structure, pearlite structure and austenite structure, will increase, which is sufficient. sufficient strength or toughness cannot be obtained, and the mechanical properties cannot be satisfied. The total volume fraction of ferrite and bainite may be 100%.
　ここで、前記フェライトは、焼戻しを受ける前の冷却過程で生成したフェライトを意味し、前記ベイナイトは、焼戻しを受ける前の冷却過程で生成したベイナイトを意味する。また、板厚中心部でのミクロ組織を規定するのは、板厚中心部でのミクロ組織が、かかる板厚中心部の強度特性に影響を与えるためであり、また、かかる板厚中心部の強度特性が、鋼板全体の強度に影響を与えるためである。 Here, the ferrite means ferrite generated in the cooling process before tempering, and the bainite means bainite generated in the cooling process before tempering. In addition, the reason why the microstructure at the center of thickness is defined is that the microstructure at the center of thickness affects the strength characteristics of the center of thickness. This is because the strength properties affect the strength of the steel plate as a whole.
　体積率で４０％以下を占める残部組織は、パーライト組織およびオーステナイト組織の他、マルテンサイト組織が含まれていてもよい。残部組織における各組織の分率は特に限定する必要はないが、残部組織はパーライト組織であることが好ましい。
　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。 The remaining structure occupying 40% or less in volume fraction may include martensite structure in addition to pearlite structure and austenite structure. The fraction of each structure in the remaining structure is not particularly limited, but the remaining structure is preferably a pearlite structure.
Incidentally, the volume ratio of various microstructures can be measured by the method described in Examples below.
（３）製造条件について
　本発明における製造方法は、Ｃ：０．０１０～０．２００％、Ｓｉ：０．０１～０．５０％、Ｍｎ：０．５０～２．５０％、Ａｌ：０．０６０％以下、Ｎ：０．００１０～０．０１００％、Ｐ：０．０２０％以下、Ｓ：０．０１００％以下およびＯ：０．０１００％以下を含有し、さらに、Ｃｕ：０．０１～０．５０％、Ｃｒ：０．０１～１．００％、Ｓｂ：０．０１～０．５０％およびＳｎ：０．０１～０．５０％のうち１種または２種以上を含有し、かつ前記式（１）によって求められるＣＲ値を０．７０以上とし、加えて、必要に応じ、Ｎｉ：０．０１～２．００％、Ｍｏ：０．０１～０．５０％およびＷ：０．０１～１．００％のうちから選ばれる１種以上並びに／またはＶ：０．０１～１．００％、Ｔｉ：０．００５～０．１００％、Ｃｏ：０．０１～１．００％、Ｎｂ：０．００５～０．１００％、Ｂ：０．０００１～０．０１００％、Ｃａ：０．０００５～０．０２００％、Ｍｇ：０．０００５～０．０２００％およびＲＥＭ：０．０００５～０．０２００％のうちから選ばれる１種以上を含有し、残部がＦｅおよび不可避的不純物である成分組成を有する鋼素材について、加熱し熱間圧延を行った後、本発明に従う所定の冷却を行うものである。以下に、鋼板の製造条件の限定理由について説明する。
　まず、鋼素材の製造条件は、特に限定する必要はないが、例えば、上述した成分組成を有する溶鋼を、転炉等の公知の溶製方法で溶製し、連続鋳造法等の公知の鋳造方法にて、所定寸法のスラブ等の鋼素材とすることが好ましい。なお、造塊－分解圧延法により、所定寸法のスラブ等の鋼素材としても何ら問題はない。 (3) Manufacturing conditions The manufacturing method in the present invention is C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, Al: 0.50%. 060% or less, N: 0.0010 to 0.0100%, P: 0.020% or less, S: 0.0100% or less and O: 0.0100% or less, and Cu: 0.01 to 0.50%, Cr: 0.01 to 1.00%, Sb: 0.01 to 0.50% and Sn: 0.01 to 0.50% containing one or more, and The CR value obtained by the above formula (1) is set to 0.70 or more, and in addition, if necessary, Ni: 0.01 to 2.00%, Mo: 0.01 to 0.50% and W: 0.5%. 01 to 1.00% and/or V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005-0.100%, B: 0.0001-0.0100%, Ca: 0.0005-0.0200%, Mg: 0.0005-0.0200% and REM: 0.0005- A steel material having a chemical composition containing one or more selected from 0.0200% with the balance being Fe and inevitable impurities is heated and hot-rolled, and then subjected to predetermined cooling according to the present invention. It is something to do. Reasons for limiting the manufacturing conditions of the steel sheet will be described below.
