Steel sheet and method for manufacturing same

Abstract

The purpose 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. A steel sheet having a component composition containing, in mass%, 0.03% to 0.15% (inclusive) of C, 0.05% or less of Si, 0.10% to 0.60% (inclusive) of Mn, 0.025% or less of P, 0.020% or less of S, 0.20% or less of Al, 0.0001% to 0.0200% (inclusive) of N, 0.005% to 0.030% (inclusive) of Nb, and more than 0.020% but 0.200% (inclusive) of Cu, the remainder being Fe and unavoidable impurities, the microstructure of the steel sheet having a ferrite content of 80% or more in area fraction, 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 at break of 15% or more, and a yield elongation of 4.5% or less.

Description

TECHNICAL FIELD

[0001] The present application relates to a steel sheet particularly suitable as a steel sheet for a can, which realizes both high strength and high ductility, and a method for manufacturing the same. BACKGROUND

[0002] In recent years, in order to reduce CO2 emission at the time of transporting a can body, a can body weight reduction based on thinning of a steel sheet for a can is required. With the thinning, the strength of the can body decreases, and thus high strength of the steel sheet is indispensable.

[0003] As a high-strength steel sheet for a can, a DR (Double Reduce) material is known. The DR material is a steel sheet for a can which is high-strengthed by performing cold rolling and annealing again after cold rolling. However, there are the following problems: the workability is low because the elongation is small, and the manufacturing cost becomes high because cold rolling is performed again after annealing. In order to solve these problems, development of an SR (Single Reduce) material which is equal in strength to the DR material and has high ductility is required.

[0004] Patent Document 1 proposes a steel sheet having the following composition: containing, in 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 inevitable impurities, and containing, in 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 Literature 2 proposes a steel sheet for a tank having a composition consisting of, 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 0.020% 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, B: 0.0010% or more and 0.0050% or less, a ratio B / N of a content (mass %) of B to a content (mass %) of N being 0.80 or more, and the balance being Fe and inevitable impurities, and having a ferrite structure containing 1.0% or more of pearlite in an area fraction, the steel sheet for a tank 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] Prior Art Documents Patent Literature Patent Literature 1: International Publication No. 2021 / 167023 Patent Literature 2: International Publication No. 2020 / 105406 SUMMARY

[0007] Problems to be Solved by the Invention The following problems can be cited in relation to the above-described prior art. In the technology described in Patent Literature 1, the lower limit of the upper yield stress is 400 MPa, and thus there is a problem that the strength of the tank body becomes insufficient when the steel sheet is thinned. In the technology described in Patent Literature 2, a steel sheet having high strength and high ductility with a yield stress of 500 MPa or more and a uniform elongation of 10% or more can be obtained, but since the yield elongation is allowed to be as high as 5.0% or less, there is a problem that wrinkles due to tensile strain occur. In addition, in order to ensure the strength when the steel sheet is used for the main body portion of the tank, a Rockwell surface hardness of a certain level or more is required, but neither Patent Literature 1 nor Patent Literature 2 describes the hardness.

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

[0009] Means for Solving the Problems The present application was made in order to solve the aforementioned problems, and the gist thereof is as follows.

[0010] [1] A steel sheet having a composition consisting of, 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, the balance being Fe and inevitable impurities, the aforementioned steel sheet has a microstructure of 80% or more of ferrite in terms of area fraction, the aforementioned steel sheet has 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.

[0011] [2] The steel sheet according to [1], wherein, in addition to the aforementioned component composition, 1 or more kinds selected from the following components are contained 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, V: 0.02% or less.

[0012] [3] The steel sheet according to [1] or [2], wherein, in addition to the aforementioned component composition, Sn: 0.020% or less is contained in mass %.

[0013] [4] A method for manufacturing a steel sheet according to any one of the aforementioned [1] to [3], comprising: a heating step of heating a steel raw material having the aforementioned component composition at 1150°C or more; a hot rolling step of hot rolling the steel raw material after the aforementioned heating step under conditions of a finish rolling temperature of 800°C or more and 950°C or less and a coiling temperature of 450°C or more and 700°C or less, and performing pickling; a cold rolling step of cold rolling a hot rolled sheet after the aforementioned hot rolling step under a rolling rate of 80% or more; and an annealing step of, after the cold rolled sheet after the aforementioned cold rolling step is held at an annealing temperature of 680°C or more and 780°C or less for 5 s or more and 90 s or less, cooling to a cooling stop temperature which is a temperature range of 600°C or less at an average cooling speed of 50°C / s or more.

