Steel sheet and method for manufacturing same

Abstract

The present invention provides a steel sheet having high strength and high ductility, and a method for manufacturing the same.　The steel sheet has a specific component composition and a structure mainly composed of ferrite. The amount of N present as AlN and the total amount of N satisfy relational expression (1), and the amount of Nb present as Nb precipitates and the total amount of Nb satisfy relational expression (2). The yield strength is 500 MPa or greater, the HR30T hardness is 66 or greater, and the total elongation is 7.0% or more. (1): (Amount of N present as AlN) / (total amount of N) ≤ 0.40 The total amount of N is the amount of all N included in the steel sheet. (2): 0.10 ≤ (amount of Nb present as Nb precipitates) / (total amount of Nb) ≤ 0.80 The total amount of Nb is the amount of all Nb included in the steel sheet.

Description

Title of Invention: STEEL SHEET AND METHOD FOR PRODUCING SAME Technical Field

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

[0002] In recent years, weight reduction of cans by downgauging the steel sheets for cans has been pursued in order to cut the CO2 emission during the transportation of the cans. However, downgauging the steel sheets decreases the can strength, and thus it is essential to increase the strength of the steel sheet in order to reduce the weight of cans and maintain the strength of the cans simultaneously.

[0003] Double reduced (DR) steel is known as high-strength steel sheets for cans. DR steel is a steel sheet for cans that has been strengthened by performing cold rolling again after previous cold rolling and annealing. However, the DR steel has a problem of low elongation and thus low workability. In order to solve this problem, it is effective to decrease the rolling reduction during the second cold rolling after the annealing; however, the concern is that decreasing the rolling reduction may decrease the strength of the steel sheet. In order to achieve both high strength and high ductility in a steel sheet for cans, it is important to increase the strength of the steel sheet by utilizing the strengthening mechanisms such as solid solution strengthening and precipitation strengthening, and to optimize the rolling reduction during the second cold rolling.

[0004] Patent Literature 1 proposes a steel sheet for cans, the steel sheet having a chemical composition containing, in mass%, C: 0.010% or more and 0.080% or less, Si: 0.05% or less, Mn: 0.10% or more and 0.70% or less, P: 0.03% or less, S: 0.020% or less, Al: 0.005% or more and 0.020% or less, N: 0.0120% or more and 0.0180% or less, and the balance Fe and inevitable impurities, in which Ar, which is the in-plane anisotropy of the r value, is -0.3 or more and 0.3 or less, and the tensile strength in the rolling direction after the aging process is 650 MPa or more.

[0005] Patent Literature 2 proposes a steel sheet for cans, the steel sheet having a microstructure that is substantially a ferrite single phase, and a composition containing, in mass%, C: 0.01 to 0.12%, Si: 0.005 to 0.5%, Mn: 0.3 to 1.5%, P: 0.005 to 0.2%, Al: 0.10% or less, N: 0.012% or less, Nb: 0.005 to 0.10%, and the balance Fe and inevitable impurities, in which the ferrite average crystal grain size is 7 pm or less, and, after the bake coating process, the yield strength is 500 MPa or more, the yield ratio is 0.9 or more, the total elongation is 10% or more, and Ar is -0.50 to 0. Citation List Patent Literature

[0006] PTL 1: International Publication No. 2020 / 203052 PTL 2: Japanese Unexamined Patent Application Publication No. 2008-214658 Summary of Invention Technical Problem

[0007] The aforementioned related art has the following problems. According to the related art disclosed in Patent Literature 1, second cold rolling needs to be performed at a rolling reduction of more than 20% but not more than 40% in order to obtain the target properties, and thus the concern is that the ductility of the steel sheet may decrease.

[0008] According to the related art disclosed in Patent Literature 2, the temper rolling ratio after continuous annealing needs to be adjusted to 1.5% or less in order to obtain the target properties, and thus the Rockwell superficial hardness of the steel sheet may become deficient.

[0009] In order to improve the strength of cans that include can bodies and can lids made of a steel sheet, it is necessary to obtain a particular Rockwell superficial hardness or higher by optimizing the rolling reduction during the second cold rolling of the steel sheet. However, both Patent Literature 1 and Patent Literature 2 lack the disclosures related to hardness.