First, the manufacturing conditions of the steel material need not be particularly limited. It is preferable to use a steel material such as a slab of predetermined dimensions in the method. It should be noted that there is no problem in making a steel material such as a slab having a predetermined size by the ingot casting-decomposition rolling method.
　かようにして得られた鋼素材は、冷却することなく直接熱間圧延するか、あるいは再度加熱してから熱間圧延する。かかる熱間圧延は、圧延終了温度をＡｒ３変態点の温度（以下、単にＡｒ３変態点という）以上として行う。熱間圧延に次いで、Ａｒ３変態点以上の冷却開始温度から６００℃以下の冷却停止温度までの冷却を所定条件で行う。 The steel material thus obtained is directly hot-rolled without cooling or hot-rolled after reheating. Such hot rolling is performed at a rolling end temperature equal to or higher than the Ar 3 transformation point (hereinafter simply referred to as the Ar 3 transformation point). After hot rolling, cooling is performed under predetermined conditions from a cooling start temperature above the Ar 3 transformation point to a cooling stop temperature below 600°C.
　鋼素材の加熱温度（熱間圧延に供する際の温度）は特に限定されないが、加熱温度が低すぎると変形抵抗が高くなって、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、１３００℃を超える高温になると、酸化が著しくなって酸化ロスが増大し、歩留りが低下するおそれが増える。このような理由から、加熱温度は、９５０℃以上１３００℃以下にすることが好ましい。 The heating temperature of the steel material (the temperature at which it is subjected to hot rolling) is not particularly limited, but if the heating temperature is too low, the deformation resistance increases, the load on the hot rolling mill increases, and hot rolling becomes difficult. may become On the other hand, if the temperature exceeds 1300° C., the oxidation becomes significant, the oxidation loss increases, and the yield increases. For these reasons, the heating temperature is preferably 950° C. or higher and 1300° C. or lower.
（熱間圧延）
［圧延終了温度：Ａｒ３変態点以上］
　本発明では、上記温度に加熱後、熱間圧延を開始して、Ａｒ3変態点以上で当該熱間圧延を終了する。
　圧延終了温度が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。従って、熱間圧延における圧延終了温度は、Ａｒ3変態点以上とする。好ましくは、熱間圧延における圧延終了温度は、Ａｒ3変態点＋１０℃以上の温度である。一方、圧延終了温度が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、圧延終了温度は、９５０℃以下であることが好ましい。
　ここで、Ａｒ3変態点は、次式で求めることが可能である。
　Ａｒ3（℃）＝９１０－３１０×Ｃ－８０×Ｍｎ－２０×Ｃｕ－１５×Ｃｒ－５５×Ｎｉ－８０×Ｍｏ
　　ただし、各元素は当該元素の鋼中含有量（質量％）を示す。 (hot rolling)
[Rolling end temperature: Ar 3 transformation point or higher]
In the present invention, after heating to the above temperature, hot rolling is started and finished at the Ar 3 transformation point or higher.
When the rolling end temperature is lower than the Ar 3 transformation point, ferrite is generated, and the generated ferrite is affected by working, resulting in deterioration of toughness. Furthermore, the load on the hot rolling mill increases. Therefore, the rolling end temperature in hot rolling should be the Ar 3 transformation point or higher. Preferably, the rolling end temperature in hot rolling is a temperature of Ar 3 transformation point +10°C or higher. On the other hand, if the rolling end temperature exceeds 950°C, the structure may coarsen and the toughness may deteriorate, so the rolling end temperature is preferably 950°C or less.
Here, the Ar 3 transformation point can be obtained by the following formula.
Ar 3 (° C.)=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo
However, each element indicates the content of the element in steel (% by mass).
（冷却）
［冷却開始温度：Ａｒ３変態点以上］
　次に、熱間圧延後の鋼板について、Ａｒ3変態点以上の冷却開始温度からの冷却を行う。冷却開始温度がＡｒ3変態点未満では、フェライトが過剰に生成し、強度不足を招く。そのため、冷却開始温度はＡｒ3変態点以上とする。 (cooling)
[Cooling start temperature: Ar 3 transformation point or higher]
Next, the hot-rolled steel sheet is cooled from a cooling start temperature equal to or higher than the Ar 3 transformation point. If the cooling start temperature is lower than the Ar 3 transformation point, excessive ferrite is formed, resulting in insufficient strength. Therefore, the cooling start temperature should be the Ar 3 transformation point or higher.