[0014] Inventive Effects The present application enables the production of a steel sheet having high strength, high ductility, and low yield elongation. According to the present application, a can steel sheet can be further thinned, and thus the lightening of a can body can be achieved. Furthermore, not only is the ductility high and the workability excellent, but also the yield elongation is low, and thus the generation of wrinkles due to tensile strain is suppressed, and more complicated working can be performed on the can body. DETAILED DESCRIPTION

[0015] The component composition, structure, mechanical properties, and production conditions of the present application are described. The percentage in the description of the component composition indicates mass. In addition, the case where the tensile strength, yield stress, and HR30T are excellent is referred to as high strength.

[0016] C: 0.03% or more and 0.15% or less C is an element that contributes to the increase in yield stress, tensile strength, and HR30T. If the C content is less than 0.03%, the solid solution strengthening amount due to solid solution into ferrite and the precipitation strengthening amount due to the precipitation of fine carbides decrease, and the yield stress, tensile strength, and HR30T decrease, and thus 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 further 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 elongation at break decreases, and the yield elongation increases due to the increase in solid solution C. Thus, the C content needs to be 0.15% or less. In order to produce a steel sheet having 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.

[0017] Si: 0.05% or less Si is an element that contributes to the increase in yield stress and tensile strength, but if the content exceeds 0.05%, the corrosion resistance decreases. Thus, the Si content needs to be 0.05% or less, and the content is preferably 0.04% or less, more preferably 0.02% or less. The lower limit is not particularly limited, but in order to increase the strength of the steel sheet, the Si content is preferably 0.01% or more.

[0018] Mn: 0.10% or more and 0.60% or less Mn is an element that improves the quenching property and promotes the solid solution of C into ferrite. In addition, it is also known to contribute to the improvement of the yield stress, tensile strength and HR30T through solid solution strengthening by Mn itself. If the Mn content is less than 0.10%, sufficient yield stress, tensile strength and HR30T cannot be obtained, and therefore the Mn content is 0.10% or more. The Mn content is preferably 0.12% or more, more preferably 0.15% or more. In order to achieve both high strength and high ductility, the Mn content is further preferably 0.30% or more. The Mn content is most preferably 0.32% or more. On the other hand, if the Mn content exceeds 0.60%, the elongation at break decreases, and therefore the Mn content is 0.60% or less. The Mn content is preferably 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.

[0019] P: 0.025% or less P reduces the ductility by grain boundary segregation and hardening of the steel sheet, and therefore the P content is 0.025% or less. It is preferably 0.020% or less. The P content is more preferably 0.019% or less, and further preferably 0.018% or less. On the other hand, the lower limit is not particularly limited, but P contributes to the improvement of the yield stress and tensile strength of the steel sheet, and therefore it is preferable to contain 0.001% or more. The P content is more preferably 0.002% or more, and further preferably 0.003% or more. In order to increase the strength of the steel sheet, it is more preferably 0.010% or more.

[0020] S: 0.020% or less S reduces the ductility by forming sulfides such as MnS, CuS, TiS, and the like in the steel, and therefore the S content is 0.020% or less. The S content is preferably 0.018% or less. The S content is more preferably 0.017% or less. The S content is further preferably 0.016% or less, and most preferably 0.015% or less. The lower limit is not particularly limited, but in order to reduce the manufacturing load, the S content is preferably 0.005% or more. The S content is more preferably 0.006% or more.

[0021] Al: 0.20% or less Al is an element contained in order to remove oxygen in the steel. In addition, by forming AlN in the steel, the solid solution N is reduced, and the yield elongation is reduced. Therefore, the lower limit is not limited, but it is preferable to contain 0.02% or more. The Al content is more preferably 0.03% or more. On the other hand, if the Al content exceeds 0.20%, aluminum oxide is excessively generated, and the ductility decreases, and therefore the Al content is 0.20% or less. The Al content is preferably 0.15% or less, and more preferably 0.12% or less. In order to achieve both high strength and high ductility, the Al content is further preferably 0.09% or less. Note that the Al referred to here means the total Al amount.

