Classifications

• • 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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

• the present invention relates to a steel sheet that combines high strength and high ductility, and is particularly suitable as a steel sheet for cans, and a method for manufacturing the same.
• DR (double reduce) material is known as a high-strength steel sheet for cans.
• DR material is a steel sheet for cans that has been strengthened by cold rolling again after cold rolling and annealing.
• it has issues with low workability due to its small elongation, and high manufacturing costs due to the need to perform cold rolling again after annealing.
• SR single reduce
• Patent Document 1 proposes a steel plate containing, by mass%, C: 0.03% to 0.13%, Si: 0.05% or less, Mn: 0.01% to 0.6%, P: 0.025% or less, S: 0.020% or less, Al: 0.01% to 0.20%, N: 0.0001% to 0.02%, Ti: 0.005% to 0.02%, and B: 0.0005% to 0.02%, with the remainder being iron and unavoidable impurities, and containing, by area percentage, 84.0% or more ferrite, 0.5% to 10.0% martensite, and 0.1% to 10.0% bainite.
• Patent document 2 states, in mass%, C: 0.085% to 0.130%, Si: 0.04% or less, Mn: 0.10% to 0.60%, P: 0.02% or less, S: over 0.010% to 0.020%, Al: 0.02% to 0.10%, N: 0.0005% to 0.0040%, Nb: 0.007% to 0.030%, B: 0.0010% to 0.0050%.
• the proposed steel sheet for cans contains the following, has a B/N ratio (mass%) of 0.80 or more, with the balance being Fe and unavoidable impurities, has a ferrite structure containing pearlite at an area fraction of 1.0% or more, has a yield stress of 500 MPa or more, a tensile strength of 550 MPa or more, a uniform elongation of 10% or more, and a yield elongation of 5.0% or less.
• Patent Document 1 the lower limit of the upper yield stress is 400 MPa, which causes an issue that the can body strength is insufficient when the steel plate is gauged down.
• Patent Document 2 a high-strength, highly ductile steel plate with a yield stress of 500 MPa or more and a uniform elongation of 10% or more is obtained, but since a yield elongation of up to 5.0% is permitted, there is an issue that wrinkles occur due to stretcher strain. Furthermore, a certain level of Rockwell superficial hardness is required to ensure the strength when the steel plate is used in the body of a can, but neither Patent Document 1 nor Patent Document 2 describes hardness.
• the present invention aims to provide a steel plate with high strength, high ductility and low yield elongation that solves the above problems, and a manufacturing method thereof.
• the present invention has been made to solve the above problems, and the gist of the present invention is as follows.
• C 0.03% or more and 0.15% or less, Si: 0.05% or less, Mn: 0.10% or more and 0.60% or less, P: 0.025% or less, S: 0.020% or less, Al: 0.20% or less, N: 0.0001% or more and 0.0200% or less, Nb: 0.005% or more and 0.030% or less, Cu: more than 0.020% and not more than 0.200%, with the balance being Fe and unavoidable impurities;
• the microstructure has an area fraction of ferrite of 80% or more, A steel plate having a yield stress of 500 MPa or more, a tensile strength of 500 MPa or more, an HR30T of 68 or more, a breaking elongation of 15% or more, and a yield elongation of 4.5% or less.
• the present invention makes it possible to manufacture steel sheets that are high strength, high ductility and low yield point elongation.
• the present invention makes it possible to further reduce the gauge of steel sheets for cans, which allows for the weight of can bodies to be reduced.
• the low yield point elongation suppresses the occurrence of wrinkles caused by stretcher strain, making it possible to perform more complex processing on the can body.
• % means mass %.
• high strength when the tensile strength, yield stress and HR30T are excellent, it is called high strength.
• C 0.03% or more and 0.15% or less C is an element that contributes to the improvement of yield stress, tensile strength, and HR30T. If the C content is less than 0.03%, the amount of solid solution strengthening due to solid solution in ferrite and the amount of precipitation strengthening due to precipitation of fine carbides decrease, and the yield stress, tensile strength, and HR30T decrease, so the C content needs to be 0.03% or more.
• the C content is preferably 0.06% or more, and more preferably 0.08% or more.
• the C content is further preferably 0.09% or more.
• the C content is most preferably 0.10% or more.