［冷却停止温度：６００℃以下］
　本発明では、熱間圧延終了後に、６００℃以下の任意に設定した冷却停止温度まで、所定条件で冷却を行うことにより、板厚中心部にてフェライトおよびベイナイトを所定の体積率にすることができる。ここで、冷却停止温度が６００℃を超えると、フェライト組織やパーライト組織が過剰に生成して、強度不足を招くおそれがある。従って、冷却停止温度は６００℃以下に規定する。かかる冷却停止温度の下限は、特に限定されないが、冷却停止温度が過度に低くなると、島状マルテンサイトの組織の体積率が多くなりすぎてしまい、靭性が低下する。そのため、冷却停止温度は、２００℃以上とすることが好ましい。
　なお、上記冷却停止温度は、鋼板の１／２位置における温度である。 [Cooling stop temperature: 600°C or less]
In the present invention, after hot rolling is completed, cooling is performed under predetermined conditions to an arbitrarily set cooling stop temperature of 600 ° C. or less, so that the ferrite and bainite can be made to have a predetermined volume ratio at the center of the plate thickness. can. Here, if the cooling stop temperature exceeds 600° C., excessive formation of ferrite structure and pearlite structure may lead to insufficient strength. Therefore, the cooling stop temperature is specified at 600° C. or less. The lower limit of the cooling stop temperature is not particularly limited, but if the cooling stop temperature is excessively low, the volume fraction of the island-shaped martensite structure becomes too large, resulting in a decrease in toughness. Therefore, the cooling stop temperature is preferably 200° C. or higher.
The cooling stop temperature is the temperature at the 1/2 position of the steel plate.
［１．０ｍｍ位置における冷却速度：１５０℃／ｓ以下］
　上記冷却において、１．０ｍｍ位置における冷却速度が１５０℃／ｓを超えると、かかる１．０ｍｍ位置における硬さがＨｖ３００超となって、耐アンモニアＳＣＣ性が劣化する。従って、１．０ｍｍ位置における冷却速度を１５０℃／ｓ以下に規定する。
　一方、かかる冷却速度の下限は、特に限定されないが、冷却速度が過度に小さくなるとフェライト組織やパーライト組織が過剰に生成して強度不足や靭性の劣化を招くおそれがある。そのため、これをより確実に防ぐ観点からは、上記冷却速度を５０℃／ｓ以上とすることが好ましい。
　なお、冷却停止期間を含む間欠的な冷却による制御冷却により、上記冷却速度を制御することができる。また、１．０ｍｍ位置における温度は、物理的に直接測定することは困難である。しかし、放射温度計にて測定された冷却開始時の表面温度と目標の冷却停止時の表面温度とをもとに、例えばプロセスコンピューターを用いて差分計算を行うことにより、板厚断面内の温度分布、特には１．０ｍｍ位置における温度を、をリアルタイムに求めることができる。 [Cooling rate at 1.0 mm position: 150° C./s or less]
In the cooling described above, if the cooling rate at the 1.0 mm position exceeds 150° C./s, the hardness at the 1.0 mm position exceeds Hv300, degrading the ammonia SCC resistance. Therefore, the cooling rate at the 1.0 mm position is specified at 150° C./s or less.
On the other hand, the lower limit of the cooling rate is not particularly limited, but if the cooling rate is excessively low, excessive generation of ferrite structure and pearlite structure may lead to insufficient strength and deterioration of toughness. Therefore, from the viewpoint of preventing this more reliably, the cooling rate is preferably 50° C./s or higher.
The cooling rate can be controlled by controlled cooling through intermittent cooling including a cooling stop period. Also, it is difficult to physically and directly measure the temperature at the 1.0 mm position. However, based on the surface temperature at the start of cooling measured by a radiation thermometer and the target surface temperature at the end of cooling, for example, by using a process computer to calculate the difference, the temperature in the thickness cross section The distribution, especially the temperature at the 1.0 mm position, can be obtained in real time.