[0022] N: 0.0001% or more and 0.0200% or less N is an element that contributes to an increase in yield stress, tensile strength by solid solution strengthening. Therefore, the N content is 0.0001% or more. The N content is preferably 0.0003% or more. The N content is more preferably 0.0010% or more. The N content is further preferably 0.0011% or more, and most preferably 0.0012% or more. On the other hand, if the N content exceeds 0.0200%, the yield elongation increases due to the solid solution N, and therefore the N content is 0.0200% or less. The N content is preferably 0.0150% or less, and more preferably 0.0100% or less. In order to achieve both high strength and low yield elongation, the N content is further preferably 0.0040% or less. The N content is most preferably 0.0035% or less.

[0023] Nb: 0.005% or more and 0.030% or less Nb is an element that contributes to precipitation strengthening and fine-grain strengthening by forming fine NbC in the steel. In order to ensure sufficient strength, the Nb content is 0.005% or more. The Nb content is preferably 0.007% or more. In order to achieve both high strength and high ductility, the Nb content is more preferably 0.010% or more. The Nb content is further preferably 0.011% or more, and the Nb content is most preferably 0.012% or more. On the other hand, if the Nb content exceeds 0.030%, it is difficult to ensure sufficient ductility due to an increase in recrystallization temperature. Therefore, the Nb content is 0.030% or less. In order to achieve both high strength and high ductility, the Nb content is preferably 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.

[0024] Cu: more than 0.020% and 0.200% or less Cu contributes to an increase in yield stress, tensile strength, and HR30T by solid solution strengthening, fine-grain strengthening, and precipitation strengthening. If the Cu content is 0.020% or less, sufficient strength is not obtained, and therefore 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. In order 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, if the Cu content exceeds 0.200%, it leads to a decrease in ductility and can also cause slab cracking due to segregation of Cu. Therefore, the Cu content is 0.200% or less. In order 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.

[0025] The steel sheet in the present application preferably contains, in addition to the above-described composition, one or more elements selected from the group consisting of the following elements.

[0026] 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: 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 quenchability, and Sn is an element that contributes to the high strength of the steel sheet by solid solution strengthening. On the other hand, the excessive inclusion of these elements decreases ductility. In order to simultaneously achieve sufficient quenchability and high strength and high ductility, in the case of containing Ni, the Ni content is 0.15% or less. The Ni content is preferably 0.14% or less, more preferably 0.13% or less. In the case of containing Mo, the Mo content is 0.05% or less. The Mo content is preferably 0.04% or less, more preferably 0.03% or less. In the case of containing Cr, the Cr content is 0.10% or less. The Cr content is preferably 0.08% or less, 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, further preferably 0.08% or more, most preferably 0.12% or more. The Mo content is preferably 0.01% or more, more preferably 0.03% or more. The Cr content is preferably 0.01% or more, more preferably 0.04% or more, further preferably 0.06% or more.

[0027] Ti: 0.02% or less, B: 0.02% or less Ti contributes to a decrease in yield elongation by reducing solid-solution N by forming TiN in the steel. In addition, since TiN is preferentially formed over BN, by containing both Ti and B, sufficient solid-solution B is ensured, and the hardenability is improved. On the other hand, if the Ti content exceeds 0.02% and the B content exceeds 0.02%, it is difficult to ensure sufficient ductility. In order to simultaneously achieve the improvement in hardenability by B and high ductility, in the case of containing Ti, 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 further preferably 0.016% or less. In addition, in the case of containing B, 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 further 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, further preferably 0.0018% or more, and most preferably 0.002% or more.

[0028] V: 0.02% or less V contributes to the promotion of the solid-solution of C into ferrite by improving solid-solution strengthening, precipitation strengthening, and hardenability, thereby increasing the yield stress, tensile strength, and HR30T. On the other hand, if the V content exceeds 0.02%, the ductility decreases with high strength. In order to simultaneously achieve high strength and high ductility, in the case of containing V, it is 0.02% or less. The V content is preferably 0.018% or less. The V content is more preferably 0.017% or less, and further preferably 0.016% or less. The lower limit is not particularly limited, but the V content is preferably 0.004% or more.

[0029] In addition, it is preferable to appropriately contain Sn: 0.020% or less in addition to the above components.

