Classifications

• • 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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
  • C22C38/105—Ferrous alloys, e.g. steel alloys containing cobalt containing Co and Ni
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • 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
• • 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
  • C21D2211/001—Austenite

Definitions

• the present invention relates to a steel sheet having excellent stress corrosion cracking resistance, which is suitable for structural steel used in an extremely low temperature environment, such as a tank for a liquefied gas storage tank, and a method for producing the same.
• Patent Documents 1, 2 and 3 propose a low-temperature steel sheet having a Ni content lower than 9% and having performance equal to or higher than that of a 9% Ni steel sheet.
• Ni-containing steel materials described in Patent Documents 1, 2 and 3 are excellent in low-temperature toughness, stress corrosion cracking caused by hydrogen is not mentioned, and there is still room for examination. That is, for example, in the case of a marine LNG tank, since sulfide and chloride are contained in the usage environment, there is a high possibility that stress corrosion cracking due to hydrogen will occur, so that it is durable against stress corrosion cracking. It is also required to have stress corrosion cracking resistance.
• the present invention has been made in view of the above problems, and an object of the present invention is to provide a steel sheet having excellent stress corrosion cracking resistance, which is particularly suitable for use in a low temperature environment.
• the present inventors have conducted intensive studies on the composition and structure of the steel sheet, and obtained the following findings.
• the structure up to 1 mm below the surface of the steel sheet can be oriented. It is a microstructure with a grain size of 5 ⁇ m or less surrounded by large-angle grain boundaries with a difference of 15 ° or more. Then, the microstructure disperses hydrogen trap sites, so that crack growth due to hydrogen embrittlement can be reduced.
• the present invention has been made by further studying the above findings, and the gist thereof is as follows. 1.
• C By mass% C: 0.01% or more and 0.15% or less, Si: 0.01% or more and 1.00% or less, Mn: 0.10% or more and 3.00% or less, Al: 0.002% or more and 0.100% or less, Ni: 5.0% or more and 10.0% or less, N: 0.0010% or more and 0.0080% or less, Co: More than 0% and less than 1.50%, It contains P: 0.030% or less and S: 0.0050% or less, and has a component composition of the balance Fe and unavoidable impurities.
• the average circle equivalent diameter of the crystal grains surrounded by the large angle grain boundaries with an orientation difference of 15 ° or more is 5 ⁇ m or less, and the maximum circle equivalent diameter of the retained austenite grains is 1 ⁇ m.
• composition of the components is further increased by mass%.
• Nb 0.001% or more and 0.030% or less
• V 0.01% or more and 0.10% or less
• Ti 0.003% or more and 0.050% or less
• B 0.0003% or more and 0.0100% or less
• Cu 0.01% or more and 1.00% or less
• Cr 0.01% or more and 1.50% or less
• Sn 0.01% or more and 0.50% or less
• Sb 0.01% or more and 0.50% or less
• the steel sheet according to 1 above which contains one or more selected from Mo: 0.03% or more and 1.00% or less and W: 0.05% or more and 2.00% or less.
• composition of the components is further increased by mass%.
• Ca 0.0005% or more and 0.0050% or less
• Zr 0.0005% or more and 0.0050% or less
• a method for producing a steel sheet in which a steel material having the component composition according to any one of 1 to 3 is heated, hot-rolled, and then heat-treated and then cooled, the temperature is 600 ° C. or lower in the cooling treatment.
• the present invention it is possible to provide a steel sheet having high durability against stress corrosion cracking due to hydrogen.
• a steel structure used in a low temperature environment such as a tank for a liquefied gas storage tank, the safety of the steel structure can be improved, which brings about a remarkable industrial effect.
• C 0.01% or more and 0.15% or less C is effective for increasing the strength, and in order to obtain the effect, it is necessary to contain C at 0.01% or more. On the other hand, if it is contained in excess of 0.15%, the low temperature toughness is lowered. Therefore, C is set to 0.01% or more and 0.15% or less. Preferably, it is 0.03% or more. Preferably, it is 0.10% or less.
• Si acts as an antacid and is not only necessary for steelmaking, but also has the effect of dissolving in steel and increasing the strength of the steel sheet by solid solution strengthening. .. In order to obtain this effect, it is necessary to contain Si at 0.01% or more. On the other hand, if it is contained in excess of 1.00%, the low temperature toughness deteriorates. Therefore, Si is set to 0.01% or more and 1.00% or less. Preferably, it is 0.03% or more. Preferably, it is 0.5% or less.