• the C content is preferably 0.14% or less.
• the C content is more preferably 0.13% or less, and further preferably 0.12% or less.
• Si 0.05% or less Si is an element that contributes to improving yield stress and tensile strength, but if the content exceeds 0.05%, corrosion resistance decreases. Therefore, the Si content must be 0.05% or less, and the content is preferably 0.04% or less, and more preferably 0.02% or less. There is no need to particularly limit the lower limit, but in order to increase the strength of the steel sheet, the Si content is preferably 0.01% or more.
• Mn 0.10% or more and 0.60% or less Mn is an element that improves hardenability and promotes the solid solution of C in ferrite. Furthermore, it is known that Mn itself contributes to improving yield stress, tensile strength, and HR30T by solid solution strengthening. If the Mn content is less than 0.10%, sufficient yield stress, tensile strength, and HR30T cannot be obtained, so the Mn content is set to 0.10% or more. The Mn content is preferably set to 0.12% or more, and more preferably set to 0.15% or more. In order to achieve both high strength and high ductility, the Mn content is further preferably set to 0.30% or more. The Mn content is most preferably set to 0.32% or more.
• the Mn content exceeds 0.60%, the fracture elongation decreases, so the Mn content is set to 0.60% or less.
• the Mn content is preferably set to 0.58% or less.
• the Mn content is more preferably 0.57% or less, further preferably 0.56% or less, and most preferably 0.55% or less.
• the P content is set to 0.025% or less. It is preferable to set the P content to 0.020% or less. It is more preferable to set the P content to 0.019% or less, and even more preferable to set the P content to 0.018% or less.
• the lower limit is not particularly limited, since P contributes to improving the yield stress and tensile strength of the steel sheet, it is preferable to contain 0.001% or more. It is more preferable to set the P content to 0.002% or more, and even more preferable to set the P content to 0.003% or more. In order to increase the strength of the steel sheet, it is more preferable to set the P content to 0.010% or more.
• S 0.020% or less S forms sulfides such as MnS, CuS, TiS, etc. in steel and reduces ductility, so the S content is set to 0.020% or less.
• the S content is preferably set to 0.018% or less.
• the S content is more preferably set to 0.017% or less.
• the S content is further preferably set to 0.016% or less, and most preferably set to 0.015% or less.
• the lower limit is not particularly limited, in order to reduce the manufacturing load, the S content is preferably set to 0.005% or more.
• the S content is more preferably set to 0.006% or more.
• Al 0.20% or less
• Al is an element contained to remove oxygen in steel.
• it reduces the amount of solute N and reduces the yield elongation. Therefore, although there is no lower limit, it is preferable to contain 0.02% or more. It is more preferable that the Al content is 0.03% or more.
• the Al content is 0.15% or less, and more preferable that it is 0.12% or less. In order to achieve both high strength and high ductility, it is even more preferable that the Al content is 0.09% or less.
• Al here refers to the total Al content.
• N is an element that contributes to improving yield stress and tensile strength by solid solution strengthening. Therefore, the N content is set to 0.0001% or more.
• the N content is preferably set to 0.0003% or more.
• the N content is more preferably set to 0.0010% or more.
• the N content is further preferably set to 0.0011% or more, and most preferably set to 0.0012% or more.
• the N content is set to 0.0200% or less.
• the N content is preferably set to 0.0150% or less, and more preferably set to 0.0100% or less. In order to achieve both high strength and low yield elongation, the N content is more preferably set to 0.0040% or less.
• the N content is most preferably set to 0.0035% or less.
• Nb is an element that contributes to precipitation strengthening and grain refinement strengthening by forming fine NbC in steel.
• the Nb content is set to 0.005% or more.
• the Nb content is preferably set to 0.007% or more.
• the Nb content is more preferably set to 0.010% or more.
• the Nb content is more preferably set to 0.011% or more, and most preferably set to 0.012% or more.
• the Nb content is set to 0.030% or less.
• the Nb content is preferably set to 0.028% or less.
• the Nb content is more preferably 0.026% or less, further preferably 0.024% or less, and most preferably 0.022% or less.
