STEEL PLATES AND METHODS OF STEEL PLATE MANUFACTURING

Abstract

The purpose of the invention is to propose a steel plate that has high strength, high ductility, and low yield strength, and to propose a method for manufacturing the steel plate. Steel plates with a chemical composition containing, by mass percentage, C: 0.03% or greater and 0.15% or less, Si: 0.05% or less, Mn: 0.10% or greater and 0.60% or less, P: 0.025% or less, S: 0.020% or less, Al: 0.20% or less, N: 0.0001% or greater and 0.0200% or less, Nb: 0.005% or greater and 0.030% or less, and Cu: greater than 0.020% and 0.200% or less, with the remainder being Fe and unavoidable impurities, in which the steel plate has microstructures in which ferrite is present with an area ratio of 80% or greater, and the steel plate has a yield stress of 500 MPa or greater. Furthermore, the tensile strength is 500 MPa or greater, HR30T is 68 or greater, the elongation at break is 15% or greater, and the yield elongation is 4.5% or less.

Description

Steel plate and its manufacturing method

[0001] The present invention relates to a steel sheet that is particularly suitable as a steel sheet for cans and that has both high strength and high ductility, and a method for producing the same.

[0002] In recent years, CO emissions during can transport have increased ２ To reduce CO2 emissions, there is a need to reduce the weight of can bodies by reducing the gauge of steel sheets used for cans. However, as the strength of can bodies decreases as the gauge is reduced, it is essential to increase the strength of steel sheets.

[0003] 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, annealing, and then cold rolling again. However, DR material has problems such as low workability due to small elongation, and high manufacturing costs due to the need to cold roll again after annealing. To solve these problems, there is a need to develop SR (Single Reduce) material that has the same strength as DR material but high ductility.

[0004] Patent Document 1 proposes a steel plate containing, by mass%, C: 0.03% or more and 0.13% or less, Si: 0.05% or less, Mn: 0.01% or more and 0.6% or less, P: 0.025% or less, S: 0.020% or less, Al: 0.01% or more and 0.20% or less, N: 0.0001% or more and 0.02% or less, Ti: 0.005% or more and 0.02% or less, and B: 0.0005% or more and 0.02% or less, with the balance being iron and unavoidable impurities, and containing, by area ratio, 84.0% or more of ferrite, 0.5% or more and 10.0% or less of martensite, and 0.1% or more and 10.0% or less of bainite.

[0005] Patent Document 2 describes, in mass %, C: 0.085% or more and 0.130% or less, Si: 0.04% or less, Mn: 0.10% or more and 0.60% or less, P: 0.02% or less, S: more than 0.010% and 0.0 20% or less, Al: 0.02% or more and 0.10% or less, N: 0.0005% or more and 0.0040% or less, Nb: 0.007% or more and 0.030% or less, B: 0.0010% or more and 0.0050% or less The present invention proposes a steel sheet for cans, which contains the following: a steel sheet for cans containing the above elements, in which B / N, which is the ratio of the B content (mass%) to the N content (mass%), is 0.80 or more, with the balance being Fe and unavoidable impurities; a steel sheet for cans having a ferrite structure containing pearlite in an area fraction of 1.0% or more; and a steel sheet for cans having 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.

[0006] International Publication No. WO 2021 / 167023 International Publication No. WO 2020 / 105406

[0007] The above-mentioned conventional technologies have the following problems. The technology described in Patent Document 1 has a lower limit of upper yield stress of 400 MPa, which results in insufficient can body strength when the steel sheet is gauged down. The technology described in Patent Document 2 produces high-strength, high-ductility steel sheets with a yield stress of 500 MPa or more and a uniform elongation of 10% or more, but allows a yield elongation of up to 5.0%, which results in the problem of wrinkles occurring due to stretcher strain. Furthermore, while a certain level of Rockwell superficial hardness is considered necessary to ensure strength when the steel sheet is used for the body of a can, neither Patent Document 1 nor Patent Document 2 describes hardness.

[0008] An object of the present invention is to provide a steel sheet having high strength, high ductility and low yield point elongation, which solves the above problems, and a method for manufacturing the same.

