Steel Plate and Method for Producing the Same

Abstract

An object of the present invention is to provide a steel sheet having high strength, high ductility, and low yield elongation, and a method for manufacturing the same. In 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, and the balance consists of Fe and inevitable impurities, and the microstructure has ferrite with an area fraction of 80% or more, the yield stress is 500 MPa or more, the tensile strength is 500 MPa or more, HR30T is 68 or more, the elongation at break is 15% or more, and the yield elongation is 4.5% or less.

Description

Technical Field

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

Background Art

[0002] In recent years, in order to reduce the CO 2 emission during can body transportation, weight reduction of the can body by reducing the gauge of the steel sheet for cans has been demanded. Since the strength of the can body decreases as the gauge is reduced, it is essential to increase the strength of the steel sheet.

[0003] As a high-strength steel sheet for cans, DR (Double Reduce) material is known. The DR material is a steel sheet for cans that has been strengthened by performing cold rolling and annealing and then performing cold rolling again. However, since it has low elongation and thus low workability, and since it is necessary to perform cold rolling again after annealing, there is a problem that the manufacturing cost becomes high. In order to solve these problems, development of an SR (Single Reduce) material having the same strength as the DR material and high ductility has been demanded.

[0004] Patent Document 1 proposes a steel sheet 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 ferrite, 0.5% or more and 10.0% or less martensite, and 0.1% or more and 10.0% or less bainite.

[0005] Patent Document 2 discloses a steel sheet for cans, which contains, by 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 0.020% 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, and B: 0.0010% or more and 0.0050% or less. The ratio of the content of B (by mass) to the content of N (by mass), i.e., B / N, is 0.80 or more. The component composition consists of the balance Fe and inevitable impurities, and it has a ferrite structure containing pearlite with an area fraction of 1.0% or more. The yield stress is 500 MPa or more, the tensile strength is 550 MPa or more, the uniform elongation is 10% or more, and the yield elongation is 5.0% or less.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] The following problems exist in the above prior art. In the technology described in Patent Document 1, since the lower limit of the upper yield stress is 400 MPa, there is a problem that the strength of the can body becomes insufficient when the gauge of the steel sheet is reduced. In the technology described in Patent Document 2, although a high-strength and high-ductility steel sheet with a yield stress of 500 MPa or more and a uniform elongation of 10% or more can be obtained, there is a problem of the occurrence of wrinkles due to stretcher strain because a yield elongation of up to 5.0% is allowed. Furthermore, in order to ensure the strength when the steel sheet is used for the body of the can, a certain Rockwell superficial hardness or more is required, but neither Patent Document 1 nor Patent Document 2 describes anything about hardness.

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

Means for Solving the Problems

[0009] The present invention has been made to solve the above problems, and the gist is as follows. [1] 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, containing more than 0.020% and 0.200% or less of Cu, and the balance consisting of Fe and inevitable impurities, having a component composition, the microstructure has ferrite with an area fraction of 80% or more, a steel plate having a yield stress of 500 MPa or more, a tensile strength of 500 MPa or more, HR30T of 68 or more, an elongation at break of 15% or more, and a yield elongation of 4.5% or less. [2] In addition to the above component composition, further, 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, the steel plate according to [1], containing one or more selected from the group consisting of V: 0.02% or less. [3] In addition to the above component composition, by mass%, the steel plate according to [1] or [2], containing Sn: 0.020% or less. [4] A method for manufacturing the steel plate according to any one of [1] to [3] above, a heating step of heating a steel material having the above component composition at 1150 °C or higher, A hot rolling step of performing hot rolling on 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, and pickling; A cold rolling step of performing cold rolling on the hot-rolled sheet after the hot rolling step under conditions of a rolling reduction rate 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 longer and 90 seconds or shorter, and then cooling 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. A method for manufacturing a steel sheet.

Advantages of the Invention

[0010] The present invention enables the production of a steel sheet having high strength, high ductility, and low yield elongation. According to the present invention, further gauge reduction of the steel sheet for cans becomes possible, enabling weight reduction of the can body. Furthermore, in addition to high ductility and excellent workability, the low yield elongation suppresses the generation of wrinkles caused by stretcher strain, enabling more complex processing of the can body.

Embodiments for Carrying Out the Invention

[0011] The component composition, structure, mechanical properties, and manufacturing conditions of the present invention will be described. % in the description of the component composition means mass%. Also, the case where it is excellent in tensile strength, yield stress, and HR30T is referred to as high strength.