[0030] Sn: 0.020% or less Sn is an element that contributes to the high-strengthening of the steel sheet by solid-solution strengthening. In the case of containing Sn, the Sn content is 0.020% or less. The Sn content is preferably 0.018% or less. More preferably 0.015% or less, and further preferably 0.012% or less. The lower limit is not particularly limited, but the Sn content is preferably 0.001% or more. More preferably 0.002% or more, and further preferably 0.003% or more.

[0031] The steel sheet according to one embodiment of the present application has the following component composition: contains the above components, and the balance is Fe and inevitable impurities. Here, as the inevitable impurities, Ca, O, H, Co, W, Zn, Pb, As, Sb, Bi, Ca, O, and the like can be given.

[0032] The microstructure and mechanical properties of the steel sheet of the present application are described.

[0033] Ferrite: 80% or more in area fraction If the ferrite is less than 80% in area fraction, the ductility decreases, and therefore the ferrite is 80% or more in area fraction. In order to obtain a steel sheet having high ductility, the ferrite is preferably 85% or more in area fraction. The ferrite is more preferably 86% or more in area fraction, and further preferably 87% or more in area fraction. The upper limit is not particularly limited, but the ferrite is preferably 95% or less in area fraction. The ferrite is more preferably 94% or less in area fraction. Note that, although it is desirable to make the ferrite complete recrystallization, it is also possible to have unrecrystallized ferrite. In the case of having unrecrystallized ferrite, in order to obtain higher ductility, 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 lower limit is not particularly limited, but the unrecrystallized ferrite is preferably 0.1% or more in area fraction, and more preferably 0.2% or more in area fraction.

[0034] In addition, the average crystal grain diameter of the ferrite is preferably 4 μm or less in diameter of an equivalent circle. The average crystal grain diameter of the ferrite is more preferably 3.9 μm or less in diameter of an equivalent circle, and further preferably 3.8 μm or less in diameter of an equivalent circle. The lower limit is not particularly limited, but the average crystal grain diameter of the ferrite is preferably 1 μm or more in diameter of an equivalent circle.

[0035] The area of the unrecrystallized ferrite can also be added to the area fraction of the ferrite, and the sum of the area fractions of the recrystallized ferrite and the unrecrystallized ferrite is only required to be 80% or more, and thus the requirements of the present application are satisfied.

[0036] In addition, as the balance other than the ferrite, cementite, pearlite, bainite, martensite, residual austenite, and the like can be contained. The area fraction of the balance is only required to be 20% or less, and thus the requirements of the present application are satisfied, and the area fraction of the balance can also be 0%, but from the aspect of further increasing the strength, it is preferable to contain 0.1% or more of martensite. The martensite is more preferably 0.2% or more, and further preferably 0.3% or more. The upper limit of the martensite is preferably 20% or less.

[0037] 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 on the basis of thinning of the steel sheet for the can body portion, the yield stress (yield strength) of the steel sheet needs to be 500 MPa or higher, the tensile strength needs to be 500 MPa or higher, and the HR30T needs to be 68 or higher. Note that the yield stress is preferably 520 MPa or higher, more preferably 530 MPa or higher, and further preferably 540 MPa or higher. In addition, the tensile strength is preferably 550 MPa or higher, more preferably 555 MPa or higher, and further preferably 560 MPa or higher. In addition, the HR30T is preferably 70 or higher, more preferably 71 or higher, and further preferably 72 or higher. In order to ensure the workability of the steel sheet, the elongation at break needs to be 15% or higher, and more preferably 16% or higher. The elongation at break is further preferably 17% or higher, and most preferably 18% or higher. There is no particular upper limit, but the yield stress is preferably 700 MPa or lower, more preferably 695 MPa or lower, and further preferably 690 MPa or lower. The tensile strength is preferably 800 MPa or lower, more preferably 795 MPa or lower, and further preferably 790 MPa or lower. The HR30T is preferably 80 or lower, more preferably 79 or lower, and further preferably 78 or lower. The elongation at break is preferably 25% or lower, more preferably 24% or lower, and further preferably 23% or lower. In order to suppress the occurrence of wrinkles due to tensile strain at the time of can making and at the time of working on the can body, the yield elongation needs to be 4.5% or lower. The yield elongation is preferably 4.4% or lower, more preferably 4.3% or lower, and further preferably 3.0% or lower. There is no particular lower limit, but the yield elongation is preferably 1.0% or higher. The yield elongation is more preferably 1.1% or higher.