• Cu more than 0.020% and not more than 0.200% Cu contributes to the improvement of yield stress, tensile strength and HR30T by solid solution strengthening, grain refinement strengthening and precipitation strengthening. If the Cu content is 0.020% or less, sufficient strength cannot be obtained, so the Cu content is made to be more than 0.020%.
• the Cu content is preferably 0.025% or more, and more preferably 0.030% or more. In order to achieve both high strength and high ductility, the Cu content is more preferably 0.100% or more. The Cu content is most preferably 0.105% or more. On the other hand, if the Cu content exceeds 0.200%, it may lead to a decrease in ductility and cause slab cracking due to segregation of Cu.
• the Cu content is made to be 0.200% or less.
• the Cu content is preferably 0.180% or less.
• the Cu content is more preferably 0.178% or less, further preferably 0.175% or less, and most preferably 0.172% or less.
• the steel sheet of the present invention preferably contains, in addition to the above-mentioned composition, one or more elements selected from the following. Ni: 0.15% or less, Mo: 0.05% or less, Cr: 0.10% or less, Ti: 0.02% or less, B: 0.02% or less, V: 0.02% or less
• Ni, Mo, and Cr are elements that promote the solid solution of C in ferrite by improving hardenability, and Sn is an element that contributes to the high strength of steel plate by solid solution strengthening. On the other hand, excessive inclusion of these elements reduces ductility.
• the Ni content is 0.15% or less.
• the Ni content is preferably 0.14% or less, and more preferably 0.13% or less.
• Mo is contained, the Mo content is 0.05% or less.
• the Mo content is preferably 0.04% or less, and more preferably 0.03% or less.
• the Cr content is 0.10% or less.
• the Cr content is preferably 0.08% or less, and more preferably 0.07% or less.
• the Ni content is preferably 0.02% or more, more preferably 0.05% or more, even more preferably 0.08% or more, and most preferably 0.12% or more.
• the Mo content is preferably 0.01% or more, and more preferably 0.03% or more.
• the Cr content is preferably 0.01% or more, more preferably 0.04% or more, and even more preferably 0.06% or more.
• Ti 0.02% or less
• B 0.02% or less
• Ti contributes to reducing the yield elongation by forming TiN in the steel and reducing the amount of solute N.
• TiN is generated preferentially over BN, by containing both Ti and B, sufficient solute B is ensured and hardenability is improved.
• the Ti content exceeds 0.02% and the B content exceeds 0.02%, it becomes difficult to ensure sufficient ductility.
• the Ti content is 0.02% or less.
• the Ti content is preferably 0.018% or less.
• the Ti content is more preferably 0.017% or less, and even more preferably 0.016% or less.
• the B content when B is contained, the B content is 0.02% or less.
• the B content is preferably 0.018% or less.
• the B content is more preferably 0.017% or less, and even more preferably 0.016% or less.
• the Ti content is preferably 0.005% or more, and more preferably 0.008% or more.
• the B content is preferably 0.0005% or more, more preferably 0.0015% or more, even more preferably 0.0018% or more, and most preferably 0.002% or more.
• V 0.02% or less V contributes to improving yield stress, tensile strength, and HR30T by promoting the solid solution of C in ferrite through solid solution strengthening, precipitation strengthening, and improved hardenability.
• V content exceeds 0.02%, ductility decreases with increasing strength. In order to achieve both high strength and high ductility, when V is contained, it is set to 0.02% or less.
• the V content is preferably set to 0.018% or less.
• the V content is more preferably set to 0.017% or less, and even more preferably set to 0.016% or less.
• the lower limit is not particularly limited, the V content is preferably set to 0.004% or more.
• Sn 0.020% or less
• Sn is an element that contributes to increasing the strength of steel sheet by solid solution strengthening.
• the Sn content is 0.020% or less.
• the Sn content is preferably 0.018% or less. It is more preferably 0.015% or less, and even more preferably 0.012% or less.
• the Sn content is preferably 0.001% or more. It is more preferably 0.002% or more, and even more preferably 0.003% or more.
• the steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the remainder being Fe and unavoidable impurities.
• unavoidable impurities include Ca, O, H, Co, W, Zn, Pb, As, Sb, Bi, Ca, and O.