[0012] 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. When 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 a decrease in yield stress, tensile strength, and HR30T. Therefore, the C content needs to 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, when the C content exceeds 0.15%, the elongation at fracture decreases, and the yield elongation increases due to the increase in solid solution C. Therefore, the C content needs to 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 even more preferably 0.12% or less.

[0013] Si: 0.05% or less Si is an element that contributes to the improvement of yield stress and tensile strength, but when the content exceeds 0.05%, the corrosion resistance decreases. Therefore, the Si content needs to be 0.05% or less, preferably 0.04% or less, and more preferably 0.02% or less. The lower limit does not particularly need to be limited, but for the purpose of increasing 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 into ferrite. Furthermore, it is also known to contribute to the improvement of yield stress, tensile strength, and HR30T by solid solution strengthening of Mn itself. When the Mn content is less than 0.10%, sufficient yield stress, tensile strength, and HR30T cannot be obtained, so the Mn content should be 0.10% or more. The Mn content is preferably 0.12% or more, and more preferably 0.15% or more. To achieve both high strength and high ductility, the Mn content is more preferably 0.30% or more. The Mn content is most preferably 0.32% or more. On the other hand, when the Mn content exceeds 0.60%, the elongation at break decreases, so the Mn content should be 0.60% or less. The Mn content is preferably 0.58% or less. The Mn content is more preferably 0.57% or less, even more preferably 0.56% or less, and most preferably 0.55% or less.

[0015] P: 0.025% or less P reduces ductility due to grain boundary segregation and hardening of the steel sheet, so the P content should be 0.025% or less. It is preferably 0.020% or less. The P content is more preferably 0.019% or less, and even more preferably 0.018% or less. On the other hand, the lower limit is not particularly limited, but since P contributes to the improvement of the yield stress and tensile strength of the steel sheet, it is preferably contained at 0.001% or more. The P content is more preferably 0.002% or more, and even more preferably 0.003% or more. For high strength of the steel sheet, it is more preferably 0.010% or more.

[0016] S: 0.020% or less Since S forms sulfides such as MnS, CuS, and TiS in steel, reducing ductility, the S content should be 0.020% or less. The S content is preferably 0.018% or less. More preferably, the S content is 0.017% or less. Even more preferably, the S content is 0.016% or less, and most preferably 0.015% or less. The lower limit is not particularly limited, but for reducing manufacturing load, the S content is preferably 0.005% or more. More preferably, the S content is 0.006% or more.

[0017] Al: 0.20% or less Al is an element contained to remove oxygen in steel. Also, by forming AlN in steel, the dissolved N is reduced, reducing the yield elongation. Therefore, although the lower limit is not limited, it is preferably contained at 0.02% or more. The Al content is more preferably 0.03% or more. On the other hand, when the Al content exceeds 0.20%, excessive alumina is generated, reducing ductility, so the Al content is 0.20% or less. The Al content is preferably 0.15% or less, more preferably 0.12% or less. To achieve both high strength and high ductility, the Al content is even more preferably 0.09% or less. Note that Al here refers to the total Al amount.

[0018] N: 0.0001% or more and 0.0200% or less N is an element that contributes to the improvement of yield stress and tensile strength through solid solution strengthening. Therefore, the N content shall be 0.0001% or more. It is preferable that the N content be 0.0003% or more. It is more preferable that the N content be 0.0010% or more. It is even more preferable that the N content be 0.0011% or more, and most preferably 0.0012% or more. On the other hand, when the N content exceeds 0.0200%, the yield elongation increases due to the solid solution N, so the N content shall be 0.0200% or less. It is preferable that the N content be 0.0150% or less, and more preferable that the N content be 0.0100% or less. To achieve both high strength and low yield elongation, it is even more preferable that the N content be 0.0040% or less. Most preferably, the N content is 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 shall be 0.005% or more. It is preferable that the Nb content be 0.007% or more. To achieve both high strength and high ductility, it is more preferable that the Nb content be 0.010% or more. It is even more preferable that the Nb content be 0.011% or more, and most preferably 0.012% or more. On the other hand, when the Nb content exceeds 0.030%, it becomes difficult to ensure sufficient ductility due to the increase in the recrystallization temperature. Therefore, the Nb content shall be 0.030% or less. To achieve both high strength and high ductility, it is preferable that the Nb content be 0.028% or less. It is more preferable that the Nb content be 0.026% or less, even more preferable that the Nb content be 0.024% or less, and most preferably 0.022% or less.

