WO2022168686A1 - 鋼材およびその製造方法、タンクおよびその製造方法 - Google Patents
鋼材およびその製造方法、タンクおよびその製造方法 Download PDFInfo
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- WO2022168686A1 WO2022168686A1 PCT/JP2022/002735 JP2022002735W WO2022168686A1 WO 2022168686 A1 WO2022168686 A1 WO 2022168686A1 JP 2022002735 W JP2022002735 W JP 2022002735W WO 2022168686 A1 WO2022168686 A1 WO 2022168686A1
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 173
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- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D51/00—Making hollow objects
- B21D51/16—Making hollow objects characterised by the use of the objects
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- 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
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- 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
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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- C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
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- C—CHEMISTRY; METALLURGY
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- 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
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- C—CHEMISTRY; METALLURGY
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C—CHEMISTRY; METALLURGY
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- C22C—ALLOYS
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
- F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
- F17C3/00—Vessels not under pressure
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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
- C21D2211/004—Dispersions; Precipitations
Definitions
- the present invention relates to a steel material suitable for structural steel used in an extremely low temperature environment, such as a tank for storing liquid hydrogen, liquid helium, liquefied gas, etc., and a method for manufacturing the same.
- the present invention also relates to a tank using this steel material and a method for manufacturing the same.
- hot-rolled steel sheets In order to use hot-rolled steel sheets as materials for storage tanks for liquid hydrogen, liquid helium, and liquefied gas, it is required that hot-rolled steel sheets have excellent toughness at low temperatures because the operating environment is extremely cold. be done. For example, when a hot-rolled steel plate is used for a liquid helium storage tank, it is necessary that the boiling point of helium is ⁇ 269° C. or lower and excellent toughness is ensured. If the low-temperature toughness of the steel material is inferior, it may become impossible to maintain the safety of the structure for cryogenic storage tanks.
- Patent Document 1 proposes a technique for ensuring low-temperature toughness in the weld heat-affected zone by controlling the crystal grain size, carbide coverage, and the like.
- liquefied gas storage tank structures are manufactured by linearly heating steel materials.
- Linear heating is a processing method that uses plastic deformation due to local thermal stress to form a curved surface.
- the carbon equivalent (Ceq) in shipbuilding is for high-strength steel with Ceq > 0.38%, and the surface when water cooling immediately after heating.
- the maximum heating temperature of is 650° C. or less. If the maximum temperature is exceeded, the maximum surface heating temperature should be 900°C or less, and water cooling should be performed after air cooling to 500°C.
- carbides are formed after linear heating, the low-temperature toughness is lowered, but Patent Document 1 does not verify the low-temperature toughness after linear heating.
- the present invention has been made in view of the above problems, and an object of the present invention is to provide a steel material having excellent low-temperature toughness after linear heating, a method for manufacturing the same, a tank using the steel material, and a method for manufacturing the same. do.
- the above-mentioned "excellent low-temperature toughness after linear heating” means that in a tank obtained by subjecting a steel material to linear heating treatment described later, 1 mm below the steel material surface in the linear heating part (from the steel material surface in the plate thickness direction) 1 mm position) at -269°C or higher, the absorbed energy in the Charpy impact test is 41 J or higher.
- the above-mentioned “linearly heated portion” refers to a heat-affected area after linearly heating the steel material. The absorbed energy of the linear heating portion in the Charpy impact test can be measured by the method described in Examples below.
- high Mn steel material refers to steel material having a Mn content of 20 to 40% by mass.
- the maximum grain size at the time of steel production is less than 200 ⁇ m.
- the maximum grain size is less than 180 ⁇ m.
- a high Mn austenitic steel contains a large amount of C, and therefore has more carbides than stainless steel. Furthermore, since carbides are formed at grain boundaries, the grain boundary strength is lowered. If the C concentration in the grain boundary after linear heating of the high-Mn steel material is less than 0.100%, the grain boundary becomes the starting point of fracture, resulting in deterioration of low-temperature toughness. Therefore, in order to suppress the deterioration of the low-temperature toughness after linearly heating the high-Mn steel, it is effective to increase the C concentration in the grain boundaries of the high-Mn steel. For this purpose, it is effective to set the maximum crystal grain size to less than 200 ⁇ m in the high-Mn steel used as the raw material.
- hot rolling at the time of steel production, after rolling is performed at 950 ° C. or higher with a total rolling reduction of 40% or more, hot rolling is performed one or more times at less than 950 ° C., and the finish rolling end temperature is 750 ° C. or higher.
- the above a and b can be realized by carrying out under the conditions of .
- the present invention was made by further studying the above knowledge, and the gist thereof is as follows. [1] in % by mass, C: 0.200% or more and 0.700% or less, Si: 0.05% or more and 1.00% or less, Mn: 20.0% or more and 40.0% or less, P: 0.030% or less, S: 0.0050% or less, Al: 5.00% or less, Cr: 7.0% or less, N: 0.0500% or less, O: 0.0050% or less, Ti: less than 0.005%, Nb: contains less than 0.005%, Contains one or more selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less, having a component composition with the balance being iron and inevitable impurities, A steel material in which the microstructure has a maximum grain size of less than 200 ⁇ m at a position 1 mm below the surface of the steel material.
- the component composition further includes, in % by mass, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less, W: The steel material according to [1] above, containing one or more selected from 2.0% or less.
- [5] A method for manufacturing a steel material according to any one of [1] to [4], heating the steel material having the above chemical composition to a temperature range of 1100° C. or higher and 1300° C. or lower; Hot rolling is performed under the following conditions: total rolling reduction at 950°C or higher: 40% or higher, number of hot rolling passes below 950°C: 1 or higher, and finish rolling finish temperature: 750°C or higher, A method of manufacturing a steel material, followed by cooling.
- a method for manufacturing the tank of [6] The surface of the steel material according to any one of the above [1] to [4] is heated to 900 ° C. or less, the steel material is air-cooled to a surface temperature of 500 ° C. or less, and then water-cooled.
- the steel material of the present invention is suitably used as a material for steel structures (tanks for liquefied gas storage tanks, etc.) used in low-temperature environments, and thus has excellent low-temperature toughness even after linear heating. can provide a method. Therefore, it can greatly contribute to the improvement of the safety and life of the steel structure, and has a remarkable industrial effect.
- the production method of the present invention does not cause a decrease in productivity and an increase in production costs, it is possible to provide an economical production method.
- FIG. 1 is a schematic diagram illustrating a linear heating specimen used in Examples of the present invention.