[0009] The present invention has been made to solve the above-mentioned problems, and is summarized as follows: [1] A steel sheet having a chemical composition containing, by mass%, C: 0.03% to 0.15%, Si: 0.05% or less, Mn: 0.10% to 0.60%, P: 0.025% or less, S: 0.020% or less, Al: 0.20% or less, N: 0.0001% to 0.0200%, Nb: 0.005% to 0.030%, Cu: more than 0.020% to 0.200%, and the balance consisting of Fe and unavoidable impurities, wherein the microstructure has an area fraction of ferrite of 80% or more, and wherein the steel sheet has a yield stress of 500 MPa or more, a tensile strength of 500 MPa or more, an HR30T of 68 or more, an elongation to fracture of 15% or more, and a yield elongation of 4.5% or less. [2] The steel sheet according to [1], further containing, in addition to the chemical composition, one or more selected from, by mass%, 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, and V: 0.02% or less. [3] The steel sheet according to [1] or [2], further containing, in addition to the chemical composition, by mass%, Sn: 0.020% or less. [4] A method for producing a steel sheet according to any one of [1] to [3] above, comprising: a heating step of heating a steel material having the chemical composition at 1150°C or higher; a hot rolling step of hot rolling the steel material after the heating step under conditions of a finishing temperature of 800°C or higher and 950°C or lower and a coiling temperature of 450°C or higher and 700°C or lower, followed by pickling; a cold rolling step of cold rolling the hot-rolled sheet after the hot rolling step under conditions of a rolling ratio of 80% or higher; and an annealing step of holding the cold-rolled sheet after the cold rolling step at an annealing temperature of 680°C or higher and 780°C or lower for 5 seconds or higher and 90 seconds or lower, and then cooling it at an average cooling rate of 50°C / s or higher to a cooling stop temperature in a temperature range of 600°C or lower.

[0010] The present invention has made it possible to manufacture steel sheets with high strength, high ductility, and low yield point elongation. The present invention also enables the further reduction in gauge of steel sheets for cans, thereby enabling the weight of can bodies to be reduced. Furthermore, in addition to high ductility and excellent formability, the low yield point elongation suppresses the occurrence of wrinkles due to stretcher strain, making it possible to apply more complex processing to can bodies.

[0011] The component composition, structure, mechanical properties, and manufacturing conditions of the present invention will be described. In the component composition description, % means mass %. Furthermore, when the tensile strength, yield stress, and HR30T are excellent, it is called high strength.

[0012] C: 0.03% or more and 0.15% or less C is an element that contributes to improving 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, resulting in lower yield stress, tensile strength, and HR30T. Therefore, the C content must be 0.03% or more. The C content is preferably 0.06% or more, more preferably 0.08% or more. The C content is even more preferably 0.09% or more. The C content is most preferably 0.10% or more. On the other hand, if the C content exceeds 0.15%, the fracture elongation decreases, and the yield elongation increases due to the increase in solute C. Therefore, the C content must be 0.15% or less. To obtain a steel sheet with high strength, high ductility, and low yield elongation, 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.

[0013] 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, 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.

[0014] 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 through solid solution strengthening. Since a Mn content of less than 0.10% does not provide sufficient yield stress, tensile strength, and HR30T, 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. 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. On the other hand, a Mn content exceeding 0.60% reduces fracture elongation, 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.

[0015] P: 0.025% or less Since P reduces ductility due to grain boundary segregation and hardening of the steel sheet, the P content is set to 0.025% or less. Preferably, it is set to 0.020% or less. The P content is more preferably set to 0.019% or less, and even more preferably set to 0.018% or less. On the other hand, although there is no particular lower limit, P contributes to improving the yield stress and tensile strength of the steel sheet, so it is preferably contained in an amount of 0.001% or more. The P content is more preferably set to 0.002% or more, and even more preferably set to 0.003% or more. In order to increase the strength of the steel sheet, it is more preferably set to 0.010% or more.

[0016] S: 0.020% or less S forms sulfides such as MnS, CuS, and TiS in steel, reducing 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. There is no particular lower limit, but 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.

[0017] Al: 0.20% or less Al is an element contained to remove oxygen from steel. It also forms AlN in steel, thereby reducing solute N and lowering yield point elongation. Therefore, although there is no lower limit, a content of 0.02% or more is preferable. The Al content is more preferably 0.03% or more. On the other hand, if the Al content exceeds 0.20%, excessive alumina is generated, reducing ductility. Therefore, the Al content is set to 0.20% or less. The Al content is preferably set to 0.15% or less, and more preferably 0.12% or less. To achieve both high strength and high ductility, the Al content is even more preferably set to 0.09% or less. Note that Al here refers to the total Al content.