[0020] Cu: More than 0.020% and 0.200% or less Cu contributes to the improvement of yield stress, tensile strength, and HR30T through solid solution strengthening, grain refinement strengthening, and precipitation strengthening. When the Cu content is 0.020% or less, sufficient strength cannot be obtained, so the Cu content should exceed 0.020%. The Cu content is preferably 0.025% or more, more preferably 0.030% or more. To achieve both high strength and high ductility, the Cu content is more preferably 0.100% or more. The most preferred Cu content is 0.105% or more. On the other hand, when the Cu content exceeds 0.200%, it leads to a decrease in ductility and may cause slab cracking due to Cu segregation. Therefore, the Cu content should be 0.200% or less. To achieve both high strength and high ductility, the Cu content is preferably 0.180% or less. More preferably, the Cu content is 0.178% or less, still more preferably 0.175% or less, and most preferably 0.172% or less.

[0021] In addition to the above component composition, the steel sheet in the present invention preferably contains one or more elements selected from the following as appropriate. 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

[0022] Ni: 0.15% or less, Mo: 0.05% or less, Cr: 0.10% or less Ni, Mo, and Cr are elements that promote the solid solution of C into ferrite by improving hardenability, and Sn is an element that contributes to the high strength of the steel sheet by solid solution strengthening. On the other hand, excessive content of these elements reduces ductility. In order to achieve both sufficient hardenability, high strength, and high ductility, when Ni is contained, the Ni content should be 0.15% or less. It is preferably 0.14% or less, and more preferably 0.13% or less. When Mo is contained, the Mo content should be 0.05% or less. The Mo content is preferably 0.04% or less, and more preferably 0.03% or less. When Cr is contained, the Cr content should be 0.10% or less. The Cr content is preferably 0.08% or less, and more preferably 0.07% or less. The lower limit is not particularly limited, but 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 forms TiN in steel to reduce the dissolved N, contributing to the reduction of yield elongation. Also, since TiN is preferentially formed over BN, ensuring sufficient dissolved B by containing both Ti and B improves hardenability. On the other hand, when the Ti content exceeds 0.02% and the B content exceeds 0.02%, it becomes difficult to ensure sufficient ductility. To achieve both the improvement of hardenability by B and high ductility, when containing Ti, the Ti content should be 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. Also, when containing B, the B content should be 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 lower limit is not particularly limited, but 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 the improvement of yield stress, tensile strength, and HR30T by solid solution strengthening, precipitation strengthening, and promoting the solid solution of C into ferrite by improving hardenability. On the other hand, when the V content exceeds 0.02%, the ductility decreases with the increase in strength. To achieve both high strength and high ductility, when containing V, it should be 0.02% or less. The V content is preferably 0.018% or less. The V content is more preferably 0.017% or less, and even more preferably 0.016% or less. The lower limit is not particularly limited, but the V content is preferably 0.004% or more.

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

[0026] Sn: 0.020% or less Sn is an element that contributes to the strengthening of the steel sheet by solid solution strengthening. When Sn is contained, the Sn content shall be 0.020% or less. It is preferable that the Sn content be 0.018% or less. More preferably, it is 0.015% or less, and even more preferably, it is 0.012% or less. The lower limit is not particularly limited, but the Sn content is preferably 0.001% or more. More preferably, it is 0.002% or more, and even more preferably, it is 0.003% or more.

[0027] The steel sheet according to an embodiment of the present invention contains the above components and has a component composition in which the balance consists of Fe and unavoidable impurities. Here, examples of unavoidable impurities include Ca, O, H, Co, W, Zn, Pb, As, Sb, Bi, Ca, O, and the like.

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

[0029] Ferrite is 80% or more in area fraction When ferrite is less than 80% in area fraction, the ductility decreases. Therefore, ferrite is 80% or more in area fraction. In order to obtain a steel sheet with high ductility, it is preferable that ferrite be 85% or more in area fraction. More preferably, ferrite is 86% or more in area fraction, and even more preferably, it is 87% or more. The upper limit is not particularly limited, but ferrite is preferably 95% or less in area fraction. More preferably, ferrite is 94% or less in area fraction. Note that it is desirable to complete the recrystallization of ferrite, but it may have unrecrystallized ferrite. When it has unrecrystallized ferrite, in order to obtain higher ductility, it is preferable that the unrecrystallized ferrite be 30% or less in area fraction. Preferably, the unrecrystallized ferrite is 28% or less in area fraction, and more preferably, it is 25% or less. The lower limit is not particularly limited, but the unrecrystallized ferrite is preferably 0.1% or more in area fraction, and more preferably, it is 0.2% or more.