- austenitic steels for example, high-Mn steels
- high-Mn steels that are inexpensive and have excellent low-temperature toughness.
- steel structures for example, tanks
- the present inventors newly found that the C concentration at the grain boundary is caused by the decrease in absorbed energy.
- the relationship between the decrease in absorbed energy and the C concentration at grain boundaries will be described below.
- the low temperature toughness is improved by reducing the number of grain boundaries, that is, by coarsening the grains.
- C around the carbides is depleted and the grain boundary strength is lowered.
- high Mn steel has a large amount of C added, in the process of forming and growing carbides on grain boundaries, C, which has a high diffusion rate, is sufficiently supplied from inside the grains away from the grain boundaries, resulting in a healing phenomenon. do. Thereby, sharp C depletion on the grain boundary can be suppressed.
- the crystal grains become too coarse, the supply of C from within the grains cannot keep up, and the C on the grain boundaries becomes deficient.
- the present invention by setting the maximum crystal grain size to less than 200 ⁇ m in the hot rolling process described later, it is possible to ensure a C concentration of 0.100% or more even when carbides are formed, and absorb energy. Decrease can be suppressed.
- the steel material of the present invention has the chemical composition described later, and the microstructure has a maximum grain size of less than 200 ⁇ m at a position of 1 mm below the surface of the steel material.
- the C concentration at the grain boundaries can be 0.100% or more even after the steel is linearly heated.
- concentration points out that it is "mass %.”
- the chemical composition of the steel material (austenitic steel material) of the present invention will be described.
- the austenitic steel material for example, high Mn steel material
- the steel material used for its production have the above-described composition.
- the chemical composition of the austenitic steel material of the present invention and the reasons for its limitation will be described.
- the display of "%" regarding a component composition means “mass %” unless otherwise indicated.
- C 0.200% or more and 0.700% or less
- C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite.
- 0.200% or more of C is contained in order to prevent the depletion of C on the grain boundaries described above.
- the content of C is set to 0.200% or more and 0.700% or less.
- the content of C is preferably 0.250% or more, more preferably 0.300% or more.
- the C content is preferably 0.600% or less, more preferably 0.550% or less.
- Si acts as a deoxidizer and is not only necessary for steelmaking, but also dissolves in steel and has the effect of increasing the strength of the steel sheet by solid solution strengthening. .
- Si contains 0.05% or more.
- the content of Si is set to 0.05% or more and 1.00% or less.
- the Si content is preferably 0.07% or more, more preferably 0.10% or more, and still more preferably 0.15% or more.
- the Si content is preferably 0.80% or less, more preferably 0.75% or less, and still more preferably 0.70% or less.
- Mn 20.0% or more and 40.0% or less
- Mn is a relatively inexpensive austenite stabilizing element.
- Ni is an important element for achieving both strength and low temperature toughness.
- Mn contains 20.0% or more.
- the Mn content exceeds 40.0%, the low temperature toughness deteriorates.
- weldability and cuttability deteriorate.
- it promotes segregation and promotes the occurrence of stress corrosion cracking. Therefore, the content of Mn is set to 20.0% or more and 40.0% or less.
- the Mn content is preferably 23.0% or more, more preferably 23.3% or more, and still more preferably 23.5% or more.
- the Mn content is preferably 35.0% or less, more preferably 30.0% or less.
- the P content is preferably 0.002% or more.
- the P content is more preferably 0.005% or more, more preferably 0.007% or more.
- the P content is preferably 0.028% or less, more preferably 0.024% or less, and still more preferably 0.020% or less.
- S 0.0050% or less S deteriorates the low-temperature toughness and ductility of the base metal, so it is desirable to reduce it as much as possible, with the upper limit set to 0.0050%. Therefore, the S content should be 0.0050% or less.
- the content is preferably 0.0045% or less, more preferably 0.0043% or less.
- the S content is preferably 0.0010% or more.
- the S content is more preferably 0.0012% or more.
- Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process for steel sheets. In addition, the yield strength and local elongation during tensile tests are improved. In order to obtain such an effect, it is preferable to contain 0.01% or more of Al. On the other hand, if the Al content exceeds 5.00%, a large amount of inclusions will be present, degrading the low temperature toughness, so the Al content is made 5.00% or less.
- the Al content is preferably 0.01% or more, more preferably 0.02% or more.
- the Al content is preferably 4.00% or less, more preferably 3.00% or less.
- Cr 7.0% or less
- Cr is an element effective in improving low-temperature toughness because it improves grain boundary strength. In order to obtain such an effect, it is preferable to contain 0.5% or more of Cr.
- the Cr content is set to 7.0% or less.
- the Cr content is preferably 0.5% or more, more preferably 1.0% or more, and still more preferably 1.2% or more.
- the Cr content is preferably 6.7% or less, more preferably 6.5% or less. Further, in order to further improve stress corrosion cracking resistance, it is even more preferable to set the Cr content to 2.0% or more and 6.0% or less.
- N is an austenite stabilizing element and is an element effective in improving low temperature toughness. In order to obtain such an effect, it is preferable to contain 0.0050% or more of N. On the other hand, if the N content exceeds 0.0500%, the nitrides or carbonitrides may become coarse and the low temperature toughness may deteriorate. Therefore, the N content is set to 0.0500% or less.
- the N content is preferably 0.0050% or more, more preferably 0.0060% or more.
- the N content is preferably 0.0400% or less, more preferably 0.0300% or less.
- O oxygen
- oxygen oxygen
- the content of O is made in the range of 0.0050% or less.
- the content is preferably 0.0045% or less, more preferably 0.0040% or less.
- the O content is preferably 0.0010% or more.
- the O content is more preferably 0.0012% or more.
- Ti and Nb form carbonitrides with a high melting point in the steel, which lowers the low-temperature toughness. Since Ti and Nb are components that are unavoidably mixed from raw materials, etc., Ti: 0.005% or more and 0.010% or less and Nb: 0.005% or more and 0.010% or less are mixed. It is customary. Therefore, it is necessary to avoid unavoidable contamination of Ti and Nb and suppress the contents of Ti and Nb to less than 0.005%, respectively, according to the melting method described later.
- the Ti and Nb contents are each 0.003% or less, more preferably 0.002% or less.
- the content of Ti and Nb may be 0%.
- One or more selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less It is a useful element.
- the morphology control of inclusions means turning expanded inclusions into granular inclusions. Ductility, low temperature toughness and stress corrosion cracking resistance are improved through the morphology control of inclusions. In order to obtain such effects, it is preferable to contain Ca and Mg at 0.0005% or more and REM at 0.0010% or more.