[0010] An object of the present invention is to provide a high-strength and high-ductility steel sheet that resolves the problems described above, and a method for producing the same. Solution to Problem

[0011] The present invention has been made to solve the aforementioned problems and the gist thereof is as follows. [1] A steel sheet which comprises a chemical composition containing, in mass%: C: 0.010% or more and 0.060% or less, Si: 0.10% or less, Mn: 0.10% or more and 0.80% or less, P: 0.025% or less, S: 0.035% or less, Al: 0.100% or less, N: 0.0050% or more and 0.0120% or less, Cr: 0.010% or more and 0.150% or less, Nb: 0.003% or more and 0.030% or less, and the balance being Fe and inevitable impurities, and a microstructure composed mainly of ferrite. An amount of N present as AlN and a total amount of N satisfy formula (1) below, an amount of Nb present as Nb precipitates and a total amount of Nb satisfy formula (2) below, and the steel sheet has a yield strength of 500 MPa or more, HR30T of 66 or more, and a total elongation of 7.0% or more: (amount of N present as AlN) / (total amount of N) ^ 0.40 ••• (1) where the total amount of N is an total amount of N contained in the steel sheet, and 0.10 ^ (amount of Nb present as Nb precipitates) / (total amount of Nb) ^ 0.80 ••• (2) where the total amount of Nb is an total amount of Nb contained in the steel sheet. [2] The steel sheet described in [1], in which the chemical composition further contains, in mass%, at least one selected from: Cu: 0.30% or less, Sn: 0.03% or less, Ni: 0.15% or less, and Mo: 0.10% or less. [3] The steel sheet described in [1] or [2], in which the ferrite has an average crystal grain size of 8.0 pm or less. [4] A method for producing the steel sheet described in any one of [1] to [3] which comprises: a heating step of heating a steel material having the chemical composition at 1150°C or more; a hot rolling step, after the heating step, of hot rolling the steel material at a finishing temperature of 800°C or more and 950°C or less, cooling at an average cooling rate of 20°C / s or more from 800°C to a coiling temperature of 450°C or more and 700°C or less, and coiling at the coiling temperature; a first cold rolling step of cold rolling a hot rolled sheet obtained by the hot rolling step under a condition of a rolling reduction of 80% or more; an annealing step of holding a cold rolled sheet obtained by the first cold rolling step at an annealing temperature of 660°C or more and 850°C or less for 5 s or more and 90 s or less and then cooling at an average cooling rate of 15°C / s or more to a cooling stop temperature range of 600°C or less; and a second cold rolling step of cold rolling an annealed sheet obtained by the annealing step under a condition of a rolling reduction of 5% or more and 20% or less. Advantageous Effects of Invention

[0012] The present invention has made it possible to produce a steel sheet that is particularly suitable as a steel sheet for cans and that has high strength and high ductility. Since the present invention makes downgauging of steel sheets for cans possible, the cans become more light-weight, and thus the CO2 emission during the transportation of the cans can be decreased. Furthermore, since the steel sheet obtains high ductility by optimization of the rolling reduction during the second cold rolling, it has become possible to perform a more complicated process when the steel sheet is used to make can bodies and can lids. Description of Embodiments

[0013] The chemical composition, the microstructure, the mechanical properties, and the production conditions of the steel sheet according to the present invention will now be described. In the description of the chemical composition, % means mass%. In addition, in the present invention, the case in which a yield strength of 500 MPa or more and HR30T of 66 or more are obtained is considered high strength. The case in which a total elongation of 7.0% or more is obtained is considered high ductility.

[0014] C: 0.010% or more and 0.060% or less C is an element that contributes to improving the strength.  At a C content of less than 0.010%, the strength decreases due to coarsening of the ferrite grain size and the decrease in the amount of solute C; thus, the C content needs to be 0.010% or more. The C content is preferably 0.015% or more. The C content is more preferably 0.017% or more, yet more preferably 0.019% or more, and most preferably 0.020% or more. At a C content exceeding 0.060%, the ductility of the steel sheet decreases. Thus, the C content needs to be 0.060% or less. In order to achieve both high strength and high ductility in a steel sheet, the C content is preferably 0.050% or less. The C content is more preferably 0.045% or less, yet more preferably 0.040% or less, and most preferably 0.039% or less.