• Mn 0.10% or more and 3.00% or less
• Mn is an element effective for enhancing the hardenability of steel and increasing the strength of steel sheets. In order to obtain the effect, Mn needs to be contained in an amount of 0.01% or more. On the other hand, if it is contained in excess of 3.00%, the corrosion cracking resistance is lowered. Therefore, Mn is set in the range of 0.10% or more and 3.00% or less. Preferably, it is 0.20% or more. It is preferably 2.00% or less, more preferably 1.00% or less.
• Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process of steel sheets. Further, it has the effect of fixing the solid solution N in the steel to form AlN and suppressing the deterioration of toughness due to the reduction of the solid solution N. On the other hand, if it is contained in excess of 0.100%, the toughness is deteriorated, so the content is set to 0.100% or less. Preferably, it is 0.010% or more. Preferably, it is 0.070% or less. More preferably, it is 0.020% or more. More preferably, it is 0.060% or less.
• Ni 5.0% or more and 10.0% or less Ni is an extremely effective element for improving the low temperature toughness of steel sheets.
• the Ni content is set to 10.0% or less.
• the Ni content is set to 5.0% or more and 10.0% or less.
• it is 9.5% or less.
• it is 6.0% or more.
• N 0.0010% or more and 0.0080% or less N forms a precipitate in steel, and when the content exceeds 0.0080%, when the steel plate is welded to form a welded structure, the base material And it causes a decrease in toughness of the weld heat affected zone.
• N is also an element that contributes to the refinement of the base material by forming AlN, and such an effect can be obtained by setting the N content to 0.0010% or more. Therefore, the N content is set to 0.0010% or more and 0.0080% or less. It is preferably 0.0020% or more. More preferably, it is 0.0060% or less.
• Co More than 0% and less than 1.50% Co is an important element that concentrates on the surface layer of steel sheets in a corrosive environment and contributes to suppressing corrosion cracking by reducing the invasion of hydrogen. Therefore, it needs to be contained in excess of 0%.
• the amount of Co is preferably 0.05% or more, more preferably 0.1% or more. However, even if it is contained in excess of 1.50%, the effect is saturated and Co is an expensive element, so the maximum addition amount is 1.50%.
• P 0.030% or less
• the content of P is preferably 0.025% or less, and more preferably 0.020% or less. It goes without saying that the content of P may be 0%, but since removing P requires a high cost, it is preferably 0.002% or more from the viewpoint of cost.
• S 0.0050% or less S forms MnS in steel and significantly deteriorates low temperature toughness. Therefore, it is desirable to limit S to 0.0050% and reduce it as much as possible. It is preferably 0.0020% or less. It goes without saying that the S content may be 0%, but since removing S requires a high cost, it is preferably 0.0005% or more from the viewpoint of cost.
• Nb 0.001% or more and 0.030% or less
• Nb is an element effective for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to add Nb in an amount of 0.001% or more. On the other hand, if it is contained in excess of 0.030%, coarse carbonitride may be precipitated and the toughness of the base metal may be deteriorated. Therefore, when Nb is contained, it is set to 0.001% or more and 0.030% or less. It is preferably 0.005% or more, more preferably 0.007% or more. It is preferably 0.025% or less, more preferably 0.022% or less.
• V 0.01% or more and 0.10% or less
• V is an element effective for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to add V at 0.01% or more.
• V is set to 0.01% or more and 0.10% or less. It is preferably 0.02% or more, more preferably 0.03% or more. It is preferably 0.09% or less, more preferably 0.08% or less.
• Ti 0.003% or more and 0.050% or less
• Ti is an element that precipitates as a nitride or carbonitride and is effective for improving the strength of a steel sheet. In order to obtain such an effect, it is preferable to add Ti at 0.003% or more. On the other hand, if it is contained in excess of 0.050%, the precipitate may become coarse and the toughness of the base metal may be deteriorated. In addition, coarse carbonitride may precipitate and serve as a starting point for fracture. Therefore, when Ti is contained, it is set to 0.003% or more and 0.050% or less. It is preferably 0.005% or more, more preferably 0.007% or more. It is preferably 0.035% or less, more preferably 0.032% or less.
• B 0.0003% or more and 0.0100% or less B is an element effective for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to add B at 0.0003% or more. On the other hand, if it is contained in excess of 0.0100%, coarse B precipitates may be formed and the toughness may be lowered. Therefore, B is in the range of 0.0003% or more and 0.0100% or less. Preferably, it is 0.0030% or less.