- any element is included in a large amount, the amount of non-metallic inclusions increases, and ductility, low-temperature toughness, and stress corrosion cracking resistance rather decrease. Moreover, it becomes disadvantageous economically.
- Ca and Mg are contained, they are each set to 0.0100% or less, and when REM is contained, they are set to 0.0200% or less.
- Ca is 0.0005% to 0.0090%
- Mg is 0.0005% to 0.0090%
- REM is 0.0010% to 0.0180%.
- Ca is 0.0010% or more and 0.0080% or less
- Mg is 0.0010% or more and 0.0080% or less
- REM is 0.0020% or more and 0.0150% or less.
- Ca is 0.0015% or more and 0.0050% or less
- Mg is 0.0015% or more and 0.0050% or less
- REM is 0.0030% or more and 0.0100% or less.
- the balance other than the above components is iron (Fe) and unavoidable impurities.
- the unavoidable impurities here include H, B, etc., and if the total content of each element is 0.01% or less, it is permissible.
- This basic component composition provides the desired properties of the present invention.
- the following elements can be contained as necessary.
- each component of Cu, Ni, Mo, V, and W shown below can be contained as needed, these components may be 0%.
- Cu and Ni are elements that not only increase the strength of the steel sheet by solid-solution strengthening, but also improve the mobility of dislocations and improve the low-temperature toughness. .
- Cu and Ni are preferably contained in an amount of 0.01% or more.
- the content of Cu and Ni exceeds 1.0%, the surface quality deteriorates during rolling and the manufacturing cost is increased. Therefore, when these alloying elements are contained, the content thereof is preferably 1.0% or less.
- Cu and Ni are each more preferably 0.03% or more and more preferably 0.7% or less. More preferably, it is 0.5% or less.
- Mo 2.0% or less
- V 2.0% or less
- W 2.0% or less Mo
- V, and W contribute to stabilizing austenite and improving base metal strength.
- each of Mo, V and W is preferably contained in an amount of 0.001% or more.
- Mo, V and W are each more preferably 0.003% or more and more preferably 1.7% or less. It is more preferably 0.1% or more, more preferably 1.5% or less.
- Maximum crystal grain size at a position 1 mm below the surface of the steel material less than 200 ⁇ m
- the C concentration at the grain boundaries can be 0.100% or more even after the steel material is linearly heated. That is, it is possible to manufacture a steel material having excellent low-temperature toughness, such as an absorbed energy of 41 J or more in a Charpy impact test at -269°C or higher in a linearly heated portion of a structure (for example, a tank) obtained after linear heating. can.
- the maximum crystal grain size is preferably 150 ⁇ m or less, more preferably 100 ⁇ m or less, and even more preferably 80 ⁇ m or less.
- the lower limit of the maximum crystal grain size is not particularly specified.
- the maximum crystal grain size is preferably 50 ⁇ m or more, more preferably 60 ⁇ m or more, in order to ensure the toughness of the hot-rolled steel sheet (steel material).
- the crystal grains refer to grains exposed by etching. In the present invention, the maximum crystal grain size can be measured by the method described in Examples below.
- the maximum crystal grain size of the steel material can be controlled within the above numerical range by performing hot rolling according to the conditions described later. As a result, it is possible to secure the C concentration at the crystal grain boundary even after linear heating, and to realize the absorption energy described above.
- Number density of crystal grains of 50 ⁇ m or more at a position 1 mm below the surface of the steel material is the grain boundary, and cracks propagate along the grain boundaries. can be done.
- the number of austenite grains having a grain size of 50 ⁇ m or more is preferably 1.0 or more, more preferably 2.0 or more per 1 mm 2 .
- the number of austenite grains exceeds 10.0 per 1 mm 2 the strength is lowered. Therefore, it is preferably 10.0 or less, more preferably 9.0 or less, per 1 mm 2 .
- the number of austenite grains having a grain size of 50 ⁇ m or more per 1 mm 2 can be measured by the method described in Examples below. This number density can be controlled within the above numerical range by performing hot rolling, which will be described later.
- Grain size of inclusions at 1 mm below the surface of steel If coarse inclusions are present at a position of 1 mm below the surface of the steel material, stress corrosion cracking resistance is lowered. If the grain size of the top 10% inclusions in the grain size distribution of inclusions (grain size of the top 10% inclusions) exceeds 3.5 ⁇ m at a position 1 mm below the surface of the steel material, stress corrosion cracking resistance may decrease. Do you get it. For this reason, the grain size of the top 10% inclusions is preferably 3.5 ⁇ m or less, more preferably 3.0 ⁇ m or less.
- the grain size of the top 10% inclusions is preferably as small as possible, but from the viewpoint of manufacturability, it is preferably 1.5 ⁇ m or more, more preferably 2.0 ⁇ m or more.
- the "upper 10% inclusion grain size” is the grain size corresponding to the 10% position when the inclusion grain sizes are sorted in descending order in the inclusion grain size distribution.
- the grain size of inclusions can be measured by the method described in Examples below.
- steel material refers to a steel plate having a thickness of 6 mm or more. From the viewpoint of suitable use as a material for structural steel used in extremely low temperature environments, the plate thickness is preferably more than 9 mm, more preferably 12 mm or more. The upper limit of the plate thickness is not particularly limited and may be any thickness, but is preferably 40 mm or less.
- the steel material (austenitic steel material) of the present invention can be produced by melting molten steel having the chemical composition described above by a melting method such as a converter or an electric furnace. Secondary refining may also be performed in a vacuum degassing furnace.
- a steel material such as a slab with a predetermined size by a casting method such as a continuous casting method, an ingot casting-blooming rolling method, or the like.
- the steel material having the above-described chemical composition is heated to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and then rolled at a temperature of 950 ° C. or higher to a total rolling reduction of 40% or more.
- the temperature control here is based on the surface temperature of the steel material.
- the temperature "°C" indicates the surface temperature of the steel material or steel plate.
- the surface temperature can be measured, for example, with a radiation thermometer.
- the temperature at the thickness center position of the slab or steel plate can be measured, for example, by attaching a thermocouple to the thickness center of the steel plate, or by calculating the temperature distribution in the steel plate cross section by heat transfer analysis, and using the results as the steel plate can be obtained by correcting with the surface temperature of
- Heating temperature of steel material 1100° C. or higher and 1300° C. or lower
- the heating temperature of the steel material before hot rolling is set to 1100° C. or higher.