[0015] Si: 0.10% or less Si is an element that contributes to improving the strength, but excessive Si degrades the ductility and corrosion resistance of the steel sheet. Thus, the Si content needs to be 0.10% or less and is preferably 0.08% or less.  The Si content is more preferably 0.05% or less, yet more preferably 0.03% or less, and most preferably 0.02% or less.  Although there is no particular need to limit the lower limit, the Si content is preferably 0.01% or more to improve the strength of the steel sheet.

[0016] Mn: 0.10% or more and 0.80% or less Mn is an element that improves hardenability and promotes C to solidly dissolve into ferrite. Furthermore, Mn is known to contribute to improving the strength through the solid solution strengthening of Mn itself and reduction of ferrite grain size by the increase in Mn content. At a Mn content of less than 0.10%, the strength of the steel sheet is insufficient; thus, the Mn content is to be 0.10% or more. The Mn content is preferably 0.20% or more. The Mn content is more preferably 0.25% or more, yet more preferably 0.30% or more, and most preferably 0.35% or more. Meanwhile, at a Mn content exceeding 0.80%, the ductility of the steel sheet decreases; thus, the Mn content is to be 0.80% or less. The Mn content is preferably 0.70% or less. The Mn content is more preferably 0.65% or less, yet more preferably 0.63% or less, and most preferably 0.60% or less.

[0017] P: 0.025% or less P decreases the ductility and corrosion resistance of the steel sheet, and thus the P content is 0.025% or less. The P content is preferably 0.023% or less. The P content is more preferably 0.021% or less, yet more preferably 0.020% or less, and most preferably 0.018% or less. Meanwhile, P contributes to improving the strength of the steel sheet, and thus a P content of 0.001% or more is preferable. Although there is no particular need to limit the lower limit, the P content is preferably 0.005% or more to improve the strength of the steel sheet. The P content is more preferably 0.006% or more.

[0018] S: 0.035% or less S forms MnS in the steel and decreases the amount of Mn contributing to improving the strength; thus, the S content is to be 0.035% or less. The S content is preferably 0.030% or less.  The S content is more preferably 0.028% or less, yet more preferably 0.025% or less, and most preferably 0.020% or less.  Although there is no particular need to limit the lower limit, the S content is preferably 0.003% or more to decrease the production load in the desulfurization process. The S content is more preferably 0.005% or more and yet more preferably 0.006% or more.

[0019] Al: 0.100% or less Al is an element contained to remove oxygen in the steel. However, it is known that Al forms AlN in the steel and decreases the amount of solute N that contributes to improving the strength of the steel sheet. Thus, the Al content is to be 0.100% or less. The Al content is preferably 0.080% or less. The Al content is more preferably 0.075% or less, yet more preferably 0.070% or less, and most preferably 0.065% or less. Although there is no particular need to limit the lower limit, from the viewpoint of decreasing the production load in processing of Al-based inclusions, the Al content is preferably 0.010% or more, more preferably 0.012% or more, and still more preferably 0.015% or more.

[0020] N: 0.0050% or more and 0.0120% or less N is an element that contributes to improving the strength of the steel sheet through solid solution strengthening. Thus, the N content is to be 0.0050% or more. The N content is preferably 0.0060% or more. The N content is more preferably 0.0065% or more, yet more preferably 0.0070% or more, and most preferably 0.0071% or more. At a N content exceeding 0.0120%, the ductility of the steel sheet decreases. Thus, the N content is to be 0.0120% or less.  In order to achieve both high strength and high ductility in a steel sheet, the N content is preferably 0.0110% or less.  The N content is more preferably 0.0108% or less, yet more preferably 0.0105% or less, and most preferably 0.0101% or less.

[0021] Cr: 0.010% or more and 0.150% or less Cr accelerates solid dissolution of C into ferrite by improving the hardenability and improves the strength of the steel sheet; thus, the Cr content is to be 0.010% or more. The Cr content is preferably 0.020% or more. The Cr content is more preferably 0.025% or more, yet more preferably 0.030% or more, and most preferably 0.031% or more. Meanwhile, at a Cr content exceeding 0.150%, the precipitation amount of Cr nitrides increases, the amount of solute N that contributes to improving the strength decreases, and thus the strength of the steel sheet decreases. Thus, the Cr content is to be 0.150% or less. The Cr content is preferably 0.120% or less. The Cr content is more preferably 0.100% or less, yet more preferably 0.080% or less, and most preferably 0.070% or less.