[0018] N: 0.0001% or more and 0.0200% or less N is an element that contributes to improving yield stress and tensile strength through 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. On the other hand, if the N content exceeds 0.0200%, the yield elongation increases due to solid solution N, so 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.

[0019] Nb: 0.005% or more and 0.030% or less Nb is an element that contributes to precipitation strengthening and grain refinement strengthening by forming fine NbC in steel. To ensure sufficient strength, the Nb content is set to 0.005% or more. The Nb content is preferably set to 0.007% or more. To achieve both high strength and high ductility, the Nb content is more preferably set to 0.010% or more. The Nb content is further preferably set to 0.011% or more, and most preferably set to 0.012% or more. On the other hand, if the Nb content exceeds 0.030%, it becomes difficult to ensure sufficient ductility due to an increase in the recrystallization temperature. Therefore, the Nb content is set to 0.030% or less. To achieve both high strength and high ductility, 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.

[0020] Cu: More than 0.020% and Not More than 0.200% Cu contributes to improving yield stress, tensile strength, and HR30T through solid solution strengthening, grain refinement strengthening, and precipitation strengthening. Since a Cu content of 0.020% or less does not provide sufficient strength, the Cu content is set to more than 0.020%. The Cu content is preferably 0.025% or more, and more preferably 0.030% or more. To achieve both high strength and high ductility, the Cu content is further preferably 0.100% or more. The Cu content is most preferably 0.105% or more. On the other hand, a Cu content exceeding 0.200% leads to a decrease in ductility and may cause slab cracking due to Cu segregation. Therefore, the Cu content is set to 0.200% or less. To achieve both high strength and high ductility, 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.

[0021] In addition to the above-mentioned chemical composition, the steel sheet of the present invention preferably contains 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, and V: 0.02% or less.

[0022] Ni: 0.15% or less, Mo: 0.05% or less, Cr: 0.10% or less. Ni, Mo, and Cr are elements that improve hardenability and promote the solid solution of C in ferrite, and Sn is an element that contributes to increasing the strength of steel sheets through solid solution strengthening. On the other hand, excessive inclusion of these elements reduces ductility. In order to achieve sufficient hardenability, high strength, and high ductility at the same time, when Ni is contained, the Ni content is set to 0.15% or less. The Ni content is preferably set to 0.14% or less, and more preferably set to 0.13% or less. When Mo is contained, the Mo content is set to 0.05% or less. The Mo content is preferably set to 0.04% or less, and more preferably set to 0.03% or less. When Cr is contained, the Cr content is set to 0.10% or less. The Cr content is preferably set to 0.08% or less, and more preferably set to 0.07% or less. Although there is no particular lower limit, 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.

[0023] Ti: 0.02% or less, B: 0.02% or less. Ti contributes to reducing yield point elongation by forming TiN in steel and reducing solute N. Furthermore, because TiN is formed preferentially over BN, containing both Ti and B ensures sufficient solute B and improves hardenability. On the other hand, if the Ti content exceeds 0.02% and the B content exceeds 0.02%, it becomes difficult to ensure sufficient ductility. To achieve both improved hardenability due to B and high ductility, when Ti is contained, the Ti content is set to 0.02% or less. The Ti content is preferably set to 0.018% or less. The Ti content is more preferably set to 0.017% or less, and even more preferably set to 0.016% or less. Furthermore, when B is contained, the B content is set to 0.02% or less. The B content is preferably set to 0.018% or less. The B content is more preferably set to 0.017% or less, and even more preferably set to 0.016% or less. Although there is no particular lower limit, 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.

[0024] V: 0.02% or less V contributes to improving yield stress, tensile strength, and HR30T by promoting solid solution strengthening, precipitation strengthening, and improving hardenability, thereby promoting the solid solution of C in ferrite. On the other hand, if the V content exceeds 0.02%, ductility decreases as strength increases. 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. Although there is no particular lower limit, the V content is preferably set to 0.004% or more.

[0025] Furthermore, in addition to the above components, it is preferable to appropriately contain Sn: 0.020% or less.