- the stability of austenite can be ensured even in the Mn negative segregation part.
- the stability of austenite can be ensured even in the linear heating portion, and brittle fracture can be prevented. That is, the absorbed energy at -269°C can be secured.
- the heating temperature exceeds 1300.degree.
- the heating temperature of the steel material is preferably 1130° C. or higher and preferably 1270° C. or lower. It is more preferably 1150° C. or higher, and more preferably 1250° C. or lower.
- Hot rolling Total rolling reduction at 950° C. or higher 40% or higher
- the total rolling reduction in the recrystallization zone is preferably 50% or more, more preferably 52% or more. Although the upper limit of the total rolling reduction in the recrystallization zone is not particularly specified, the total rolling reduction in the recrystallization zone is preferably 85% or less, more preferably 70% or less, for the purpose of ensuring strength. .
- Number of hot rolling passes at less than 950°C: 1 or more, and finish rolling temperature: 750°C or more It is important that the number of hot rolling passes is one or more. Preferably, it is twice or more. If there is no hot rolling pass below 950°C, the maximum grain size will be 200 ⁇ m or more. In addition, the number density of crystal grains of 50 ⁇ m or more exceeds 10.0/mm 2 . The upper limit of the number of hot rolling passes is not specified. From the viewpoint of manufacturability, the number of hot rolling passes is preferably 10 or less, more preferably 8 or less. When hot rolling is performed at less than 750°C, the crystal grain size becomes excessively fine and the low-temperature toughness decreases.
- the finish rolling end temperature is 775°C or less, the grain size becomes finer, and as a result, the maximum grain size may be less than 50 ⁇ m. More preferably, it is 780°C or higher.
- the upper limit of the finish rolling end temperature is not particularly specified. From the viewpoint of ensuring strength, the finishing temperature of finish rolling is preferably 930° C. or lower, more preferably 900° C. or lower.
- Cooling Cooling is performed after hot rolling is completed. No particular cooling conditions are specified. In the present invention, it is preferable to cool from a temperature of (temperature at the end of hot rolling - 100°C) or higher to 600°C or lower at an average cooling rate of 1.0°C/s or higher. This suppresses the formation of carbides and grain boundary segregation of P, and further enhances the properties of the steel material.
- the above-mentioned "temperature at the end of hot rolling” refers to the finishing temperature of finish rolling.
- the upper limit of the average cooling rate is not particularly defined. From the viewpoint of cooling stop temperature control, it is preferably 30.0° C./s or less.
- the tank of the present invention is manufactured by linearly heating the steel material described above under specific linear heating conditions to form a curved surface, and welding the steel materials processed into the curved surface.
- the tank of the present invention manufactured in this way has the same component composition and microstructure in the base metal portion as those of the above steel material (austenitic steel material).
- the C concentration at the grain boundary at a position 1 mm below the surface of the base material portion after linear heating is 0.100% or more. Grain boundary strength cannot be ensured if the C concentration in the crystal grain boundary at the above position of the base material portion after linear heating is less than 0.100%. Therefore, the C concentration at the crystal grain boundary at the above position of the base material portion after linear heating is set to 0.100% or more. It is preferably 0.200% or more, more preferably 0.250% or more. The upper limit of the C concentration in the crystal grain boundary at the above position of the base material portion after linear heating is not particularly defined. From the viewpoint of deterioration of low-temperature toughness due to excessive Cr carbide formation, the content is preferably 0.600% or less, more preferably 0.550% or less.
- the tank of the present invention manufactured in this way can absorb energy of 41 J or more in a Charpy impact test at -269°C or higher at a position 1 mm below the surface of the linear heating portion after linear heating.
- the absorbed energy in the Charpy impact test can be measured by the method described in Examples below. That is, the absorbed energy in the Charpy impact test at ⁇ 269° C. or higher in the linear heating portion is 41 J or more for the full size, and 27 J or more for the 5 mm sub-size.
- stress corrosion cracking resistance can also be provided.
- the tank of the present invention is manufactured by linearly heating the above steel material under the following conditions to form a curved surface, and welding the steel material processed into the curved surface. Since the manufacturing method of the steel material (austenitic steel material), which is the raw material, has already been described, it will be omitted. Here, preferred linear heating conditions and welding conditions will be described.
- the steel material is linearly heated under the condition that the target heating temperature (heating target temperature) of the surface of the steel material is 900° C. or less. After heating, the steel material is air-cooled to a surface temperature of 500° C. or less, and then water-cooled.
- the above linear heat treatment of heating and air cooling may be performed once, or may be repeated (repeated) one or more times. The number of times of repetition is preferably one or more in order to change the microstructure. The number of iterations is preferably 5 or less because the local thermal cycle history becomes complicated.
- the heating temperature is preferably above 800°C.
- a steel slab with the chemical composition shown in Table 1 was produced by the converter - ladle refining - continuous casting method.
- "-" shown in Table 1 indicates that the element is not intentionally added, and means that not only the case where the element is not contained (0%) but also the case where the element is unavoidably contained is included. do.
- the obtained steel slabs were hot rolled under the conditions shown in Table 2-1 and then cooled to produce steel materials (hot rolled steel sheets) having a thickness of 6 to 40 mm.
- the crystal grain size and inclusion grain size were evaluated in the following manner.
- the obtained steel sheets were linearly heated, and the steel sheets after the linear heating were evaluated for C concentration, low-temperature toughness, and stress corrosion cracking resistance in the following manner.
- the above linear heating will be described.
- plate linear heating shown in FIG. 1 was performed.
- a linear heating test specimen with a length of 1000 mm and a width of 500 mm was prepared from the obtained steel plate, and the specimen was fixed with a restraining plate at a half position in the width direction (rolling direction).
- sheet wire heating was performed under the following conditions.
- the target heating temperature of the surface of the steel material was set to 900° C., the steel material was heated to that temperature, air-cooled to a surface temperature of the steel material of 500° C. or less, and then water-cooled. Linear heating of the same area was repeated under the conditions shown in Table 2-2. Welding of the steel sheets after the linear heat treatment was performed using a solid wire (1.2 mm in diameter) as an electrode, without preheating, in a downward position, and under the welding conditions shown in Table 2-2.