［１／２位置における冷却速度：１０℃／ｓ以上］
　１／２位置における冷却速度を１０℃／ｓ以上で行う冷却は、高強度で高靱性の鋼板を得るために不可欠なプロセスであり、高い冷却速度で冷却することで変態強化による強度上昇効果が得られる。かかる効果を得るため、本発明に従う冷却時の１／２位置における冷却速度を、１０℃／ｓ以上に規定する。上記冷却速度が１０℃／ｓ未満では、フェライト、パーライトが過剰に生成し、十分な強度が得られない。従って、板厚１／２位置における冷却速度を１０℃以上に規定する。
　一方、かかる冷却速度の上限は、特に限定されないが、冷却速度が過度に大きくなると島状マルテンサイトの体積率が多くなりすぎてしまい、靭性の劣化を招くおそれがある。そのため、１／２位置における冷却速度は、８０℃／ｓ以下とすることが好ましい。
　なお、冷却停止期間を含む間欠的な冷却による制御冷却により、上記冷却速度を制御することができる。また、１／２位置における温度は、物理的に直接測定することは困難である。しかし、放射温度計にて測定された冷却開始時の表面温度と目標の冷却停止時の表面温度とをもとに、例えばプロセスコンピューターを用いて差分計算を行うことにより、板厚断面内の温度分布、特には１／２位置における温度を、リアルタイムに求めることができる。 [Cooling rate at 1/2 position: 10 ° C./s or more]
Cooling performed at a cooling rate of 10°C/s or more at the 1/2 position is an essential process for obtaining a high-strength and high-toughness steel sheet, and cooling at a high cooling rate has the effect of increasing strength due to transformation strengthening. can get. In order to obtain such effects, the cooling rate at the 1/2 position during cooling according to the present invention is specified to be 10° C./s or more. If the cooling rate is less than 10°C/s, ferrite and pearlite are excessively formed, and sufficient strength cannot be obtained. Therefore, the cooling rate at the plate thickness 1/2 position is specified to be 10° C. or higher.
On the other hand, the upper limit of the cooling rate is not particularly limited, but if the cooling rate is excessively high, the volume fraction of island-shaped martensite becomes too large, which may lead to deterioration of toughness. Therefore, the cooling rate at the 1/2 position is preferably 80° C./s or less.
The cooling rate can be controlled by controlled cooling through intermittent cooling including a cooling stop period. Also, the temperature at the 1/2 position is physically difficult to measure directly. However, based on the surface temperature at the start of cooling measured by a radiation thermometer and the target surface temperature at the end of cooling, for example, by using a process computer to calculate the difference, the temperature in the thickness cross section The distribution, in particular the temperature at the 1/2 position, can be determined in real time.
　１．０ｍｍ位置における冷却速度、および１／２位置における冷却速度は、例えば、冷却開始温度、水量などを複合的に調節することで、それぞれ変化し得る。 The cooling rate at the 1.0 mm position and the cooling rate at the 1/2 position can each be changed by, for example, adjusting the cooling start temperature, the amount of water, etc. in a complex manner.
　上記した成分組成を有する鋼素材を、上記した製造条件に従って製造することによって、本発明に従う成分組成ならびに硬さ特性および金属組織を有する鋼板を得ることができる。かくして得られた鋼板は、優れた強度特性と靭性とを具えることになる。ここで、優れた強度特性とは、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）：３６０ＭＰａ以上および引張強さ（ＴＳ）：４９０ＭＰa以上である。また、優れた靭性とは、ＪＩＳ　Ｚ　２２４１に準拠するｖＴｒｓが－３０℃以下である。 By producing a steel material having the above chemical composition according to the above manufacturing conditions, it is possible to obtain a steel plate having the chemical composition, hardness characteristics and metallographic structure according to the present invention. The steel sheet thus obtained will have excellent strength properties and toughness. Here, the excellent strength characteristics are yield strength YS (yield point YP when there is a yield point, 0.2% yield strength σ0.2 when there is no yield point): 360 MPa or more and tensile strength (TS): 490 MPa or more is. In addition, excellent toughness means that vTrs conforming to JIS Z 2241 is -30°C or less.
　なお、本発明に従う製造方法では、本明細書に記載のない項目は、いずれも常法を用いることができる。 In addition, in the manufacturing method according to the present invention, any item not described in this specification can be used by a conventional method.
　表１に示す成分組成の鋼（鋼種Ａ～ＡＨ、残部はＦｅおよび不可避的不純物）を連続鋳造法によりスラブとし、これを用いて板厚３０ｍｍの厚鋼板（Ｎｏ．１～４４）とした。次いで、表２に示す条件で、熱間圧延、冷却を順次行い、鋼板を得た。得られた鋼板について、板厚の１／２位置における金属組織の組織分率の測定、鋼板表面から１．０ｍｍ位置における硬さの測定、強度特性および靭性の評価、耐アンモニアＳＣＣ性の評価をそれぞれ実施した。各試験方法は次のとおりである。また、これらの結果を、表２に併記する。 Slabs were made from steels (steel grades A to AH, the balance being Fe and unavoidable impurities) having the chemical compositions shown in Table 1, and used to make thick steel plates (No. 1 to 44) with a thickness of 30 mm. Then, hot rolling and cooling were sequentially performed under the conditions shown in Table 2 to obtain steel sheets. The obtained steel plate was subjected to measurement of the metal structure fraction at the position of 1/2 of the plate thickness, measurement of hardness at a position of 1.0 mm from the steel plate surface, evaluation of strength characteristics and toughness, and evaluation of ammonia SCC resistance. implemented each. Each test method is as follows. These results are also shown in Table 2.