• the ferrite is 80% or more in area fraction. If the ferrite is less than 80% in area fraction, the ductility decreases, so the ferrite is 80% or more in area fraction. In order to obtain a steel sheet with high ductility, the ferrite is preferably 85% or more in area fraction. The ferrite is more preferably 86% or more in area fraction, and even more preferably 87% or more in area fraction. Although the upper limit is not particularly limited, the ferrite is preferably 95% or less in area fraction. The ferrite is more preferably 94% or less in area fraction. It is preferable that the recrystallization of ferrite is completed, but unrecrystallized ferrite may be present.
• the unrecrystallized ferrite is preferably 30% or less in area fraction.
• the unrecrystallized ferrite is preferably 28% or less in area fraction, and more preferably 25% or less in area fraction.
• the area fraction of unrecrystallized ferrite is preferably 0.1% or more, and more preferably 0.2% or more.
• the average ferrite crystal grain size is preferably 4 ⁇ m or less in equivalent circle diameter.
• the average ferrite crystal grain size is more preferably 3.9 ⁇ m or less in equivalent circle diameter, and even more preferably 3.8 ⁇ m or less. There is no particular lower limit, but the average ferrite crystal grain size is preferably 1 ⁇ m or more in equivalent circle diameter.
• the area of unrecrystallized ferrite can also be added to the area fraction of ferrite, and the requirements of the present invention are met if the sum of the area fractions of recrystallized ferrite and unrecrystallized ferrite is 80% or more.
• the remainder other than ferrite may include cementite, pearlite, bainite, martensite, retained austenite, etc. If the area fraction of the remainder is 20% or less, the requirements of the present invention are met.
• the area fraction of the remainder may be 0%, but from the viewpoint of further increasing strength, it is preferable for the material to contain 0.1% or more of martensite. It is more preferable for the martensite to be 0.2% or more, and even more preferable for it to be 0.3% or more.
• the upper limit of martensite is preferably 20% or less.
• Yield stress 500 MPa or more, tensile strength: 500 MPa or more, HR30T: 68 or more, breaking elongation: 15% or more, yield elongation: 4.5% or less
• yield stress yield strength
• the yield stress is preferably 520 MPa or more, more preferably 530 MPa or more, and even more preferably 540 MPa or more.
• the tensile strength is preferably 550 MPa or more, more preferably 555 MPa or more, and even more preferably 560 MPa or more.
• the HR30T is preferably 70 or more, more preferably 71 or more, and even more preferably 72 or more.
• the breaking elongation needs to be 15% or more, and more preferably 16% or more.
• the breaking elongation is more preferably 17% or more, and most preferably 18% or more.
• the yield stress is preferably 700 MPa or less, more preferably 695 MPa or less, and even more preferably 690 MPa or less.
• the tensile strength is preferably 800 MPa or less, more preferably 795 MPa or less, and even more preferably 790 MPa or less.
• the HR30T is preferably 80 or less, more preferably 79 or less, and even more preferably 78 or less.
• the breaking elongation is preferably 25% or less, more preferably 24% or less, and even more preferably 23% or less.
• the yield elongation is preferably 4.4% or less, more preferably 4.3% or less, and even more preferably 3.0% or less. Although there is no particular lower limit, the yield elongation is preferably 1.0% or more, and more preferably 1.1% or more.
• the method for manufacturing steel sheet in the present invention is characterized by including a heating step in which a steel material having the above-mentioned composition is heated to 1150°C or higher, a hot rolling step in which the steel after the heating step is hot rolled under conditions of a finishing temperature of 800°C to 950°C and a coiling temperature of 450°C to 700°C and then pickled, a cold rolling step in which the hot rolled sheet after the hot rolling step is cold rolled under conditions of a rolling ratio of 80% or higher, and an annealing step in which the cold rolled sheet after the cold rolling step is held at an annealing temperature of 680°C to 780°C for 5s to 90s and then cooled to a cooling stop temperature in the temperature range of 600°C or lower.
• Heating temperature 1150°C or higher If the heating temperature in the heating step is low, coarse nitrides such as AlN may be formed, which may reduce the strength and ductility of the steel sheet, so the heating temperature is set to 1150°C or higher.
• the heating temperature is preferably set to 1170°C or higher, and more preferably set to 1200°C or higher.