[0022] Nb: 0.003% or more and 0.030% or less Nb is an element that contributes to precipitation strengthening and grain refinement strengthening through formation of fine NbC in the steel. In order to obtain a sufficient steel sheet strength, the Nb content is to be 0.003% or more. The Nb content is preferably 0.005% or more. The Nb content is more preferably 0.007% or more, yet more preferably 0.009% or more, and most preferably 0.011% or more. Meanwhile, at a Nb content exceeding 0.030%, the ductility of the steel sheet decreases due to the increase in recrystallization temperature after the cold rolling. Thus, the Nb content is to be 0.030% or less. In order to achieve both high strength and high ductility in a steel sheet, the Nb content is preferably 0.025% or less. The Nb content is more preferably 0.023% or less, yet more preferably 0.021% or less, and most preferably 0.019% or less.

[0023] In the present invention, at least one selected from the following elements may be contained in addition to the chemical composition described above.

[0024] Cu: 0.30% or less, Sn: 0.03% or less, Ni: 0.15% or less, and Mo: 0.10% or less Cu, Sn, Ni, and Mo improve the strength of the steel sheet through solid solution strengthening. However, when these elements are excessively contained, the ductility of the steel sheet decreases. Thus, when Cu, Sn, Ni, and / or Mo is to be contained, the Cu content is 0.30% or less, the Sn content is 0.03% or less, the Ni content is 0.15% or less, and the Mo content is 0.10% or less. In order to obtain sufficient ductility, the Cu content is preferably 0.25% or less, the Sn content is preferably 0.02% or less, the Ni content is preferably 0.12% or less, and the Mo content is preferably 0.08% or less. The Cu content is more preferably 0.22% or less, the Sn content is more preferably 0.01% or less, the Ni content is more preferably 0.10% or less, and the Mo content is more preferably 0.06% or less. The Cu content is yet more preferably 0.20% or less, the Ni content is yet more preferably 0.08% or less, and the Mo content is yet more preferably 0.04% or less. Although the lower limit is not particularly limited, the Cu content is preferably 0.01% or more, the Sn content is preferably more than 0%, the Ni content is preferably 0.01% or more, and the Mo content is preferably 0.01% or more.

[0025] A steel sheet according to an embodiment of the present invention has a chemical composition that contains the aforementioned components and the balance Fe and inevitable impurities. Examples of the inevitable impurities include Ca, O, H, Ti, Co, W, Zn, Pb, As, Sb, and Bi.

[0026] The microstructure and the mechanical properties of the steel sheet according to the present invention will now be described.

[0027] Microstructure composed mainly of ferrite The steel sheet according to the present invention has a microstructure composed mainly of ferrite. Here, a microstructure is considered to be composed mainly of ferrite when the area ratio of ferrite is 70% or more. The ferrite area ratio is preferably 70% or more. The ferrite area ratio is more preferably 80% or more, yet more preferably 85% or more, and most preferably 90% or more. The upper limit is not particularly limited and may be 100%. The remaining portion other than ferrite may include cementite, pearlite, bainite, martensite, and retained austenite. The requirement of the present invention is considered satisfied as long as the area ratio of the remaining portion is 30% or less, and the area ratio of the remaining portion may be 0%.

[0028] As described below, ferrite is preferably recrystallized by annealing, and it is preferable to obtain recrystallized ferrite. Moreover, since the strength increases smoothly by obtaining fine ferrite, the average crystal grain size of ferrite is preferably 8.0 pm or less. The average crystal grain size of ferrite is more preferably 7.5 pm or less and yet more preferably 7.0 pm or less. Although the lower limit is not particularly limited, the average crystal grain size of ferrite is preferably 3.0 pm or more, more preferably 3.5 pm or more, and yet more preferably 4.0 pm or more.