［１／２位置における金属組織の組織分率］
　各鋼板から１／２位置（板厚中心部）が観察面となるように、サンプルを採取した。次いで、かかるサンプルを鏡面研磨し、さらにナイタール腐食をした後、走査型電子顕微鏡（ＳＥＭ）を用いて１０ｍｍ×１０ｍｍの範囲を倍率：５００～３０００倍で撮影した。そして、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、ミクロ組織の面分率（金属組織の組織分率）を求めた。ミクロ組織の異方性が小さい場合、面分率は体積率に相当するため、本発明では面分率を体積率と見なした。 [Structural fraction of metal structure at 1/2 position]
A sample was taken from each steel plate so that the 1/2 position (plate thickness center) was the observation surface. Then, the sample was mirror-polished, and after being subjected to nital corrosion, a scanning electron microscope (SEM) was used to photograph an area of 10 mm×10 mm at a magnification of 500 to 3000. Then, the photographed image was analyzed using an image analyzer to determine the area fraction of the microstructure (structure fraction of the metal structure). When the anisotropy of the microstructure is small, the area fraction corresponds to the volume fraction, so in the present invention the area fraction is regarded as the volume fraction.
　なお、本実施例において、サンプルの金属組織の分率を求める際の判別は、次のとおりに行った。すなわち、上述の撮影された像において、ポリゴナル状のフェライトをフェライト（表２におけるＦ）と判別し、また細長く成長したラス状のフェライトを有し、円相当径で０．０５μｍ以上の炭化物を含む組織をベイナイト（表２におけるＢ）と判別した。 In addition, in this example, the determination when obtaining the fraction of the metal structure of the sample was performed as follows. That is, in the photographed image described above, the polygonal ferrite is discriminated as ferrite (F in Table 2), and it has elongated lath-shaped ferrite and contains carbide with an equivalent circle diameter of 0.05 μm or more. The texture was identified as bainite (B in Table 2).
［１．０ｍｍ位置における硬さ］
　各鋼板の圧延方向に直角な断面について、ＪＩＳ Ｚ ２２４４に準拠して、１．０ｍｍ位置において１００点のビッカース硬さ（ＨＶ１０）を測定し、その最大値を求めた。 [Hardness at 1.0 mm position]
The cross section perpendicular to the rolling direction of each steel plate was measured for 100 points of Vickers hardness (HV10) at a position of 1.0 mm according to JIS Z 2244, and the maximum value was obtained.
［強度特性］
　各鋼板の全厚から、圧延方向に直角かつ板厚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に記載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。そして降伏強さが３６０ＭＰａ以上、引張強さが４９０ＭＰa以上のものを、強度特性に優れた鋼板と評価した。 [Strength characteristics]
From the full thickness of each steel plate, a JIS Z 2201 No. 1B test piece was taken in a direction perpendicular to the rolling direction and perpendicular to the plate thickness direction, and a tensile test was performed in accordance with JIS Z 2241. Yield strength YS (Yield point YP when there is a yield point, 0.2% yield strength σ0.2 when there is no yield point) and tensile strength (TS) were measured. A steel sheet having a yield strength of 360 MPa or more and a tensile strength of 490 MPa or more was evaluated as having excellent strength characteristics.
［靭性］
　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（破面遷移温度）を測定した。そして、かかるｖＴｒｓが－３０℃以下のものを、靭性に優れた鋼板と評価した。 [Toughness]
A JIS Z 2202 V-notch test piece was taken in the rolling direction from a portion shaved 1 mm from the surface side of each steel plate, and a Charpy impact test was performed according to JIS Z 2242 to measure vTrs (fracture surface transition temperature). . A steel sheet with such vTrs of -30°C or less was evaluated as a steel sheet having excellent toughness.