[0030] Also, the average ferrite crystal grain size is preferably 4 μm or less in terms of the equivalent circle diameter. The average ferrite crystal grain size is more preferably 3.9 μm or less and even more preferably 3.8 μm or less in terms of the equivalent circle diameter. The lower limit is not particularly limited, but the average ferrite crystal grain size is preferably 1 μm or more in terms of the equivalent circle diameter.

[0031] The area of the unrecrystallized ferrite can also be added to the area fraction of the ferrite, and if the sum of the area fractions of the recrystallized ferrite and the unrecrystallized ferrite is 80% or more, the requirements of the present invention are satisfied.

[0032] In addition, as the remainder other than ferrite, it may contain 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 satisfied. The area fraction of the remainder may be 0%, but from the viewpoint of further increasing the strength, it is preferable to contain 0.1% or more of martensite. Martensite 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, Elongation at break: 15% or more, Yield elongation: 4.5% or less In order to maintain sufficient can body strength after reducing the gauge of the steel sheet used for the can body, the yield stress (yield strength) of the steel sheet should be 500 MPa or more, the tensile strength should be 500 MPa or more, and HR30T should be 68 or more. Note that the yield stress is preferably 520 MPa or more, more preferably 530 MPa or more, and even more preferably 540 MPa or more. Also, the tensile strength is preferably 550 MPa or more, more preferably 555 MPa or more, and even more preferably 560 MPa or more. Further, 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 elongation at break should be 15% or more, and more preferably 16% or more. The elongation at break is even more preferably 17% or more, and most preferably 18% or more. The upper limit is not particularly limited, but 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. HR30T is preferably 80 or less, more preferably 79 or less, and even more preferably 78 or less. The elongation at break is preferably 25% or less, more preferably 24% or less, and even more preferably 23% or less. In order to suppress the generation of wrinkles caused by stretcher strain during can manufacturing and when processing the can body, the yield elongation should 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. The lower limit is not particularly limited, but the yield elongation is preferably 1.0% or more. The yield elongation is more preferably 1.1% or more.

[0034] The manufacturing method of the steel sheet in the present invention will be described.

[0035] The manufacturing method of the steel sheet in the present invention includes a heating step of heating a steel material having the above-described component composition at 1150°C or higher, a hot rolling step of performing hot rolling on the steel after the heating step under the 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, and pickling, and a cold rolling step of performing cold rolling on the hot-rolled sheet after the hot rolling step under the condition of a rolling reduction rate 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 longer and 90 seconds or shorter, and then cooling to a cooling stop temperature which is in a temperature range of 600°C or lower.

[0036] Heating temperature: 1150°C or higher When the heating temperature in the heating step is low, coarse nitrides such as AlN are formed, which may reduce the strength and ductility of the steel sheet. Therefore, the heating temperature is set to 1150°C or higher. The heating temperature is preferably 1170°C or higher, more preferably 1200°C or higher. The heating temperature is even more preferably 1210°C or higher. Although the upper limit of the heating temperature is not limited, from the viewpoint of manufacturing cost, it is preferably 1300°C or lower. The heating temperature is more preferably 1290°C or lower, and even more preferably 1270°C or lower.

[0037] Finishing temperature: 800°C or higher and 950°C or lower When the finishing temperature in the hot rolling step exceeds 950°C, the ferrite grain size of the hot-rolled sheet becomes coarse, and the ferrite grain size of the steel sheet after subsequent processes also becomes coarse, making it difficult to ensure sufficient strength. Therefore, the finishing temperature is set to 950°C or lower. The finishing temperature is preferably 930°C or lower, more preferably 920°C or lower. The finishing temperature is even more preferably 900°C or lower, and most preferably 890°C or lower. On the other hand, when the finishing temperature is less than 800°C, rolling is performed in the two-phase region of ferrite and austenite, resulting in coarse ferrite grains, and the strength of the steel sheet decreases due to factors such as precipitation of coarse Nb carbides during hot rolling. Therefore, the finishing temperature in the hot rolling step is set to 800°C or higher. The finishing temperature is preferably 830°C or higher, more preferably 850°C or higher.