[0038] A method for manufacturing the steel sheet in the present application will be described.

[0039] The method for manufacturing the steel sheet in the present application is characterized by comprising: a heating step of heating a steel raw material having the above-described composition at 1150°C or higher; a hot rolling step of hot rolling the steel after the heating step at a finish rolling 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 performing pickling; a cold rolling step of cold rolling a hot-rolled sheet after the hot rolling step at a rolling rate of 80% or higher; and an annealing step of holding a cold-rolled sheet after the cold rolling step at an annealing temperature of 680°C or higher and 780°C or lower for 5 s or longer and 90 s or shorter, and cooling to a cooling stop temperature that is a temperature range of 600°C or lower.

[0040] Heating temperature: 1150°C or higher In the case where the heating temperature in the heating step is low, there is a possibility that coarse nitrides such as AlN are formed, and the strength and ductility of the steel sheet are reduced, and therefore the heating temperature is 1150°C or higher. The heating temperature is preferably 1170°C or higher, more preferably 1200°C or higher. The heating temperature is further preferably 1210°C or higher. The upper limit of the heating temperature is not limited, but from the viewpoint of manufacturing cost, it is preferably 1300°C or lower. The heating temperature is more preferably 1290°C or lower, further preferably 1270°C or lower.

[0041] Finish rolling temperature: 800°C or higher, 950°C or lower If the finish rolling temperature in the hot rolling step exceeds 950°C, the ferrite grain size of the hot-rolled sheet is coarsened, and the ferrite grain size of the steel sheet after the subsequent steps is also coarsened, and therefore it is difficult to ensure sufficient strength. Therefore, the finish rolling temperature is 950°C or lower. The finish rolling temperature is preferably 930°C or lower, more preferably 920°C or lower. The finish rolling temperature is further preferably 900°C or lower, most preferably 890°C or lower. On the other hand, in the case where the finish rolling temperature is lower than 800°C, rolling is performed in the two-phase region of ferrite and austenite, and therefore the strength of the steel sheet is reduced due to the generation of coarse ferrite grains, the precipitation of coarse Nb carbides in the hot rolling, and the like. Therefore, the finish rolling temperature in the hot rolling step is 800°C or higher. Note that the finish rolling temperature is preferably 830°C or higher, more preferably 850°C or higher.

[0042] Coiling temperature: 450°C or higher, 700°C or lower If the coiling temperature exceeds 700°C, the ferrite grain size is coarsened, and the strength of the steel sheet is reduced. In addition, since the formation of coarse alloy carbides is promoted, the cementite cannot be sufficiently dissolved in the annealing step, the solid solution amount of C into ferrite is reduced, and therefore this becomes a cause of reduction in strength. Therefore, the coiling temperature is 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, most preferably 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 carbides is reduced, and therefore the strength is reduced. Therefore, the coiling temperature is 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, most preferably 520°C or higher.

[0043] After coiling, pickling is performed with an aqueous solution of H2SO4, HC1, H3PO4, or the like, for the purpose of removing scale.

[0044] Rolling rate in cold rolling: 80% or higher After the foregoing hot rolling process, cold rolling is performed. By the cold rolling process, the ferrite grain size is refined, and the yield stress and the tensile strength are increased. In order to ensure sufficient yield stress and tensile strength, the reduction ratio is 80% or more. Note that the reduction ratio is preferably 82% or more, more preferably 85% or more. The reduction ratio is further preferably 86% or more, most preferably 87% or more. The upper limit of the reduction ratio is not limited, but in order to ensure sufficient ductility, it is preferably 95% or less. The reduction ratio is more preferably 94% or less.

[0045] Annealing temperature: 680°C or higher and 780°C or lower, holding time: 5 s or more and 90 s or less, cooling stop temperature: 600°C or lower, average cooling rate: 50°C / s or more After the foregoing cold rolling process, annealing is performed. In order to obtain sufficient ductility by recrystallization of ferrite, the annealing temperature is 680°C or higher. The annealing temperature is preferably 685°C or higher, more preferably 690°C or higher, further preferably 695°C or higher, most preferably 700°C or higher. On the other hand, if the annealing temperature exceeds 780°C, the yield stress and the tensile strength are reduced due to coarsening of the ferrite grain size and fine precipitates that contribute to precipitation strengthening. Therefore, the annealing temperature is 780°C or lower. Note that the annealing temperature is preferably 760°C or lower, more preferably 740°C or lower. The annealing temperature is further preferably 735°C or lower, most preferably 730°C or lower.