[0029] Amount of N present as AlN and total amount of N satisfy formula (1) (Amount of N present as AlN) / (total amount of N) ^ 0.40 ••• (1) The total amount of N is the total amount of N contained in the steel sheet. In order to increase the strength of the steel sheet, it is critical to suppress the ratio of the amount of N present as AlN (N as AlN) to the total amount of N and to secure the solute N that contributes to strength. The amount of N present as AlN refers to the amount of N contained in AlN. When formula (1) is not satisfied, the amount of solute N becomes deficient, and the strength of the steel sheet decreases. Thus, (N as AlN) / (total amount of N) is to be 0.40 or less. To increase the strength of the steel sheet, (amount of N as AlN) / (total amount of N) is preferably 0.35 or less. The ratio is more preferably 0.32 or less, more preferably 0.30 or less, yet more preferably 0.28 or less, and most preferably 0.25 or less. Although there is no particular need to limit the lower limit, the ratio is preferably 0.01 or more to improve the ductility of the steel sheet. The ratio is more preferably 0.02 or more and yet more preferably 0.03 or more. As described below, precipitation of AlN is affected by the coiling temperature and the cooling rate from 800°C to the coiling temperature; thus, the coiling temperature and the average cooling rate from 800°C to the coiling temperature are controlled so that the scope of the present invention expressed by formula (1) is satisfied. It should be noted that the amount of N obtained by subtracting the amount of N contained in AlN from the total amount of N is the amount of solute N, and thus the amount of solute N is preferably (amount of N present as solute N) / (total amount of N) ^ 0.60. The (amount of N present as solute N) / (total amount of N) is more preferably 0.65 or more, yet more preferably 0.68 or more, and most preferably 0.70 or more. Although the upper limit is not particularly limited, the ratio is preferably 0.99 or less, more preferably 0.98 or less, and yet more preferably 0.97 or less. The value of (amount of N present as solute N) / (total amount of N) tends to increase with the increase in the average cooling rate from 800°C to the coiling temperature.

[0030] Amount of Nb present as Nb precipitates satisfies formula (2) 0.10 < (Amount of Nb present as Nb precipitates) / (total amount of Nb) < 0.80 ••• (2) The total amount of Nb is the total amount of Nb contained in the steel sheet. In order to increase the strength of the steel sheet through precipitation strengthening by addition of Nb, it is critical to increase the ratio of the amount of Nb present as Nb precipitates to the total amount of Nb. The amount of Nb present as Nb precipitates refers to the amount of Nb contained in the Nb precipitates. When the value of (amount of Nb present as Nb precipitates) / (total amount of Nb) is less than 0.10, there is not enough Nb contributing to the precipitation strengthening, and the strength of the steel sheet becomes deficient. Thus, (amount of Nb present as Nb precipitates) / (total amount of Nb) is to be 0.10 or more. This value is preferably 0.15 or more. The value is more preferably 0.17 or more, yet more preferably 0.20 or more, and most preferably 0.22 or more. When the value of (amount of Nb present as Nb precipitates) / (total amount of Nb) exceeds 0.80, the ductility of the steel sheet decreases. Thus, (amount of Nb present as Nb precipitates) / (total amount of Nb) is to be 0.80 or less. This value is preferably 0.75 or less. The value is more preferably 0.73 or less, yet more preferably 0.70 or less, still more preferably 0.68 or less, and most preferably 0.60 or less. Here, the Nb precipitates include all of Nb carbides, Nb nitrides, and Nb-containing carbonitrides. As described below, precipitation of Nb is affected by the coiling temperature and the average cooling rate from 800°C to the coiling temperature; thus, the coiling temperature and the average cooling rate from 800°C to the coiling temperature are controlled so that the scope of the present invention expressed by formula (2) is satisfied.