[0038] Coiling temperature: 450°C or higher and 700°C or lower When 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, so that cementite cannot be sufficiently dissolved in the annealing process, and the amount of C solid solution in ferrite decreases, which becomes a factor in the strength reduction. Therefore, the coiling temperature should be 700°C or lower. The coiling temperature is preferably 650°C or lower, more preferably 600°C or lower. The coiling temperature is further preferably 595°C or lower, and most preferably 590°C or lower. On the other hand, when the coiling temperature is less than 450°C, the precipitation amount of alloy carbides such as Nb carbide decreases, resulting in a decrease in strength. Therefore, the coiling temperature should be 450°C or higher. The range of the coiling temperature is preferably 470°C or higher, more preferably 500°C or higher. The coiling temperature is further preferably 510°C or higher, and most preferably 520°C or higher. After coiling, for the purpose of scale removal, H 2 SO 4 、HCl, H 3 PO 4 Perform pickling with an aqueous solution such as etc.

[0039] Rolling reduction in cold rolling: 80% or higher After the above hot rolling process, cold rolling is performed. The cold rolling process refines the ferrite grain size and increases the yield stress and tensile strength. To ensure sufficient yield stress and tensile strength, the rolling reduction should be 80% or higher. The rolling reduction is preferably 82% or higher, more preferably 85% or higher. The rolling reduction is further preferably 86% or higher, and most preferably 87% or higher. The upper limit of the rolling reduction is not limited, but it is preferably 95% or lower to ensure sufficient ductility. The rolling reduction is more preferably 94% or lower.

[0040] Annealing temperature: 680°C or higher and 780°C or lower, holding time: 5 s or longer and 90 s or shorter, 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 shall be 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, when the annealing temperature exceeds 780 °C, the yield stress and tensile strength decrease due to the coarsening of the ferrite grain size and the fine precipitates contributing to precipitation strengthening. Therefore, the annealing temperature shall be 780 °C or lower. Note that the annealing temperature is preferably 760 °C or lower, and more preferably 740 °C or lower. The annealing temperature is even more preferably 735 °C or lower, and most preferably 730 °C or lower.

[0041] When the holding time at the annealing temperature is less than 5 s, the recrystallization of ferrite does not proceed sufficiently, and the required ductility cannot be obtained. Therefore, the holding time shall be 5 s or longer. The holding time is preferably 6 s or longer, more preferably 7 s or longer, even more preferably 8 s or longer, and most preferably 9 s or longer. On the other hand, when the holding time is longer than 90 s, the ferrite grain size coarsens, and the yield stress and tensile strength decrease. Therefore, the holding time at the annealing temperature shall be 90 s or shorter. The holding time is preferably 89 s or shorter, more preferably 88 s or shorter, even more preferably 87 s or shorter, and most preferably 86 s or shorter.

[0042] After annealing and holding, cool to a temperature range of 600°C or lower at an average cooling rate of 50°C / s or higher. If the average cooling rate is less than 50°C / s, the ferrite grain size becomes coarser and the amount of solid-solved C in the ferrite decreases, resulting in a decrease in strength. Therefore, the average cooling rate should be 50°C / s or higher. The average cooling rate is preferably 60°C / s or higher, more preferably 80°C / s or higher. The average cooling rate is even more preferably 85°C / s or higher, and most preferably 90°C / s or higher. The upper limit is not particularly limited, but for reducing the manufacturing load, the average cooling rate is preferably 200°C / s or lower. The average cooling rate is more preferably 190°C / s or lower, and even more preferably 180°C / s or lower.

[0043] If the cooling stop temperature after annealing and holding exceeds 600°C, the ferrite grain size becomes coarse and the strength decreases. Therefore, the cooling stop temperature should be 600°C or lower. The cooling stop temperature is preferably 595°C or lower, more preferably 590°C or lower, and even more preferably 585°C or lower. On the other hand, even if the cooling stop temperature is less than 300°C, a significant improvement in the steel plate properties is not expected. Therefore, from the perspective of manufacturing cost, the cooling stop temperature is preferably 300°C or higher. The cooling stop temperature is preferably 305°C or higher, more preferably 310°C or higher, and even more preferably 315°C or higher.