- Microstructure evaluation [Crystal grain size] The cross section in the rolling direction of the obtained hot-rolled steel sheet was polished and then etched, and then the position 1 mm below the surface of the steel sheet was photographed with an optical microscope at a magnification of 200 times. 100 crystal grains exposed by etching were randomly selected from the photographed image, and the maximum crystal grain size ( ⁇ m) at a position of 1 mm below the surface of the steel sheet was determined using the equivalent circle diameter of the crystal grains as the crystal grain size. Also, the total area of 100 crystal grains and the number of crystal grains of 50 ⁇ m or more were determined, and the number density of crystal grains of 50 ⁇ m or more per mm 2 (mm 2 /piece) was determined. Aqua regia was used as the corrosive liquid.
- a 5 mm sub-sized Charpy V-notch test piece was taken in accordance with the provisions of JIS Z 2242 (2005). Charpy impact tests were performed at -196°C and -269°C using three Charpy V-notch specimens. An average value of three absorbed energies at each temperature was obtained. In Table 2, "*1" is shown in the item of absorbed energy for the samples performed using the sub-sized Charpy V-notch test pieces. In the case of the sub-sized Charpy V-notch test piece, an average value of 27 J or more of the absorbed energy of three specimens at -269°C was judged to be excellent in low temperature toughness.
- the austenitic steel material of the present invention satisfies the aforementioned target performance of maximum grain size in the microstructure: less than 200 ⁇ m.
- the C concentration at the grain boundary which is the above-mentioned target performance: 0.100% or more
- the absorbed energy (vE -269 ) in the Charpy impact test is 41 J or more
- the 5 mm sub-size was confirmed to satisfy 27J or more.
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Abstract
Description
[1] 質量%で、
C:0.200%以上0.700%以下、
Si:0.05%以上1.00%以下、
Mn:20.0%以上40.0%以下、
P:0.030%以下、
S:0.0050%以下、
Al:5.00%以下、
Cr:7.0%以下、
N:0.0500%以下、
O:0.0050%以下、
Ti:0.005%未満、
Nb:0.005%未満を含有し、
Ca:0.0100%以下、Mg:0.0100%以下、REM:0.0200%以下から選択される1種または2種以上を含有し、
残部が鉄および不可避不純物からなる成分組成を有し、
ミクロ組織は、鋼材の表面下1mm位置の最大結晶粒径が200μm未満である、鋼材。
[2] 前記成分組成は、さらに、質量%で、
Cu:1.0%以下、
Ni:1.0%以下、
Mo:2.0%以下、
V:2.0%以下、
W:2.0%以下
から選択される1種または2種以上を含有する、前記[1]に記載の鋼材。
[3] 前記ミクロ組織は、鋼材の表面下1mm位置における、結晶粒径50μm以上の個数密度が1.0個/mm2以上である、前記[1]または[2]に記載の鋼材。
[4] 前記ミクロ組織は、鋼材の表面下1mm位置における、介在物粒径分布の上位10%の介在物粒径が3.5μm以下である、前記[1]~[3]のいずれか1つに記載の鋼材。
[5] 前記[1]~[4]のいずれか1つに記載の鋼材の製造方法であって、
前記成分組成を有する鋼素材を、1100℃以上1300℃以下の温度域に加熱し、
950℃以上での総圧下率:40%以上、950℃未満での熱間圧延パス数:1回以上、および、仕上圧延終了温度:750℃以上の条件で熱間圧延を行い、
その後、冷却を行う、鋼材の製造方法。
[6] 前記[1]~[4]のいずれか1つに記載の鋼材を溶接してなるタンクであって、
線状加熱された母材部の表面下1mm位置での結晶粒界におけるC濃度が0.100%以上であり、
線状加熱された線状加熱部の表面下1mm位置における、-269℃以上でのシャルピー衝撃試験の吸収エネルギーが41J以上である、タンク。
[7] 前記[6]のタンクの製造方法であって、
前記[1]~[4]のいずれか1つに記載の鋼材の表面を900℃以下に加熱し、該鋼材を表面温度で500℃以下まで空冷し、その後、水冷する線状加熱処理を施して曲面加工し、
次いで、曲面加工された鋼材同士を溶接する、タンクの製造方法。
[8] 前記溶接は、ソリッドワイヤを電極として用い、パス間温度:100~150℃、シールドガス:80%Ar+20%CO2の条件で行う、前記[7]に記載のタンクの製造方法。