• Cu 0.01% or more and 1.00% or less
• Cu is an element effective for increasing the strength of a steel sheet by improving hardenability, but when the content exceeds 1.00%, the low temperature toughness of the steel sheet decreases. There is a risk of Therefore, when Cu is contained, the content thereof is preferably 1.00% or less. On the other hand, if it is less than 0.01%, the effect of increasing the strength cannot be obtained. Therefore, when it is added, it is preferably 0.01% or more. More preferably, it is 0.10% or more and 0.30% or less.
• Cr 0.01% or more and 1.50% or less Cr is an element that contributes to the improvement of low temperature toughness and corrosion resistance of high Mn steel.
• the amount of Cr is preferably 0.01% or more.
• Cr may precipitate in the form of nitrides, carbides, carbonitrides, etc. during rolling, and the formation of such precipitates causes corrosion and fracture to reduce low temperature toughness. It is preferably 1.50%. More preferably, it is 1.00% or less.
• Mo 0.03% or more and 1.00% or less
• Mo is an element effective for suppressing the temper embrittlement sensitivity of a steel sheet, and is also an element for increasing the strength of a steel sheet without impairing low temperature toughness.
• the Mo content is preferably 0.03% or more.
• Mo exceeds 1.00%, the low temperature toughness may decrease. Therefore, when Mo is contained, the content is set to 0.03% or more and 1.00% or less. More preferably, it is more than 0.05% and 0.30% or less.
• Sn 0.01% or more and 0.50% or less
• Sb 0.01% or more and 0.50% or less
• W 0.05% or more and 2.00% or less
• Sn, Sb and W are elements effective for improving corrosion resistance. is there. These effects are exhibited when Sn and Sb are 0.01% or more and W is 0.05% or more.
• the Sn amount is in the range of 0.01% or more and 0.50% or less
• the Sb amount is in the range of 0.01% or more and 0.50% or less
• the W amount is in the range of 0.05% or more and 2.00% or less.
• the Sn amount is 0.02% or more and 0.25% or less
• the Sb amount is 0.02% or more and 0.25% or less
• the W amount is 0.10% or more and 1.00% or less.
• the following elements can be contained, if necessary.
• the following Ca, Zr, Mg and REM are elements useful for morphological control of inclusions such as MnS, and can be added as needed.
• the morphological control of inclusions means that the expanded sulfide-based inclusions are made into granular inclusions. Toughness and sulfide stress corrosion cracking resistance are improved through morphological control of the inclusions.
• Ca, Zr and Mg are contained in an amount of 0.0005% or more and REM is contained in an amount of 0.0010% or more.
• the content is 0.0005% or more and 0.0050% or less, respectively, and when REM is contained, the content is 0.0010% or more and 0.0100% or less.
• the Ca amount is 0.0010% or more and 0.0040% or less
• the Zr amount is 0.0010% or more and 0.0040% or less
• the Mg amount is 0.0010% or more and 0.0040% or less
• the REM amount is 0. It shall be 0020% or more and 0.0100% or less.
• the average circle equivalent diameter of the crystal grains surrounded by the large angle grain boundaries having an orientation difference of 15 ° or more is 5 ⁇ m or less.
• the maximum circular equivalent diameter of the retained austenite grains is 1 ⁇ m or less.
• the average equivalent circle diameter of the crystal grains is preferably 4 ⁇ m or less, more preferably 3 ⁇ m or less.
• the crystal grains surrounded by large-angle grain boundaries with an orientation difference of 15 ° or more and the average circle-equivalent diameter of the crystal grains can be specified by the measurement method in the examples described later.
• the temperature is determined after hot rolling or, if heat treatment is performed after hot rolling, after the heat treatment. Perform cooling treatment with an average cooling rate of 1 ° C./s or higher in the region.
• the maximum circle-equivalent diameter of the retained austenite grains needs to be 1 ⁇ m or less. This is because by setting the diameter corresponding to the maximum circle to 1 ⁇ m or less, the hydrogen traps on the retained austenite are dispersed and the local concentration of the hydrogen traps is avoided, and as a result, the crack growth due to hydrogen embrittlement can be reduced.
• the amount of retained austenite in the surface layer structure is preferably 15% or less in terms of area ratio, and more preferably 10% or less.
• the structure of the steel sheet is preferably martensite and / or bainite. At that time, the area ratio of martensite and / or bainite is preferably 80% or more.