[0044] It is also possible to perform temper rolling after the annealing process. By performing temper rolling, the yield stress can be increased and the yield elongation can be reduced. Therefore, it is preferable to perform temper rolling with a reduction ratio of 0.5% or higher. For temper rolling, the reduction ratio is more preferably 0.6% or higher, even more preferably 0.7% or higher, and most preferably 0.8% or higher. On the other hand, if the reduction ratio in temper rolling is increased, the ductility decreases. Therefore, the reduction ratio is preferably 10% or lower. The reduction ratio is more preferably 9% or lower, even more preferably 8% or lower, and most preferably 7% or lower.

Examples

[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] The steel sheet structure was observed 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 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 position at 1 / 2 the sheet thickness in the sheet thickness direction at a magnification of 3000 times, 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. The area that can be observed as black lumps in the SEM photograph was determined to be ferrite.

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

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

[0050] From the above steel plate, a JIS No. 5 tensile test piece with the direction along the rolling direction as the tensile direction and a 30-degree test piece used for Rockwell superficial hardness measurement were taken, and aging heat treatment was carried out at 210 °C for 10 minutes in a thermostat. For the tensile test piece, a tensile test was carried out in accordance with JIS Z 2241, and the yield stress, tensile strength, elongation at break, and yield elongation were evaluated. Also, HR30T was obtained by measuring the Rockwell superficial hardness of the plate surface with HR15T and converting it using the conversion table in JIS G 3303 (2017). Table 3 shows the evaluation results of the yield stress, tensile strength, elongation at break, yield elongation, and HR30T.

[0051] In the invention examples in Table 3, all of them have a yield stress of 500 MPa or more, a tensile strength of 500 MPa or more, an HR30T of 68 or more, an elongation at break of 15% or more, and a yield elongation of 4.5% or less. Therefore, it can be said that the invention examples are steel plates suitable as materials for cans, having high strength, high ductility, and low yield elongation. On the other hand, in the comparative examples where one or more of the component composition and the area fraction of the steel plate structure, and the manufacturing conditions are outside the scope of the invention, any one of the yield stress, tensile strength, HR30T, elongation at break, and yield elongation is outside the scope of the invention.

[0052] Among these, for Steel Plate No.7 in Tables 2 and 3, since the annealing temperature was low, the fraction of unrecrystallized ferrite became high, ductility decreased, and the elongation at break was outside the scope of the invention (the preferable fraction of unrecrystallized ferrite is 30% or less, and for No.7 it is 55%). For Steel Plate No.8, since the slab heating temperature was low, the coiling temperature was high, and the annealing temperature was high, the yield stress and tensile strength decreased due to the coarsening of ferrite grains and the formation of coarse alloy carbides that did not contribute to the improvement of strength (the preferable average crystal grain size of ferrite is 4 μm or less, and for No.8 it is 6.0 μm). For Steel Plate No.9, since the finishing temperature was low, the cooling rate was slow, and the cooling stop temperature was high, in addition to the coarsening of ferrite grains, the formation of coarse Nb precipitates that did not contribute to the improvement of strength occurred, and the yield stress and tensile strength decreased (the preferable average crystal grain size of ferrite is 4 μm or less, and for No.9 it is 6.0 μm). For Steel Plate No.10, since the finishing temperature was high, the coiling temperature was low, and the cold rolling reduction rate was low, in addition to the coarsening of ferrite grains, the amount of fine Nb precipitates decreased, and the yield stress and tensile strength decreased (the preferable average crystal grain size of ferrite is 4 μm or less, and for No.10 it is 7.0 μm).

[0053]

Table 1

[0054]

Table 2

[0055]

Table 3

Claims

1. In mass percent, 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.

2. In addition to the above-mentioned component composition, further, in 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, The steel sheet according to claim 1, further comprising one or more selected from the group consisting of V: ​​0.02% or less.

3. In addition to the above-mentioned component composition, in mass%, The steel plate according to claim 1, containing Sn: 0.020% or less.

4. In addition to the above-mentioned component composition, The steel plate according to claim 2, containing Sn: 0.020% or less.

5. A method for producing a steel sheet according to any one of claims 1 to 4, A heating step of heating a steel material having the above-mentioned composition at 1150°C or higher; a hot rolling process in which the steel material after the heating process 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 method for producing a steel sheet, comprising: a cold rolling step in which the hot-rolled sheet after the hot rolling step is cold-rolled at a rolling ratio of 80% or more; and an annealing step in which the cold-rolled sheet after the cold rolling step is held at an annealing temperature of 680° C. or more and 780° C. or less for 5 s to 90 s and then cooled at an average cooling rate of 50° C. / s or more to a cooling stop temperature in a temperature range of 600° C. or less.

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