本発明の鋼材は、後述する成分組成を有し、ミクロ組織は、鋼材の表面下1mm位置での最大結晶粒径が200μm未満とする。これにより、鋼材を線状加熱した後においても、結晶粒界におけるC濃度が0.100%以上とすることができる。なお、C濃度に関する「%」の表示は、「質量%」であることを指す。
まず、本発明の鋼材(オーステナイト鋼材)における成分組成について説明する。
本発明では、オーステナイト鋼材(例えば、高Mn鋼材)およびその製造に用いられる鋼素材が、上記した成分組成を有する。本発明のオーステナイト鋼材の成分組成とその限定理由について説明する。なお、成分組成に関する「%」の表示は、特に断らない限り「質量%」を意味する。
Cは、安価なオーステナイト安定化元素であり、オーステナイトを得るために重要な元素である。上記した粒界上のCの欠乏を防ぐために、Cは0.200%以上を含有する。一方、Cは0.700%を超えて含有すると、Cr炭化物が過度に生成され、低温靱性(線状加熱後の低温靭性)が低下する。このため、Cの含有量は0.200%以上0.700%以下とする。Cの含有量は、好ましくは0.250%以上とし、より好ましくは0.300%以上とする。また、Cの含有量は、好ましくは0.600%以下とし、より好ましくは0.550%以下とする。
Siは、脱酸材として作用し、製鋼上必要であるだけでなく、鋼に固溶して固溶強化により鋼板を高強度化する効果を有する。このような効果を得るために、Siは0.05%以上を含有する。一方、Siは1.00%を超えて含有すると、非熱的応力が過度に上昇するため、低温靱性が劣化する。このため、Siの含有量は0.05%以上1.00%以下とする。Siの含有量は、好ましくは0.07%以上とし、より好ましくは0.10%以上とし、さらに好ましくは0.15%以上とする。また、Siの含有量は、好ましくは0.80%以下とし、より好ましくは0.75%以下とし、さらに好ましくは0.70%以下とする。
Mnは、比較的安価なオーステナイト安定化元素である。本発明では、強度と低温靱性を両立するために重要な元素である。その効果を得るために、Mnは20.0%以上を含有する。一方、Mnは40.0%を超えて含有した場合、低温靱性が劣化する。また、溶接性、切断性が劣化する。さらに、偏析を助長し、応力腐食割れの発生を助長する。このため、Mnの含有量は20.0%以上40.0%以下とする。Mnの含有量は、好ましくは23.0%以上とし、より好ましくは23.3%以上とし、さらに好ましくは23.5%以上とする。Mnの含有量は、好ましくは35.0%以下とし、より好ましくは30.0%以下とする。
Pは、0.030%を超えて含有すると、過度に粒界に偏析するため、低温靱性が低下する。このため、0.030%を上限とし、可能なかぎり低減することが望ましい。したがって、Pの含有量は0.030%以下とする。尚、過度のP低減は精錬コストを高騰させ経済的に不利となるため、Pの含有量は0.002%以上とすることが望ましい。Pの含有量は、より好ましくは0.005%以上とし、より好ましくは0.007%以上とする。Pの含有量は、好ましくは0.028%以下とし、より好ましくは0.024%以下とし、さらに好ましくは0.020%以下とする。
Sは、母材の低温靭性や延性を劣化させるため、0.0050%を上限とし、可能なかぎり低減することが望ましい。したがって、Sの含有量は0.0050%以下とする。好ましくは0.0045%以下とし、より好ましくは0.0043%以下とする。尚、過度のSの低減は精錬コストを高騰させ経済的に不利となるため、Sの含有量は0.0010%以上とすることが望ましい。Sの含有量は、より好ましくは0.0012%以上とする。
Alは、脱酸剤として作用し、鋼板の溶鋼脱酸プロセスに於いて、もっとも汎用的に使われる。また、引張試験時の降伏強度および局部伸びが向上する。このような効果を得るためには、Alは0.01%以上を含有することが好ましい。一方、Alは5.00%を超えて含有すると、介在物が多量に存在し、低温靭性を劣化させるため、Alの含有量は5.00%以下とする。Alの含有量は、好ましくは0.01%以上とし、より好ましくは0.02%以上とする。Alの含有量は、好ましくは4.00%以下とし、より好ましくは3.00%以下とする。
Crは、粒界強度を向上させるため、低温靱性の向上に有効な元素である。このような効果を得るためには、Crは0.5%以上を含有することが好ましい。一方、Crは7.0%を超えて含有すると、Cr炭化物の生成により、低温靭性および耐応力腐食割れ性が低下するおそれがある。このため、Crの含有量は7.0%以下とする。Crの含有量は、好ましくは0.5%以上とし、より好ましくは1.0%以上とし、さらに好ましくは1.2%以上する。Crの含有量は、好ましくは6.7%以下とし、より好ましくは6.5%以下とする。また、耐応力腐食割れをさらに向上させるためには、Crの含有量を2.0%以上6.0%以下とすることがさらに一層好ましい。
Nは、オーステナイト安定化元素であり、低温靱性の向上に有効な元素である。このような効果を得るためには、Nは0.0050%以上を含有することが好ましい。一方、Nは0.0500%を超えて含有すると、窒化物または炭窒化物が粗大化し、低温靭性が低下するおそれがある。このため、Nの含有量は0.0500%以下とする。Nの含有量は、好ましくは0.0050%以上とし、より好ましくは0.0060%以上とする。Nの含有量は、好ましくは0.0400%以下とし、より好ましくは0.0300%以下とする。
O(酸素)は、酸化物の形成により低温靱性を劣化させる。このため、Oの含有量は0.0050%以下の範囲とする。好ましくは0.0045%以下とし、より好ましくは0.0040%以下とする。尚、過度のOの低減は精錬コストを高騰させ経済的に不利となるため、Oの含有量は0.0010%以上とすることが望ましい。Oの含有量は、より好ましくは0.0012%以上とする。
TiおよびNbは、鋼中で高融点の炭窒化物を形成するため、低温靭性が低下する。TiおよびNbは、原材料などから不可避的に混入する成分であるため、Ti:0.005%以上0.010%以下およびNb:0.005%以上0.010%以下の範囲で混入するのが通例である。そこで、後述する溶製の手法に従って、TiおよびNbの不可避混入を回避し、TiおよびNbの含有量を各々0.005%未満に抑制する必要がある。TiおよびNbの含有量を各々0.005%未満に抑制することによって、上記した炭窒化物の悪影響を排除し、優れた低温靭性並びに延性を確保することができる。好ましくは、TiおよびNbの含有量を各々0.003%以下とし、より好ましくは各々0.002%以下とする。勿論、TiおよびNbの含有量は0%であってもよい。
Ca、MgおよびREM(希土類金属)は、介在物の形態制御に有用な元素である。介在物の形態制御とは、展伸した介在物を粒状の介在物とすることをいう。この介在物の形態制御を介して、延性、低温靭性および耐応力腐食割れ性を向上させる。このような効果を得るためには、CaおよびMgは0.0005%以上、REMは0.0010%以上含有することが好ましい。一方、いずれの元素も多く含有させると、非金属介在物量が増加し、かえって延性、低温靭性、耐応力腐食割れ性が低下する。また、経済的に不利になる。