[0031] Yield strength of 500 MPa or more, HR30T of 66 or more, and total elongation of 7.0% or more In order to secure a sufficient can strength with a steel sheet of a lower gauge, the yield strength of the steel sheet needs to be 500 MPa or more and HR30T needs to be 66 or more. The yield strength is preferably 520 MPa or more and HR30T is preferably 67 or more. The yield strength is more preferably 530 MPa or more and HR30T is more preferably 68 or more. The yield strength is yet more preferably 540 MPa or more and HR30T is yet more preferably 69 or more; and the yield strength is most preferably 550 MPa or more and HR30T is most preferably 70 or more. There is no particular need to limit the upper limit, but the yield strength is preferably 700 MPa or less and HR30T is preferably 80 or less. Furthermore, in order to obtain sufficient workability, the total elongation of the steel sheet is to be 7.0% or more. The total elongation is preferably 7.5% or more. The total elongation is more preferably 8.0% or more, yet more preferably 8.5% or more, and most preferably 9.0% or more. There is no particular need to limit the upper limit, but the total elongation is preferably 25.0% or less.

[0032] The aforementioned yield strength, total elongation, and HR30T are determined by testing methods described in Examples, and are the properties after the obtained steel sheet is subjected to aging heat treatment at 210°C for 10 minutes.

[0033] The method for producing the steel sheet according to the present invention will now be described.

[0034] A method for producing a steel sheet according to the present invention is characterized by including a heating step in which a steel material having the chemical composition described above is heated at 1150°C or more; a hot rolling step in which the steel material after the heating step is hot-rolled at a finishing temperature of 800°C or more and 950°C or less, cooled at an average cooling rate of 20°C / s or more from 800°C to a coiling temperature of 450°C or more and 700°C or less, and coiled under a condition of 450°C or more and 700°C or less; a first cold rolling step in which a hot rolled sheet, which is obtained by the hot rolling step, is cold-rolled under a condition of a rolling reduction of 80% or more; an annealing step in which a cold rolled sheet obtained by the first cold rolling step is held at an annealing temperature of 660°C or more and 850°C or less for 5 s or more and 90 s or less and then cooled at an average cooling rate of 15°C / s or more to a cooling stop temperature range of 600°C or less; and a second cold rolling step in which an annealed sheet, which is obtained by the annealing step, is cold- rolled under a condition of a rolling reduction of 5% or more and 20% or less. It should be noted that the steel sheet temperature described in the present invention refers to the temperature at the steel sheet surface unless otherwise noted, and is measured with a radiation thermometer.

[0035] Hot rolling step Heating temperature: 1150°C or more When the heating temperature in the heating step is low, coarse nitrides such as AlN are formed, and the amount of solute N that contributes to improving the strength of the steel sheet decreases. Thus the heating temperature is 1150°C or more. The heating temperature is preferably 1170°C or more, more preferably 1180°C or more, yet more preferably 1190°C or more, and most preferably 1200°C or more.  Although there is no particular need to limit the upper limit of the heating temperature, from the production cost viewpoint, the heating temperature is preferably 1300°C or less, more preferably 1290°C or less, yet more preferably 1280°C or less, and most preferably 1270°C or less.

[0036] Finishing temperature: 800°C or more and 950°C or less When the finishing temperature in the hot rolling step exceeds 950°C, ferrite grain size coarsens, and the strength of the steel sheet decreases. Thus, the finishing temperature is 950°C or less. The finishing temperature is preferably 940°C or less. The finishing temperature is more preferably 930°C or less, yet more preferably 920°C or less, still more preferably 910°C or less, and most preferably 900°C or less. Meanwhile, when the finishing temperature is less than 800°C, coarse ferrite grains occur during rolling, and coarse Nb(C,N) precipitates during hot rolling. Coarse Nb(C,N) does not contribute to improving the strength of the steel sheet and causes the strength of the steel sheet to decrease since coarse Nb(C,N) decreases the amounts of solute C, solute N, and fine Nb precipitates that improve the strength of the steel sheet. Thus, the finishing temperature in the hot rolling step is 800°C or more. The finishing temperature is preferably 830°C or more. The finishing temperature is more preferably 840°C or more, yet more preferably 845°C or more, and most preferably 850°C or more.