[0046] In a case where the holding time at the annealing temperature is less than 5 s, recrystallization of ferrite cannot sufficiently proceed, and desired ductility cannot be obtained. Therefore, the holding time is 5 s or more. The holding time is preferably 6 s or more, more preferably 7 s or more, further preferably 8 s or more, most preferably 9 s or more. On the other hand, in a case where the holding time is longer than 90 s, the ferrite grain size is coarsened, and the yield stress and the tensile strength are reduced. Therefore, the holding time at the annealing temperature is 90 s or less. The holding time is preferably 89 s or less, more preferably 88 s or less, further preferably 87 s or less, most preferably 86 s or less.

[0047] After the annealing holding, cooling is performed at an average cooling rate of 50°C / s or more to a temperature range of 600°C or lower. If the average cooling rate is less than 50°C / s, the ferrite grain size is coarsened, and the amount of solid-solution C in the ferrite is reduced, and thus the strength is reduced. Therefore, the average cooling rate is 50°C / s or more. The average cooling rate is preferably 60°C / s or more, more preferably 80°C / s or more. The average cooling rate is further preferably 85°C / s or more, most preferably 90°C / s or more. The upper limit is not particularly limited, but in order to reduce the manufacturing load, the average cooling rate is preferably 200°C / s or less. The average cooling rate is more preferably 190°C / s or less, further preferably 180°C / s or less.

[0048] If the cooling stop temperature after annealing exceeds 600°C, the ferrite grain size becomes coarse, and the strength decreases. Therefore, the cooling stop temperature is below 600°C. The cooling stop temperature is preferably below 595°C, more preferably below 590°C, and even more preferably below 585°C. On the other hand, even if the cooling stop temperature is below 300°C, a significant improvement in the steel sheet properties cannot be expected. Therefore, from the viewpoint of manufacturing cost, the cooling stop temperature is preferably above 300°C. The cooling stop temperature is preferably above 305°C, more preferably above 310°C, and even more preferably above 315°C.

[0049] Temper rolling can also be performed after the aforementioned annealing process. Temper rolling increases yield stress and reduces yield elongation; therefore, it is preferable to perform temper rolling with a reduction rate of 0.5% or more. In temper rolling, the reduction rate is more preferably 0.6% or more, further preferably 0.7% or more, and most preferably 0.8% or more. On the other hand, increasing the reduction rate in temper rolling reduces ductility; therefore, the reduction rate is preferably 10% or less. The reduction rate is more preferably 9% or less, further preferably 8% or less, and most preferably 7% or less.

[0050] Example The following are embodiments of the present invention. The present invention is not limited to the embodiments shown herein.

[0051] Steel containing the composition of grades 1 to 29 shown in Table 1, with the balance being Fe and unavoidable impurities, is smelted and cast to obtain steel billets. The steel billets obtained here are heated, hot-rolled, cold-rolled, and annealed under the conditions shown in Table 2, and then subjected to quenching and tempering rolling with a reduction of 1% to obtain steel plates No. 1 to 40.

[0052] The following steps were followed to observe the microstructure of the steel plate. After collecting test pieces from the steel plate, the section parallel to the rolling direction was ground and etched with nitric acid and ethanol to reveal the microstructure. Samples for microstructure observation were collected. A scanning electron microscope (SEM) was used to observe the microstructure at 3000x magnification at a position half the plate thickness in the thickness direction. Images were taken of the microstructure in three randomly selected fields of view. Table 3 shows the area fraction of ferrite in the SEM images measured using Image-J image processing software. It should be noted that the area fraction shown in Table 3 is the average of the three fields of view. It should also be noted that in the above SEM images, areas that can be observed as black blocks are identified as ferrite.

[0053] It should be noted that in the above SEM images, the portion of the white line that enters the grain is identified as non-recrystallized ferrite, and its area fraction is measured using Image-J to determine the non-recrystallized ferrite fraction.

[0054] The average ferrite crystal grain size was determined by the cutting method described in JIS G 0551, and was set as the average value of 3 fields of view.