［耐アンモニアＳＣＣ性］
　耐アンモニアＳＣＣ性は、試験溶液で４点曲げ試験を実施し、腐食を促進させるため定電位アノード電解した促進試験により評価した。
　具体的には、以下の手順で実施した：
　鋼板表面から、５ｍｍ厚×１５ｍｍ×１１５ｍｍの試験片を採取して、アセトン中で超音波脱脂を５分間行い、４点曲げにより各鋼板の実際の降伏強さの１００％ＹＳの応力を負荷した。かかる４点曲げの試験片を試験セルに設置し、カルバミン酸アンモニウム１２．５ｇと液体アンモニア１Ｌとを混合した溶液を充填した後、ポテンショスタットにより、試験片に＋２．０Ｖ ｖｓ Ｐｔが流れるように制御し、室温（２５℃）で浸漬した。１６８時間の浸漬後に、割れが認められない場合を、耐アンモニアＳＣＣ性が「良」と判定し、また割れが発生した場合を、耐アンモニアＳＣＣ性が「不良」と判定した。 [Ammonia SCC resistance]
Ammonia SCC resistance was evaluated by an accelerated test in which a four-point bending test was performed using a test solution and constant potential anodic electrolysis was performed to promote corrosion.
Specifically, we performed the following steps:
A test piece with a thickness of 5 mm x 15 mm x 115 mm was taken from the surface of the steel plate, subjected to ultrasonic degreasing in acetone for 5 minutes, and stress of 100% YS of the actual yield strength of each steel plate was applied by four-point bending. . After setting such a four-point bending test piece in a test cell and filling it with a mixed solution of 12.5 g of ammonium carbamate and 1 L of liquid ammonia, a potentiostat was used to apply +2.0 V vs Pt to the test piece. controlled and immersed at room temperature (25° C.). After immersion for 168 hours, the ammonia SCC resistance was determined to be "good" when cracks were not observed, and the ammonia SCC resistance was determined to be "poor" when cracks occurred.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-I000003
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-I000003
　表１および２から分かるように、発明例（Ｎｏ．１～２６）は、いずれも、３６０ＭＰａ以上の降伏強度ＹＳと４９０ＭＰａ以上の引張強度ＴＳとをもち、ｖＴｒｓが－３０℃以下であり低温での靭性に優れ、かつ耐アンモニアＳＣＣ性にも優れた鋼板が得られている。 As can be seen from Tables 1 and 2, the invention examples (No. 1 to 26) all have a yield strength YS of 360 MPa or more and a tensile strength TS of 490 MPa or more, and vTrs is -30 ° C. or less at low temperatures. A steel sheet with excellent toughness and excellent ammonia SCC resistance is obtained.
　一方、Ｎｏ．２７～３１は、成分組成が本発明の範囲内であるものの、製造方法が本発明の範囲外であるため、所望の金属組織または硬さ特性が得られていない。その結果、降伏強度ＹＳ、引張強度ＴＳ、低温での靱性、あるいは耐アンモニアＳＣＣ性のいずれかが劣っている。 On the other hand, No. In Nos. 27 to 31, although the component composition is within the scope of the present invention, the manufacturing method is outside the scope of the present invention, so the desired metallographic structure or hardness properties are not obtained. As a result, the yield strength YS, tensile strength TS, toughness at low temperature, or resistance to ammonia SCC is inferior.
　また、Ｎｏ．３２～４４は、鋼の成分組成が本発明の範囲外であるため、降伏強度ＹＳ、引張強度ＴＳ、低温での靱性、あるいは耐アンモニアＳＣＣ性のいずれかが劣っている。なお、本発明では、鋼の成分組成は、そのまま鋼板の成分組成と考えてよい。 Also, No. In Nos. 32 to 44, the chemical compositions of the steels are outside the range of the present invention, so they are inferior in any of yield strength YS, tensile strength TS, low temperature toughness, or ammonia SCC resistance. In addition, in the present invention, the chemical composition of the steel may be considered as the chemical composition of the steel sheet.