• the heating temperature is further preferably set to 1210°C or higher. There is no upper limit to the heating temperature, but from the viewpoint of manufacturing costs, it is preferably set to 1300°C or lower.
• the heating temperature is more preferably set to 1290°C or lower, and further preferably set to 1270°C or lower.
• finishing temperature 800°C or more and 950°C or less If the finishing temperature in the hot rolling process exceeds 950°C, the ferrite grain size of the hot rolled sheet becomes coarse, and the ferrite grain size of the steel sheet that has undergone the subsequent process also becomes coarse, making it difficult to ensure sufficient strength. Therefore, the finishing temperature is set to 950°C or less.
• the finishing temperature is preferably set to 930°C or less, and more preferably set to 920°C or less.
• the finishing temperature is more preferably set to 900°C or less, and most preferably set to 890°C or less.
• the finishing temperature in the hot rolling process is set to 800°C or more.
• the finishing temperature is preferably set to 830°C or more, and more preferably set to 850°C or more.
• Winding temperature 450°C or more and 700°C or less
• the winding temperature exceeds 700°C
• the ferrite grain size becomes coarse and the strength of the steel sheet decreases. Furthermore, the formation of coarse alloy carbides is promoted, so that cementite does not dissolve sufficiently in the annealing process and the amount of solid solution of C in ferrite decreases, which causes a decrease in strength. Therefore, the winding temperature is set to 700°C or less.
• the winding temperature is preferably set to 650°C or less, and more preferably set to 600°C or less.
• the winding temperature is more preferably set to 595°C or less, and most preferably set to 590°C or less.
• the winding temperature is set to 450°C or more.
• the range of the winding temperature is preferably set to 470°C or more, and more preferably set to 500°C or more.
• the winding temperature is more preferably set to 510°C or more, and most preferably set to 520°C or more.
• Reduction ratio in cold rolling 80% or more Cold rolling is performed after the hot rolling process.
• the cold rolling process refines the ferrite grain size and increases the yield stress and tensile strength.
• the rolling ratio is 80% or more.
• the rolling ratio is preferably 82% or more, more preferably 85% or more.
• the rolling ratio is further preferably 86% or more, and most preferably 87% or more. There is no upper limit to the rolling ratio, but it is preferably 95% or less to ensure sufficient ductility.
• the rolling ratio is more preferably 94% or less.
• Annealing temperature 680°C or more and 780°C or less, holding time: 5s or more and 90s or less, cooling stop temperature: 600°C or less, average cooling rate: 50°C/s or more Annealing is performed after the cold rolling process.
• the annealing temperature is 680°C or more.
• the annealing temperature is preferably 685°C or more, more preferably 690°C or more, even more preferably 695°C or more, and most preferably 700°C or more.
• the annealing temperature is 780°C or less.
• the annealing temperature is preferably 760°C or less, more preferably 740°C or less.
• the annealing temperature is more preferably 735°C or less, and most preferably 730°C or less.
• the holding time at the annealing temperature is set to 5 seconds or more.
• the holding time is preferably set to 6 seconds or more, more preferably set to 7 seconds or more, even more preferably set to 8 seconds or more, and most preferably set to 9 seconds or more.
• the holding time at the annealing temperature is set to 90 seconds or less.
• the holding time is preferably set to 89 seconds or less, more preferably set to 88 seconds or less, even more preferably set to 87 seconds or less, and most preferably set to 86 seconds or less.
• the material After annealing, the material is cooled to a temperature range of 600°C or less at an average cooling rate of 50°C/s or more. If the average cooling rate is less than 50°C/s, the ferrite grain size becomes coarse and the amount of dissolved carbon in the ferrite decreases, resulting in a decrease in strength. Therefore, the average cooling rate is set to 50°C/s or more.
• the average cooling rate is preferably set to 60°C/s or more, and more preferably to 80°C/s or more.
• the average cooling rate is further preferably set to 85°C/s or more, and most preferably to 90°C/s or more.
• the average cooling rate is preferably set to 200°C/s or less.
• the average cooling rate is more preferably set to 190°C/s or less, and even more preferably to 180°C/s or less.
• the cooling stop temperature is set to 600°C or less.
• the cooling stop temperature is preferably set to 595°C or less, more preferably 590°C or less, and even more preferably 585°C or less.