[0055] From the aforementioned steel sheets, JIS No. 5 tensile test pieces with the orientation along the rolling direction as the tensile direction and test pieces of 30 mm square for Rockwell surface hardness measurement were collected, and in a thermostat, aging heat treatment was performed at 210°C for 10 minutes. Tensile tests were performed on the tensile test pieces in accordance with JIS Z 2241, and the yield stress, tensile strength, elongation at break, and yield elongation were evaluated. In addition, HR30T was determined using the Rockwell surface hardness of the HR15T measurement plate, and was converted using the conversion table of JIS G 3303 (2017) to obtain. The evaluation results of the yield stress, tensile strength, elongation at break, yield elongation, and HR30T are shown in Table 3.

[0056] In the invention examples in Table 3, the yield stress was 500 MPa or more, the tensile strength was 500 MPa or more, HR30T was 68 or more, the elongation at break was 15% or more, and the yield elongation was 4.5% or less. Therefore, it can be said that the invention examples are steel sheets suitable as can materials that are high in strength, high in ductility, and low in yield elongation. On the other hand, in the comparative examples in which one or more of the composition, the area fraction of the steel sheet structure, and the manufacturing conditions were outside the invention range, any one of the yield stress, the tensile strength, HR30T, the elongation at break, and the yield elongation was outside the invention range.

[0057] Among the above, in Steel Sheet No. 7 of Table 2, Table 3, the annealing temperature was low, and therefore the fraction of unrecrystallized ferrite was high, the ductility was low, and the elongation at break was outside the invention range (the preferred fraction of unrecrystallized ferrite is 30% or less, and No. 7 was 55%). In Steel Sheet No. 8, the slab heating temperature was low, the coiling temperature was high, and the annealing temperature was high, and therefore the ferrite grains were coarse and coarse alloy carbides that do not contribute to the improvement of strength were generated, and the yield stress and the tensile strength were reduced (the preferred average ferrite crystal grain size is 4 μm or less, and No. 8 was 6.0 μm). In Steel Sheet No. 9, the finish rolling temperature was low, the cooling rate was slow, and the cooling stop temperature was high, and therefore not only were the ferrite grains coarse, but also coarse Nb precipitates that do not contribute to the improvement of strength were generated, and the yield stress and the tensile strength were reduced (the preferred average ferrite crystal grain size is 4 μm or less, and No. 9 was 6.0 μm). In Steel Sheet No. 10, the finish rolling temperature was high, the coiling temperature was low, and the cold rolling rate was low, and therefore not only were the ferrite grains coarse, but also the amount of fine Nb precipitates was reduced, and the yield stress and the tensile strength were reduced (the preferred average ferrite crystal grain size is 4 μm or less, and No. 10 was 7.0 μm).

[0058] [Table 1] [table 2] [table 3]

Claims

1. A steel plate having the following composition, wherein the composition comprises, by mass%,: C: 0.03% to 0.15% Si: below 0.05% Mn: 0.10% to 0.60% P: below 0.025% S: Below 0.020% Al: below 0.20% N: 0.0001% to 0.0200% Nb: 0.005% to 0.030% Cu: exceeding 0.020% but below 0.200%, The balance of the composition is Fe and unavoidable impurities. The microstructure of the steel plate has ferrite with an area fraction of more than 80%. 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, an elongation at break of 15% or more, and a yield elongation of 4.5% or less.

2. The steel plate as described in claim 1, wherein, In addition to the ingredients described above, it also contains, by mass%, one or more ingredients selected from the following: Ni: below 0.15% Mo: 0.05% or less, Cr: less than 0.10% Ti: below 0.02% B: Below 0.02% V: Below 0.02%.

3. The steel plate as described in claim 1 or 2, wherein, In addition to the aforementioned components, it also contains less than 0.020% Sn by mass.

4. A method for manufacturing a steel plate, comprising the method for manufacturing a steel plate according to any one of claims 1 to 3, including: The heating process involves heating the steel raw material having the aforementioned composition at a temperature above 1150°C; The hot rolling process involves hot rolling the steel raw material after the heating process at a final rolling temperature of 800°C to 950°C and a winding temperature of 450°C to 700°C, followed by pickling. The cold rolling process involves cold rolling the hot-rolled plate after the hot rolling process under a rolling rate of 80% or higher. and In the annealing process, the cold-rolled sheet after the cold rolling process is held at an annealing temperature of 680°C to 780°C for 5 seconds to 90 seconds, and then cooled 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.

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