[0037] Average cooling rate from 800°C to coiling temperature of 450°C or more and 700°C or less: 20°C / s or more When the average cooling rate from 800°C to a coiling temperature of 450°C or more and 700°C or less is less than 20°C / s, the precipitation amount of AlN increases, the amount of Nb present as Nb precipitates decreases, and the ferrite grains coarsen, thereby decreasing the strength of the steel sheet. Thus, the average cooling rate to the coiling temperature is to be 20°C / s or more. The average cooling rate is preferably 25°C / s or more to increase the strength of the steel sheet. The average cooling rate is more preferably 30°C / s or more, yet more preferably 35°C / s or more, and most preferably 40°C / s or more. Although there is no particular need to limit the upper limit of the average cooling rate to the coiling temperature, from the viewpoint of reducing the production load, the average cooling rate is preferably 80°C / s or less, more preferably 75°C / s or less, and yet more preferably 70°C / s or less. Here, the average cooling rate can be determined by dividing the temperature difference between the cooling start temperature (800°C in this case) and the cooling stop temperature (coiling temperature in this case) by the cooling time taken for this cooling.

[0038] Coiling temperature: 450°C or more and 700°C or less When the coiling temperature exceeds 700°C, the ferrite grains in the steel sheet coarsen, the amount of solute C decreases due to accelerated formation of coarse alloy carbides, and the amount of solute N decreases due to accelerated formation of coarse alloy nitrides, resulting in a decrease in strength of the steel sheet.  Thus, the coiling temperature is to be 700°C or less. The coiling temperature is preferably 650°C or less. The coiling temperature is more preferably 630°C or less, yet more preferably 625°C or less, and most preferably 620°C or less. Meanwhile, when the coiling temperature is less than 450°C, the precipitation amount of fine alloy carbides, such as Nb carbides, decreases, resulting in a decrease in strength of the steel sheet. Thus, the coiling temperature is to be 450°C or more. The coiling temperature is preferably 500°C or more. The coiling temperature is more preferably 530°C or more, yet more preferably 540°C or more, and most preferably 550°C or more. After coiling, pickling may be performed with an aqueous solution such as H2SO4, HCl, or H3PO4 to remove scales.

[0039] First cold rolling step Rolling reduction in cold rolling: 80% or more After the aforementioned hot rolling step, cold rolling (first cold rolling) is performed. Strains that accumulate by this cold rolling step act as a driving force to accelerate the recrystallization of ferrite in the subsequent annealing step; thus, ferrite grains become finer, and the strength of the steel sheet increases. In order to obtain a sufficient strength, the rolling reduction is to be 80% or more. The rolling reduction is preferably 85% or more. The rolling reduction is more preferably 86% or more, yet more preferably 87% or more, and most preferably 88% or more. Although there is no particular need to limit the upper limit of the rolling reduction, in order to obtain sufficient ductility, the rolling reduction is preferably 95% or less, more preferably 93% or less, yet more preferably 92% or less, and most preferably 91% or less.

[0040] Annealing step Annealing temperature: 660°C or more and 850°C or less, holding time: 5 s or more and 90 s or less, cooling stop temperature: 600°C or less, and average cooling rate to cooling stop temperature: 15°C / s or more After the aforementioned cold rolling step, annealing is performed. In order to obtain sufficient ductility by accelerating recrystallization of ferrite, the annealing temperature is to be 660°C or more. The annealing temperature is preferably 680°C or more. The annealing temperature is more preferably 700°C or more, yet more preferably 710°C or more, and most preferably 720°C or more. Meanwhile, at an annealing temperature exceeding 850°C, ferrite grains and fine precipitates that contribute to precipitation strengthening coarsen, resulting in a decrease in the strength of the steel sheet.  Thus, the annealing temperature is to be 850°C or less. The annealing temperature is preferably 830°C or less, more preferably 800°C or less, yet more preferably 780°C or less, and most preferably 760°C or less.

[0041] When the time for which the annealing temperature is held is less than 5 s, the recrystallization of ferrite does not sufficiently occur, and sufficient ductility cannot be obtained. Thus, the holding time is to be 5 s or more. The holding time is preferably 8 s or more, more preferably 10 s or more, yet more preferably 12 s or more, and most preferably 14 s or more. Meanwhile, when the holding time is longer than 90 s, the ferrite grain size coarsens, and the strength of the steel sheet decreases. Thus, the time for which the annealing temperature is held is to be 90 s or less. The holding time is preferably 85 s or less, more preferably 80 s or less, yet more preferably 75 s or less, still more preferably 70 s or less, and most preferably 65 s or less.