Claims (6)

  1. 　質量％で、
    　Ｃ：０．０１０～０．２００％、
    　Ｓｉ：０．０１～０．５０％、
    　Ｍｎ：０．５０～２．５０％、
    　Ａｌ：０．０６０％以下、
    　Ｎ：０．００１０～０．０１００％、
    　Ｐ：０．０２０％以下、
    　Ｓ：０．０１００％以下および
    　Ｏ：０．０１００％以下
    を含有し、さらに、
    　Ｃｕ：０．０１～０．５０％、
    　Ｃｒ：０．０１～１．００％、
    　Ｓｂ：０．０１～０．５０％および
    　Ｓｎ：０．０１～０．５０％
    のうち１種または２種以上を含有し、以下の式（１）によって求められるＣＲ値が０．７０以上であり、残部がＦｅおよび不可避的不純物である成分組成を有する鋼板であって、
    　前記鋼板の表面から１．０ｍｍ深さの位置における硬さがＨｖ３００以下である硬さ特性と、
    　前記鋼板の板厚の１／２位置において、ベイナイト組織の体積率が２０％以上で、かつフェライト組織およびベイナイト組織の合計体積率が６０％以上である金属組織と、を有する、鋼板。
    　ＣＲ＝２．３[Ｃｕ]＋２．８[Ｃｒ]＋７．３[Ｓｂ]＋３．６[Ｓｎ]・・・式（１）
    ただし、[Ｘ]はＸ元素の鋼中含有量（質量％）を示す。     in % by mass,
    C: 0.010 to 0.200%,
    Si: 0.01 to 0.50%,
    Mn: 0.50-2.50%,
    Al: 0.060% or less,
    N: 0.0010 to 0.0100%,
    P: 0.020% or less,
    S: 0.0100% or less and O: 0.0100% or less, and
    Cu: 0.01-0.50%,
    Cr: 0.01 to 1.00%,
    Sb: 0.01-0.50% and Sn: 0.01-0.50%
    A steel sheet having a chemical composition containing one or two or more of
    a hardness characteristic in which the hardness at a position 1.0 mm deep from the surface of the steel plate is Hv300 or less;
    A steel sheet having a metal structure in which a bainite structure has a volume fraction of 20% or more and a total volume fraction of a ferrite structure and a bainite structure is 60% or more at a position of 1/2 of the plate thickness of the steel plate.
    CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn] Expression (1)
    However, [X] indicates the content (mass%) of the X element in the steel.
  2. 　前記成分組成が、さらに、質量％で、
    　Ｎｉ：０．０１～２．００％、
    　Ｍｏ：０．０１～０．５０％および
    　Ｗ：０．０１～１．００％
    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 component composition further, in mass %,
    Ni: 0.01 to 2.00%,
    Mo: 0.01-0.50% and W: 0.01-1.00%
    The steel sheet according to claim 1, containing one or more selected from.
  3. 　前記成分組成が、さらに、質量％で、
    　Ｖ：０．０１～１．００％、
    　Ｔｉ：０．００５～０．１００％、
    　Ｃｏ：０．０１～１．００％、
    　Ｎｂ：０．００５～０．１００％、
    　Ｂ：０．０００１～０．０１００％、
    　Ｃａ：０．０００５～０．０２００％、
    　Ｍｇ：０．０００５～０．０２００％および
    　ＲＥＭ：０．０００５～０．０２００％
    のうちから選ばれる１種以上を含有する、請求項１または請求項２に記載の鋼板。     The component composition further, in mass %,
    V: 0.01 to 1.00%,
    Ti: 0.005 to 0.100%,
    Co: 0.01 to 1.00%,
    Nb: 0.005 to 0.100%,
    B: 0.0001 to 0.0100%,
    Ca: 0.0005 to 0.0200%,
    Mg: 0.0005-0.0200% and REM: 0.0005-0.0200%
    The steel sheet according to claim 1 or 2, containing one or more selected from.
  4. 　質量％で、
    　Ｃ：０．０１０～０．２００％、
    　Ｓｉ：０．０１～０．５０％、
    　Ｍｎ：０．５０～２．５０％、
    　Ａｌ：０．０６０％以下、
    　Ｎ：０．００１０～０．０１００％、
    　Ｐ：０．０２０％以下、
    　Ｓ：０．０１００％以下および
    　Ｏ：０．０１００％以下
    を含有し、さらに、
    　Ｃｕ：０．０１～０．５０％、
    　Ｃｒ：０．０１～１．００％、
    　Ｓｂ：０．０１～０．５０％および、
    　Ｓｎ：０．０１～０．５０％
    のうち１種または２種以上を含有し、以下の式（１）によって求められるＣＲ値が０．７０以上であり、残部がＦｅおよび不可避的不純物である成分組成を有する鋼素材について、圧延終了温度をＡｒ３変態点以上として熱間圧延を行い、次いで、Ａｒ３変態点以上の冷却開始温度から６００℃以下の冷却停止温度までの冷却を行う、鋼板の製造方法であって、
    　前記冷却では、鋼板の表面から１．０ｍｍ深さの位置における冷却速度を１５０℃／ｓ以下とし、かつ、鋼板の板厚の１／２位置における冷却速度を１０℃／ｓ以上とする、鋼板の製造方法。
    　ＣＲ＝２．３[Ｃｕ]＋２．８[Ｃｒ]＋７．３[Ｓｂ]＋３．６[Ｓｎ]・・・式（１）
    ただし、[Ｘ]はＸ元素の鋼中含有量（質量％）を示す。     in % by mass,
    C: 0.010 to 0.200%,
    Si: 0.01 to 0.50%,
    Mn: 0.50-2.50%,
    Al: 0.060% or less,
    N: 0.0010 to 0.0100%,
    P: 0.020% or less,
    S: 0.0100% or less and O: 0.0100% or less, and
    Cu: 0.01-0.50%,
    Cr: 0.01 to 1.00%,
    Sb: 0.01 to 0.50% and
    Sn: 0.01-0.50%
    of which the CR value obtained by the following formula (1) is 0.70 or more, and the balance is Fe and unavoidable impurities. A method for manufacturing a steel sheet, wherein hot rolling is performed at a temperature of the Ar 3 transformation point or higher, and then cooling is performed from a cooling start temperature of the Ar 3 transformation point or higher to a cooling stop temperature of 600 ° C. or lower,
    In the cooling, the cooling rate at a position 1.0 mm deep from the surface of the steel sheet is 150 ° C./s or less, and the cooling rate at a position 1/2 of the thickness of the steel plate is 10 ° C./s or more. manufacturing method.
    CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn] Expression (1)
    However, [X] indicates the content (mass%) of the X element in the steel.
  5. 　前記鋼素材の成分組成が、さらに、質量％で、
    　Ｎｉ：０．０１～２．００％、
    　Ｍｏ：０．０１～０．５０％および
    　Ｗ：０．０１～１．００％
    のうちから選ばれる１種以上を含有する、請求項４に記載の鋼板の製造方法。     The chemical composition of the steel material is further, in mass%,
    Ni: 0.01 to 2.00%,
    Mo: 0.01-0.50% and W: 0.01-1.00%
    The method for producing a steel sheet according to claim 4, containing one or more selected from.
  6. 　前記鋼素材の成分組成が、さらに、質量％で、
    　Ｖ：０．０１～１．００％、
    　Ｔｉ：０．００５～０．１００％、
    　Ｃｏ：０．０１～１．００％、
    　Ｎｂ：０．００５～０．１００％、
    　Ｂ：０．０００１～０．０１００％、
    　Ｃａ：０．０００５～０．０２００％、
    　Ｍｇ：０．０００５～０．０２００％および
    　ＲＥＭ：０．０００５～０．０２００％
    のうちから選ばれる１種以上を含有する、請求項４または請求項５に記載の鋼板の製造方法。     The chemical composition of the steel material is further, in mass%,
    V: 0.01 to 1.00%,
    Ti: 0.005 to 0.100%,
    Co: 0.01 to 1.00%,
    Nb: 0.005 to 0.100%,
    B: 0.0001 to 0.0100%,
    Ca: 0.0005 to 0.0200%,
    Mg: 0.0005-0.0200% and REM: 0.0005-0.0200%
    The method for producing a steel sheet according to claim 4 or 5, containing one or more selected from.
PCT/JP2023/001049 2022-02-24 2023-01-16 Steel sheet and method for producing same WO2023162507A1 (en)

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5530062B2 (en) * 1973-12-04 1980-08-08
JP2011105963A (en) * 2009-11-12 2011-06-02 Nippon Steel Corp Method for manufacturing low yield ratio high tensile strength steel plate excellent in low temperature toughness
JP2019214751A (en) * 2018-06-12 2019-12-19 日本製鉄株式会社 Low-yield-ratio thick steel plate
JP2020012168A (en) * 2018-07-19 2020-01-23 日本製鉄株式会社 Thick steel sheet for sour resistant linepipe, and manufacturing method therefor
WO2021106368A1 (en) * 2019-11-27 2021-06-03 Ｊｆｅスチール株式会社 Steel sheet and method for producing same

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5530062B2 (en) * 1973-12-04 1980-08-08
JP2011105963A (en) * 2009-11-12 2011-06-02 Nippon Steel Corp Method for manufacturing low yield ratio high tensile strength steel plate excellent in low temperature toughness
JP2019214751A (en) * 2018-06-12 2019-12-19 日本製鉄株式会社 Low-yield-ratio thick steel plate
JP2020012168A (en) * 2018-07-19 2020-01-23 日本製鉄株式会社 Thick steel sheet for sour resistant linepipe, and manufacturing method therefor
WO2021106368A1 (en) * 2019-11-27 2021-06-03 Ｊｆｅスチール株式会社 Steel sheet and method for producing same

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