[0026] Sn: 0.020% or less Sn is an element that contributes to increasing the strength of steel sheet through solid solution strengthening. When Sn is contained, the Sn content is set to 0.020% or less. The Sn content is preferably set to 0.018% or less. It is more preferably set to 0.015% or less, and even more preferably set to 0.012% or less. Although there is no particular lower limit, the Sn content is preferably set to 0.001% or more. It is more preferably set to 0.002% or more, and even more preferably set to 0.003% or more.

[0027] A steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance being Fe and unavoidable impurities, such as Ca, O, H, Co, W, Zn, Pb, As, Sb, Bi, Ca, and O.

[0028] The structure (microstructure) and mechanical properties of the steel sheet of the present invention will be described.

[0029] The area fraction of ferrite is 80% or more. If the area fraction of ferrite is less than 80%, ductility decreases, so the area fraction of ferrite is set to 80% or more. To obtain a steel sheet with high ductility, the area fraction of ferrite is preferably 85% or more. The area fraction of ferrite is more preferably 86% or more, and even more preferably 87% or more. There is no particular upper limit, but the area fraction of ferrite is preferably 95% or less. The area fraction of ferrite is more preferably 94% or less. Note that it is desirable to complete the recrystallization of ferrite, but unrecrystallized ferrite may be present. If unrecrystallized ferrite is present, the area fraction of unrecrystallized ferrite is preferably 30% or less to obtain higher ductility. The area fraction of unrecrystallized ferrite is preferably 28% or less, and more preferably 25% or less. Although there is no particular lower limit, the area fraction of unrecrystallized ferrite is preferably 0.1% or more, and more preferably 0.2% or more.

[0030] The average ferrite grain size is preferably 4 μm or less in equivalent circle diameter, more preferably 3.9 μm or less, and even more preferably 3.8 μm or less in equivalent circle diameter. Although there is no particular lower limit, the average ferrite grain size is preferably 1 μm or more in equivalent circle diameter.

[0031] 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.

[0032] 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 that the martensite content be 0.1% or more. The martensite content is more preferably 0.2% or more, and even more preferably 0.3% or more. The upper limit of martensite is preferably 20% or less.

[0033] 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. In order to maintain sufficient can body strength after the steel sheet used for the can body is gauged down, it is necessary for the steel sheet to have a yield stress (yield strength) of 500 MPa or more, a tensile strength of 500 MPa or more, and an HR30T of 68 or more. 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. In order to ensure the workability of the steel sheet, 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. Although there is no particular upper limit, 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. To suppress the occurrence of wrinkles due to stretcher strain during can making and processing of the can body, the yield elongation needs to be 4.5% 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 point elongation is preferably 1.0% or more, and more preferably 1.1% or more.

[0034] The method for producing a steel sheet according to the present invention will be described.

[0035] The method for producing a steel sheet according to the present invention is characterized by comprising: a heating step of heating a steel material having the above-described chemical composition at 1150°C or higher; a hot rolling step of hot rolling the steel after the heating step under conditions of a finishing temperature of 800°C or higher and 950°C or lower and a coiling temperature of 450°C or higher and 700°C or lower, followed by pickling; a cold rolling step of cold rolling the hot-rolled sheet after the hot rolling step under conditions of a rolling ratio of 80% or higher; and an annealing step of holding the cold-rolled sheet after the cold rolling step at an annealing temperature of 680°C or higher and 780°C or lower for 5 seconds or higher and 90 seconds or lower, and then cooling it to a cooling stop temperature in the temperature range of 600°C or lower.

[0036] 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 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 to 1270°C or lower.

[0037] Finishing temperature: 800°C or higher and 950°C or lower If the finishing temperature in the hot rolling process exceeds 950°C, the ferrite grain size of the hot-rolled sheet will become coarse, and the ferrite grain size will also become coarse in the steel sheet that has undergone subsequent processes, making it difficult to ensure sufficient strength. Therefore, the finishing temperature is set to 950°C or lower. The finishing temperature is preferably set to 930°C or lower, and more preferably to 920°C or lower. The finishing temperature is further preferably set to 900°C or lower, and most preferably to 890°C or lower. On the other hand, if the finishing temperature is lower than 800°C, rolling will occur in the two-phase region of ferrite and austenite, resulting in the generation of coarse ferrite grains, and coarse Nb carbides will precipitate during hot rolling, reducing the strength of the steel sheet. Therefore, the finishing temperature in the hot rolling process is set to 800°C or higher. Note that the finishing temperature is preferably set to 830°C or higher, and more preferably to 850°C or higher.