なお、下記に示すCu、Ni、Mo、V、Wの各成分は、必要に応じて含有できるので、これらの成分は0%であってもよい。
Cu:1.0%以下、Ni:1.0%以下
CuおよびNiは、固溶強化により鋼板を高強度化するだけでなく、転位の易動度を向上させ、低温靱性も向上する元素である。このような効果を得るためには、CuおよびNiは0.01%以上で含有することが好ましい。一方、CuおよびNiは1.0%を超えて含有すると、圧延時に表面性状が劣化する他、製造コストを圧迫する。このため、これらの合金元素を含有する場合は、その含有量は各々1.0%以下とすることが好ましい。CuおよびNiは、それぞれ、より好ましくは0.03%以上とし、より好ましくは0.7%以下とする。さらに好ましくは0.5%以下とする。
Mo、VおよびWは、オーステナイトの安定化に寄与するとともに母材強度の向上に寄与する。このような効果を得るためには、Mo、VおよびWは、各々0.001%以上を含有することが好ましい。一方、Mo、VおよびWは、各々2.0%を超えて含有すると、粗大な炭窒化物が生成し、破壊の起点となることがある他、製造コストを圧迫する。このため、これらの合金元素を含有する場合は、その含有量は各々2.0%以下とすることが好ましい。Mo、VおよびWは、それぞれ、より好ましくは0.003%以上とし、より好ましくは1.7%以下とする。さらに好ましくは0.1%以上とし、さらに好ましくは1.5%以下とする。
次に、本発明においてミクロ組織を上記のように限定した理由を説明する。
上述したように、鋼材(母材)の結晶粒径が粗大である場合、炭化物形成時にCが欠乏してしまう。鋼材の表面下1mm位置の最大結晶粒径を200μm未満にすることで、鋼材を線状加熱した後においても結晶粒界におけるC濃度を0.100%以上とすることができる。すなわち、線状加熱後に得られる構造物(例えば、タンク)の線状加熱部において、-269℃以上でのシャルピー衝撃試験の吸収エネルギーが41J以上と優れた低温靭性を有する鋼材を製造することができる。
高Mn鋼の破壊の起点は結晶粒界であり、き裂は粒界を伝播することから、粗大な結晶粒が存在することで、き裂の伝播を抑制し、低温靭性をさらに向上することができる。そのためには、結晶粒径が50μm以上のサイズを有するオーステナイト結晶粒の数が、1mm2あたりに1.0個以上であることが好ましく、2.0個以上であることがより好ましい。一方、上記オーステナイト結晶粒の数が、1mm2あたり10.0個を超える場合、強度が低下する。そのため、1mm2あたりに10.0個以下が好ましく、9.0個以下であることがより好ましい。
本発明では、上記の結晶粒径50μm以上のオーステナイト結晶粒の1mm2あたりの個数(個数密度)は、後述する実施例に記載の方法で測定できる。この個数密度は、後述する熱間圧延を行うことによって上記の数値範囲に制御することができる。
鋼材の表面下1mm位置に粗大な介在物が存在していた場合、耐応力腐食割れ性が低下する。鋼材の表面下1mm位置における、介在物粒径分布の上位10%の介在物粒径(上位10%介在物粒径)が3.5μmを超えた場合、耐応力腐食割れ性が低下することが分かった。このことから、上記の上位10%介在物粒径を3.5μm以下とすることが好ましく、3.0μm以下とすることがより好ましい。一方、上記の上位10%介在物粒径は小さいほど好ましいが、製造性の観点から、好ましくは1.5μm以上とし、より好ましくは2.0μm以上とする。
ここで、「上位10%介在物粒径」とは、介在物粒径の分布において、介在物粒径を大きい順に整理した際、10%位置にあたる粒径である。本発明では、上記の介在物粒径は、後述する実施例に記載の方法で測定できる。
次に、本発明の一実施形態における鋼材の製造方法について説明する。
熱間圧延にてMnを拡散させるために、熱間圧延前の鋼素材の加熱温度は1100℃以上とする。Mnを拡散させることで、Mn負偏析部においてもオーステナイトの安定度を確保することができる。これにより、線状加熱部においてもオーステナイトの安定度を確保することができ、脆性破壊を防ぐことができる。すなわち、-269℃での吸収エネルギーを確保できる。一方、加熱温度が1300℃を超えると鋼の溶解が始まってしまう懸念があるため、加熱温度の上限は1300℃とする。鋼素材の加熱温度は、好ましくは1130℃以上であり、好ましくは1270℃以下である。より好ましくは1150℃以上であり、より好ましくは1250℃以下である。
950℃以上での総圧下率:40%以上
上述のように、本発明では、鋼材の表面下1mm位置の最大結晶粒径を200μm未満にすることが重要である。再結晶域の圧延で等軸粒にすることができなければ、その後の未再結晶域の圧延においても粗大粒として残存してしまい、最大結晶粒径が200μm以上になる。そのうえ、結晶粒径50μm以上の個数密度が10.0個/mm2を超える。そのため、再結晶域である950℃以上の温度域で総圧下率を40%以上確保することが有効である。再結晶域での総圧下率は、好ましくは50%以上であり、より好ましくは52%以上である。再結晶域での総圧下率の上限は特に規定しないが、強度確保の理由から、再結晶域での総圧下率は、85%以下とすることが好ましく、70%以下とすることがより好ましい。
950℃以上での熱間圧延で形成した等軸粒を微細にするために、950℃未満での熱間圧延パス数を1回以上することが重要である。好ましくは、2回以上である。950℃未満での熱間圧延パスがない場合、最大結晶粒径が200μm以上になる。そのうえ、結晶粒径50μm以上の個数密度が10.0個/mm2を超える。この熱間圧延パス数の上限は特に規定しない。製造性の観点から、この熱間圧延パス数は、10回以下とすることが好ましく、8回以下がより好ましい。750℃未満で熱間圧延を行った場合、結晶粒径が過度に微細になり、低温靱性が低下するため、仕上圧延終了温度は750℃以上とする。仕上圧延終了温度が775℃以下の場合、結晶粒径が微細になり、その結果、最大結晶粒径が50μm未満となる場合があるため、仕上圧延終了温度は775℃超えとすることが好ましく、より好ましくは780℃以上である。仕上圧延終了温度の上限は特に規定しない。強度確保の観点から、仕上圧延終了温度は、930℃以下とすることが好ましく、900℃以下とすることがより好ましい。
熱間圧延が終了した後、冷却を行う。冷却条件は特に規定しない。本発明では、(熱間圧延終了時の温度-100℃)以上の温度から、1.0℃/s以上の平均冷却速度で600℃以下まで冷却することが好ましい。これにより、炭化物生成およびPの粒界偏析を抑制し、鋼材の特性がより高められる。上記の「熱間圧延終了時の温度」とは、仕上圧延終了温度を指す。
なお、上記平均冷却速度の上限は特に規定しない。冷却停止温度制御の観点から、30.0℃/s以下とすることが好ましい。
また、本発明によれば、耐応力腐食割れ性も備えることができる。