• the conditions for manufacturing the steel sheet of the present invention will be described. That is, it can be produced by heating a steel material having the above-mentioned composition and cooling it by hot rolling, or by further heat-treating it after hot rolling and cooling it. At that time, in the cooling after hot rolling, or when the heat treatment is performed after the hot rolling, the average cooling rate in a predetermined temperature range is set to 1 ° C./s or more in the cooling after the heat treatment. Needed to get.
• the manufacturing conditions will be described in the order of steps. In the following description, the temperature (° C.) means the temperature at the center of the plate thickness.
• the reheating temperature of the steel material in hot rolling is preferably 1000 ° C. or higher and 1300 ° C. or lower.
• Heating temperature of steel material 1000 ° C or higher and 1300 ° C or lower.
• the heating temperature is preferably 1000 ° C. or higher and 1300 ° C. or lower. .. That is, if the heating temperature is less than 900 ° C., the precipitate may not be sufficiently solid-solved, so that the desired characteristics may not be obtained.
• the material may be deteriorated due to the coarsening of the crystal grain size, and excessive energy may be required for production to reduce the productivity. More preferably, it is in the range of 1050 ° C. or higher and 1250 ° C. or lower, and further preferably 1100 ° C. or higher and 1250 ° C. or lower.
• cooling treatment is performed after hot rolling.
• the average cooling rate of the surface layer structure in the temperature range of 600 ° C. or lower and 200 ° C. or higher is set to 1 ° C./s or higher. That is, when the cooling rate in this cooling treatment is less than 1 ° C./s, the surface layer structure becomes an upper bainite structure, the large-angle grain boundaries contained in the structure are reduced, the structure is not sufficiently finely divided, and stress corrosion cracking resistance is deteriorated. I can't get it.
• the upper limit of the average cooling rate does not need to be particularly limited. When the heat treatment described later is performed after the hot rolling, it is not necessary to set the cooling rate after the hot rolling to 1 ° C./s or more.
• Heat treatment after hot rolling After hot rolling, the following heat treatment may be performed without cooling. As described above, in order to preferably make the surface layer structure of the steel sheet a martensite and / or bainite structure and increase the large grain boundaries contained in the structure to ensure excellent stress corrosion cracking resistance, it is hot.
• heat treatment is performed after rolling, it is preferable to perform hot rolling by heating to Acc 3 points or more and 900 ° C. or less for quenching (primary quenching). That is, if the heating temperature is less than 3 points of Ac or exceeds 900 ° C., the equivalent circle diameter of the large-angle grain boundary becomes coarse, and there is a possibility that the desired characteristics cannot be obtained.
• the average cooling rate in the temperature range of 600 ° C. or lower and 200 ° C. or higher in the surface layer structure is set to 1 ° C./s or higher.
• the low temperature toughness of the base metal can be improved.
• the average cooling rate in the temperature range of 600 ° C. or lower and 200 ° C. or higher in the surface layer structure is set to 1 ° C./s or higher.
• the retained austenite grains in the surface layer structure In order to obtain properties such as high strength and excellent low temperature toughness, it is effective to make the retained austenite grains in the surface layer structure into fine grains having a diameter of 1 ⁇ m or less.
• steel plates (Sample Nos. 1 to 26) having a thickness of 30 to 50 mm are manufactured according to the manufacturing conditions shown in Table 2, and each sample is prepared. It was subjected to the following Charpy impact test and stress corrosion cracking test. In addition, for each sample, the spacing between large-angle grain boundaries and the residual austenite grain size in the surface structure were investigated.
• the large-angle grain boundary was defined as a grain boundary with a grain boundary orientation difference of 15 ° or more, and this was specified using EBSD. Then, the crystal grain size was measured in an arbitrary range of 500 ⁇ 500 ⁇ m at a depth of 1 mm from the surface of the steel sheet, and the average value of the equivalent circle diameters of the crystal grains surrounded by the large angle grain boundaries was obtained. The range surrounded by the large-angle grain boundaries of less than 0.1 ⁇ m was excluded from the calculation.
• the retained austenite grains existing in the same EBSD measurement region were specified from the crystal structure, and the circle-equivalent diameter of the largest of the crystal grains recognized as austenite was used.
• Sample No. according to the present invention It was confirmed that Nos. 1 to 14, 23, and 26 ensure low temperature toughness and have excellent stress corrosion cracking resistance.
• the absorbed energy is lower than 34J, or the DCB test is less than 25MPa ⁇ m%, and the above-mentioned target performance is achieved. I wasn't satisfied.

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