[0038] Coiling temperature: 450°C or higher and 700°C or lower If the coiling 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, which prevents cementite from dissolving sufficiently in the annealing process and reduces the amount of C dissolved in ferrite, resulting in a decrease in strength. Therefore, the coiling temperature is set to 700°C or lower. The coiling temperature is preferably set to 650°C or lower, and more preferably to 600°C or lower. The coiling temperature is further preferably set to 595°C or lower, and most preferably to 590°C or lower. On the other hand, if the coiling temperature is lower than 450°C, the amount of precipitation of alloy carbides such as Nb carbide decreases, resulting in a decrease in strength. Therefore, the coiling temperature is set to 450°C or higher. The coiling temperature range is preferably set to 470°C or higher, and more preferably to 500°C or higher. The coiling temperature is more preferably set to 510°C or higher, and most preferably to 520°C or higher. After winding, in order to remove scale, ２ SO ４ , HCl, H ３ P.O. ４ The acid is picked with an aqueous solution such as

[0039] 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, increasing the yield stress and tensile strength. In order to ensure sufficient yield stress and tensile strength, the rolling ratio is set to 80% or more. The rolling ratio is preferably set to 82% or more, and more preferably to 85% or more. The rolling ratio is further preferably set to 86% or more, and most preferably to 87% or more. There is no upper limit to the rolling ratio, but it is preferably set to 95% or less in order to ensure sufficient ductility. The rolling ratio is more preferably set to 94% or less.

[0040] Annealing temperature: 680°C or higher and 780°C or lower, holding time: 5 seconds or higher and 90 seconds or lower, cooling stop temperature: 600°C or lower, average cooling rate: 50°C / s or higher. Annealing is performed after the cold rolling process. In order to recrystallize ferrite and obtain sufficient ductility, the annealing temperature is 680°C or higher. The annealing temperature is preferably 685°C or higher, more preferably 690°C or higher, even more preferably 695°C or higher, and most preferably 700°C or higher. On the other hand, if the annealing temperature exceeds 780°C, the ferrite grain size and fine precipitates that contribute to precipitation strengthening will become coarse, resulting in a decrease in yield stress and tensile strength. Therefore, the annealing temperature is set to 780°C or lower. The annealing temperature is preferably 760°C or lower, more preferably 740°C or lower. The annealing temperature is more preferably 735°C or lower, and most preferably 730°C or lower.

[0041] If the holding time at the annealing temperature is less than 5 seconds, ferrite recrystallization does not proceed sufficiently, and the required ductility cannot be obtained. Therefore, the holding time 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. On the other hand, if the holding time is longer than 90 seconds, the ferrite grain size becomes coarse, and the yield stress and tensile strength decrease. Therefore, 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.

[0042] After annealing, the steel 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 solute C 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. There is no particular upper limit, but in order to reduce the manufacturing load, 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.

[0043] If the cooling stop temperature after annealing exceeds 600°C, the ferrite grain size becomes coarse and the strength decreases, so 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. On the other hand, since a significant improvement in steel sheet properties is not expected even if the cooling stop temperature is less than 300°C, from the viewpoint of manufacturing costs, 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.

[0044] Temper rolling may be performed after the annealing step. Temper rolling can increase the yield stress and reduce the yield elongation, so it is preferable to perform temper rolling at a reduction of 0.5% or more. The temper rolling reduction is more preferably 0.6% or more, even 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, the reduction is preferably 10% or less. The reduction is more preferably 9% or less, even more preferably 8% or less, and most preferably 7% or less.

[0045] Examples of the present invention are shown below, but the present invention is not limited to the examples shown here.

[0046] Steel slabs were obtained by melting and casting steels containing the components of steel types 1 to 29 shown in Table 1, with the balance being Fe and unavoidable impurities. The steel slabs thus obtained were heated, hot rolled, cold rolled, and annealed under the conditions shown in Table 2, and then temper rolled at a reduction ratio of 1%, to obtain steel plates Nos. 1 to 40.

[0047] Observation of the steel sheet structure was carried out according to the following procedure. After taking a test piece from the steel sheet, the cross section parallel to the rolling direction was polished and subjected to nital etching to reveal the structure, and a sample for structure observation was taken. Using a scanning electron microscope (SEM), the position at 1 / 2 the sheet thickness in the sheet thickness direction was observed at a magnification of 3000x, and the structure of three randomly selected fields was photographed. Table 3 shows the area fraction of ferrite in the SEM image measured using Image-J, an image processing software. The area fraction shown in Table 3 is the average value of the three fields. In the SEM photograph, the area that could be observed as a black mass was determined to be ferrite.