鋼材に対して、鋼材の表面の加熱温度の目標(加熱目標温度)を900℃以下の条件で線状加熱する。加熱後、該鋼材を表面温度で500℃以下まで空冷し、その後、水冷する。上記の加熱および空冷する線状加熱処理は、1回でもよく、あるいは1回以上繰り返しても(反復しても)よい。反復回数は、ミクロ組織を変化させるため、1回以上が好ましい。反復回数は、局所的な熱サイクル履歴が複雑となるため、5回以下が好ましい。上記加熱温度は、800℃超えとすることが好ましい。
高強度および高延性で優れた極低温衝撃靭性を確保する観点から、溶接は、ソリッドワイヤ(直径1.2mm)を電極として用いて、予熱なし、下向き姿勢で、パス間温度:100~150℃、シールドガス:80%Ar+20%CO2、の条件で実施する。なお、シールドガスに関する「%」の表示は、「体積%」であることを指す。
ここで、上記の線状加熱について説明する。線状加熱として、図1に示す板線状加熱を行った。図1に示すように、得られた鋼板から、縦が1000mm、横が500mmの線状加熱試験体を作製し、幅方向(圧延方向)の1/2位置で該試験体を拘束板で固定し、次の条件で板線状加熱を行った。条件は、鋼材の表面の加熱温度目標を900℃とし、該温度に加熱し、鋼材の表面温度で500℃以下まで空冷し、その後、水冷するものとした。同一領域の線状加熱は、表2-2に示す条件で反復した。
また、線状加熱処理後の鋼板同士の溶接は、ソリッドワイヤ(直径1.2mm)を電極として用い、予熱なし、下向き姿勢とし、かつ、表2-2に示す溶接条件で行った。
[結晶粒径]
得られた熱間圧延鋼板について、圧延方向断面を研磨した後、エッチングし、次いで、鋼板表面下1mm位置を光学顕微鏡を用いて200倍の倍率で撮影した。撮影した画像から無作為にエッチングで現出した100個の結晶粒を選び、結晶粒の円相当径を結晶粒径とし、鋼板表面下1mm位置の最大結晶粒径(μm)を求めた。また、100個の結晶粒の総面積と50μm以上の結晶粒の個数を求め、1mm2当たりの結晶粒径50μm以上の個数密度(mm2/個)を求めた。なお、腐食液には王水を用いた。
得られた熱間圧延鋼板について、SEM(走査電子顕微鏡)を用いて介在物粒径を調べた。評価領域は200mm2とし、鋼板表面下1mm位置の上位10%介在物粒径(μm)を求めた。
得られた熱間圧延鋼板に線状加熱を行った後の鋼板から、12mm×10mmのTEMサンプルを作製した。該サンプルについて、TEM(透過電子顕微鏡)に付属のEDS検出器を用いて、炭化物の無い粒界を横断して組成分析を行い、得られたC濃度を評価した。鋼板の表面下1mm位置を観察対象とした。分析は、10個の粒界について行い、その平均値を求めた。
線状加熱部の低温靭性の評価は、以下の通り行った。
得られた熱間圧延鋼板から、図1に示す線状加熱試験体を作製し、該試験体に上述の条件で板線状加熱を行った後の鋼板を用いて、線状加熱部の低温靭性の評価を行った。板厚が10mm以上の線状加熱部から、JIS Z 2242(2005年)の規定に準拠してシャルピーVノッチ試験片(フルサイズのシャルピーVノッチ試験片)を採取した。3本のシャルピーVノッチ試験片を用いて、シャルピー衝撃試験を-196℃および-269℃で実施した。各温度での3本の吸収エネルギーの平均値を求めた。本実施例では、フルサイズのシャルピーVノッチ試験片の場合、-269℃での3本の吸収エネルギーの平均値が41J以上を低温靭性に優れると判定した。
耐応力腐食割れ性の評価は、ASTM G36に基づき応力腐食割れ試験を行った。板厚2.5mm、幅20mm、長さ80mmとなるサイズの試験片を、得られた熱間圧延鋼板の表面下1mm位置から採取した。溶液は沸騰塩化MgCl2とし、曲げ半径は5mmとした。上記溶液に応力を付与した試験片を400時間浸漬した後、割れの発生有無を確認した。割れの発生が無かった場合を表2-2に示す「〇(合格)」、割れの発生が有った場合を表2-2に示す「×(不合格)」と評価した。
Claims (8)
- 質量%で、
C:0.200%以上0.700%以下、
Si:0.05%以上1.00%以下、
Mn:20.0%以上40.0%以下、
P:0.030%以下、
S:0.0050%以下、
Al:5.00%以下、
Cr:7.0%以下、
N:0.0500%以下、
O:0.0050%以下、
Ti:0.005%未満、
Nb:0.005%未満を含有し、
Ca:0.0100%以下、Mg:0.0100%以下、REM:0.0200%以下から選択される1種または2種以上を含有し、
残部が鉄および不可避不純物からなる成分組成を有し、
ミクロ組織は、鋼材の表面下1mm位置の最大結晶粒径が200μm未満である、鋼材。 - 前記成分組成は、さらに、質量%で、
Cu:1.0%以下、
Ni:1.0%以下、
Mo:2.0%以下、
V:2.0%以下、
W:2.0%以下
から選択される1種または2種以上を含有する、請求項1に記載の鋼材。 - 前記ミクロ組織は、鋼材の表面下1mm位置における、結晶粒径50μm以上の個数密度が1.0個/mm2以上である、請求項1または2に記載の鋼材。
- 前記ミクロ組織は、鋼材の表面下1mm位置における、介在物粒径分布の上位10%の介在物粒径が3.5μm以下である、請求項1~3のいずれか1項に記載の鋼材。
- 請求項1~4のいずれか1項に記載の鋼材の製造方法であって、
前記成分組成を有する鋼素材を、1100℃以上1300℃以下の温度域に加熱し、
950℃以上での総圧下率:40%以上、950℃未満での熱間圧延パス数:1回以上、および、仕上圧延終了温度:750℃以上の条件で熱間圧延を行い、
その後、冷却を行う、鋼材の製造方法。 - 請求項1~4のいずれか1項に記載の鋼材を溶接してなるタンクであって、
線状加熱された母材部の表面下1mm位置での結晶粒界におけるC濃度が0.100%以上であり、
線状加熱された線状加熱部の表面下1mm位置における、-269℃以上でのシャルピー衝撃試験の吸収エネルギーが41J以上である、タンク。 - 請求項6のタンクの製造方法であって、
請求項1~4のいずれか1項に記載の鋼材の表面を900℃以下に加熱し、該鋼材を表面温度で500℃以下まで空冷し、その後、水冷する線状加熱処理を施して曲面加工し、
次いで、曲面加工された鋼材同士を溶接する、タンクの製造方法。 - 前記溶接は、ソリッドワイヤを電極として用い、パス間温度:100~150℃、シールドガス:80%Ar+20%CO2の条件で行う、請求項7に記載のタンクの製造方法。
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JP2016196703A (ja) | 2015-04-02 | 2016-11-24 | 新日鐵住金株式会社 | 極低温用高Mn鋼材 |
JP2017155300A (ja) * | 2016-03-03 | 2017-09-07 | 新日鐵住金株式会社 | 低温用厚鋼板及びその製造方法 |
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JP2020537714A (ja) * | 2017-10-18 | 2020-12-24 | ポスコPosco | 表面品質に優れた低温用高マンガン鋼材及びその製造方法 |
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