• the cooling stop temperature is preferably set to 300°C or more.
• the cooling stop temperature is preferably set to 305°C or more, more preferably 310°C or more, and even more preferably 315°C or more.
• Temper rolling may be performed after the annealing step. Since temper rolling can increase the yield stress and reduce the yield elongation, it is preferable to perform temper rolling with a reduction of 0.5% or more. In temper rolling, the reduction is preferably 0.6% or more, more preferably 0.7% or more, and most preferably 0.8% or more. On the other hand, since a large reduction in temper rolling reduces ductility, it is preferable to set the reduction to 10% or less. It is more preferable to set the reduction to 9% or less, more preferably 8% or less, and most preferably 7% or less.
• Steel slabs were obtained by melting and casting steel containing the components of steel types 1 to 29 shown in Table 1, with the remainder consisting of Fe and unavoidable impurities.
• the steel slabs obtained here were heated, hot rolled, cold rolled, and annealed under the conditions shown in Table 2, and then temper rolled with a reduction ratio of 1% to obtain steel plates No. 1 to 40.
• the steel plate structure was observed using the following procedure. After taking a test piece from the steel plate, the cross section parallel to the rolling direction was polished and etched with nital to reveal the structure, and a sample for structure observation was taken. A scanning electron microscope (SEM) was used to observe the plate at a magnification of 3000x at a position half the plate thickness in the plate thickness direction, and the structure was photographed in three randomly selected fields of view. Table 3 shows the area fraction of ferrite in the SEM image, measured using the image processing software Image-J. The area fractions shown in Table 3 are the average values for the three fields of view. The areas that could be observed as black lumps in the SEM photograph were determined to be ferrite.
• SEM scanning electron microscope
• the average grain size of ferrite was determined by the cutting method described in JIS G 0551, and was taken as the average value of three fields of view.
• JIS No. 5 tensile test pieces with the tensile direction along the rolling direction and 30 square test pieces for measuring Rockwell superficial hardness were taken from the above steel plate and aged in an incubator at 210°C for 10 minutes.
• Tensile tests were conducted on the tensile test pieces in accordance with JIS Z 2241 to evaluate the yield stress, tensile strength, fracture elongation and yield elongation.
• HR30T was determined by measuring the Rockwell superficial hardness of the plate surface at HR15T and converting it using the conversion table in JIS G 3303 (2017). The evaluation results for yield stress, tensile strength, fracture elongation, yield elongation and HR30T are shown in Table 3.
• All of the examples of the invention in Table 3 have a yield stress of 500 MPa or more, a tensile strength of 500 MPa or more, an HR30T of 68 or more, a breaking elongation of 15% or more, and a yield elongation of 4.5% or less. Therefore, it can be said that the examples of the invention are steel sheets with high strength, high ductility, and low yield elongation, suitable as a material for cans.
• the comparative examples in which one or more of the composition and area fraction of the steel sheet structure, and the manufacturing conditions are outside the range of the invention, have any of the yield stress, tensile strength, HR30T, breaking elongation, and yield elongation that are outside the range of the invention.
• steel plate No. 7 in Tables 2 and 3 had a high percentage of unrecrystallized ferrite due to the low annealing temperature, which resulted in a decrease in ductility and an elongation at break outside the scope of the invention (the preferred percentage of unrecrystallized ferrite is 30% or less, and 55% for No. 7).
• Steel plate No. 7 in Tables 2 and 3 had a high percentage of unrecrystallized ferrite due to the low annealing temperature, which resulted in a decrease in ductility and an elongation at break outside the scope of the invention (the preferred percentage of unrecrystallized ferrite is 30% or less, and 55% for No. 7).
• Steel plate No. 7 in Tables 2 and 3 had a high percentage of unrecrystallized ferrite due to the low annealing temperature, which resulted in a decrease in ductility and an elongation at break outside the scope of the invention (the preferred percentage of unrecrystall
• Steel sheet No. 10 had a high finishing temperature, a low coiling temperature, and a low cold rolling ratio, which resulted in coarsening of the ferrite grains and a decrease in the amount of fine Nb precipitates, resulting in a decrease in yield stress and tensile strength (the preferred average ferrite grain size is 4 ⁇ m or less, and No. 10 is 7.0 ⁇ m).

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