[0048] In the above SEM image, areas with white lines within the grains were determined to be unrecrystallized ferrite, and their area fraction was measured using Image-J to determine the unrecrystallized ferrite fraction.

[0049] The average ferrite grain size was determined by the cutting method described in JIS G 0551, and was taken as the average value of three fields of view.

[0050] From the steel sheets, JIS No. 5 tensile test pieces with the tensile direction along the rolling direction and 30-square test pieces used for Rockwell superficial hardness measurement were taken and subjected to aging heat treatment at 210°C for 10 minutes in an incubator. Tensile tests in accordance with JIS Z 2241 were conducted on the tensile test pieces to evaluate yield stress, tensile strength, fracture elongation, and yield elongation. HR30T was determined by measuring the Rockwell superficial hardness of the sheet surface using HR15T and converting it using the conversion table in JIS G 3303 (2017). Table 3 shows the evaluation results for yield stress, tensile strength, fracture elongation, yield elongation, and HR30T.

[0051] All of the inventive examples 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 fracture elongation of 15% or more, and a yield elongation of 4.5% or less. Therefore, the inventive examples can be said to be steel sheets with high strength, high ductility, and low yield elongation, suitable for use as can materials. On the other hand, the comparative examples, in which one or more of the chemical composition, the area fraction of the steel sheet structure, and the manufacturing conditions were outside the ranges of the invention, had any of the yield stress, tensile strength, HR30T, fracture elongation, and yield elongation outside the ranges of the invention.

[0052] Among the above, steel sheet No. 7 in Tables 2 and 3 had a low annealing temperature, which increased the fraction of unrecrystallized ferrite, reduced ductility, and resulted in a fracture elongation outside the range of the invention (the preferred fraction of unrecrystallized ferrite is 30% or less, and No. 7 was 55%). Steel sheet No. 8 had a low slab heating temperature, a high coiling temperature, and a high annealing temperature, which caused ferrite grains to coarsen and the formation of coarse alloy carbides that do not contribute to improving strength, resulting in reduced yield stress and tensile strength (the preferred average ferrite grain size is 4 μm or less, and No. 8 is 6.0 μm). Steel sheet No. 9 had a low finishing temperature, a slow cooling rate, and a high cooling stop temperature, which caused ferrite grains to coarsen and the formation of coarse Nb precipitates that do not contribute to improving strength, resulting in reduced yield stress and tensile strength (the preferred average ferrite grain size is 4 μm or less, and No. 9 is 6.0 μm). In No. 10, the finishing temperature was high, the coiling temperature was low, and the cold rolling reduction was low, 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 μm or less, and the preferred average ferrite grain size is 7.0 μm for No. 10).

[0053]

[0054]

[0055]

Claims

1. A steel plate having a composition comprising, by mass%, 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 0.200% or less, with the balance being Fe and unavoidable impurities, wherein the microstructure has an area fraction of 80% or more of ferrite, and wherein the steel plate has a yield stress of 500 MPa or more, a tensile strength of 500 MPa or more, an HR30T of 68 or more, a fracture elongation of 15% or more, and a yield elongation of 4.5% or less.

2. The steel plate according to claim 1, further containing, in addition to the above-mentioned chemical composition, one or more selected from, by mass%, 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, and V: 0.02% or less.

3. The steel sheet according to claim 1 or 2, further comprising, in addition to the above-mentioned composition, the following mass percent: Sn: 0.020% or less.

4. A method for producing a steel sheet according to any one of claims 1 to 3, comprising: a heating step of heating a steel material having the above-mentioned chemical composition at 1150°C or higher; a hot rolling step of hot rolling and pickling the steel material after the heating step under conditions of a finishing temperature of 800°C to 950°C and a coiling temperature of 450°C to 700°C; a cold rolling step of cold rolling the hot rolled sheet after the hot rolling step under conditions of a rolling ratio of 80% or higher; and an annealing step of holding the cold rolled sheet after the cold rolling step at an annealing temperature of 680°C to 780°C for 5 s to 90 s, and then cooling it at an average cooling rate of 50°C / s or higher to a cooling stop temperature in the temperature range of 600°C or lower.