[0042] When the cooling stop temperature after holding the annealing exceeds 600°C, the precipitation amount of cementite becomes excessive, and the amount of solute C that contributes to improving the strength of the steel sheet becomes deficient. Thus, the cooling stop temperature is to be 600°C or less. The cooling stop temperature is preferably 590°C or less, more preferably 580°C or less, yet more preferably 575°C or less, and most preferably 570°C or less. Although there is no particular need to limit the lower limit of the cooling stop temperature, from the production cost viewpoint, the cooling stop temperature is preferably 150°C or more. The cooling stop temperature is more preferably 300°C or more, yet more preferably 320°C or more, and most preferably 350°C or more. Furthermore, in order to improve the ductility of the steel sheet, the sheet may be held in a temperature range of 150°C or more and 600°C or less after the cooling is stopped. In order to achieve both strength and ductility in a steel sheet by securing a sufficient amount of solute C, the time for which the sheet is held in a temperature range of 150°C or more and 600°C or less is preferably 300 s or less, more preferably 280 s or less, still more preferably 260 s or less, yet more preferably 240 s or less, and most preferably 180 s or less. The lower limit of the holding time is not particularly limited and may be 0 s; however, for improving the ductility of the steel sheet, the hold preferably lasts 5 s or more, more preferably 10 s or more, yet more preferably 15 s or more, still more preferably 20 s or more, and most preferably 30 s or more.

[0043] When the average cooling rate to the cooling stop temperature is less than 15°C / s, the amount of solute C becomes deficient, and the strength of the steel sheet decreases. Thus, the average cooling rate after the annealing is to be 15°C / s or more. The average cooling rate is preferably 20°C / s or more. The average cooling rate is more preferably 25°C / s or more, yet more preferably 30°C / s or more, still more preferably 35°C / s or more, and most preferably 40°C / s or more. Although there is no particular need to limit the upper limit, in order to reduce the production load, the average cooling rate is preferably 200°C / s or less, more preferably 180°C / s or less, yet more preferably 160°C / s or less, and most preferably 120°C / s or less. Here, the average cooling rate can be determined by dividing the temperature difference between the cooling start temperature (annealing temperature in this case) and the cooling stop temperature by the cooling time taken for this cooling.

[0044] Second cold rolling step Rolling reduction in second cold rolling: 5% or more and 20% or less After the annealing step, second cold rolling is performed.  When the rolling reduction in the second cold rolling is less than 5%, the strength of the steel sheet decreases.  Thus, the rolling reduction is to be 5% or more. The rolling reduction is preferably 6% or more. The rolling reduction is more preferably 7% or more and most preferably 8% or more. Meanwhile, when the rolling reduction in the second cold rolling exceeds 20%, the ductility of the steel sheet decreases. Thus, the rolling reduction is to be 20% or less. The rolling reduction is preferably 18% or less. The rolling reduction is more preferably 15% or less, yet more preferably 12% or less, still more preferably 11% or less, and most preferably 10% or less. EXAMPLES

[0045] Examples of the present invention are described below. The present invention is not limited to the examples below.

[0046] Steels containing the components of Steel Grade Nos. 1 to 17 shown in Table 1 with the balance Fe and inevitable impurities were melted and cast to obtain steel slabs. The obtained steel slabs were heated, hot-rolled, cold-rolled for the first time, annealed, and cold-rolled for the second time under the conditions shown in Table 2 to obtain steel sheets Nos. 1 to 18.

[0047] From each of the steel sheets, a JIS No. 5 test piece for a tensile test in which the tensile direction was the direction along the rolling direction, and a 30-mm-square test specimen to be used in Rockwell superficial hardness measurement were taken, and were subjected to aging heat treatment at 210°C for 10 minutes in a thermostat. The test piece for a tensile test was subjected to a tensile test according to JIS Z 2241 to evaluate the yield strength and the total elongation. Moreover, HR30T was determined by measuring the Rockwell superficial hardness of the sheet surface with HR15T and then converting the result using a conversion table of JIS G 3303 (2017). Table 3 shows the evaluation results of the yield strength, HR30T, and the total elongation.

[0048] The amount of N present as AlN (amount of N as AlN) and the amount of Nb present as Nb precipitates were measured by taking samples from the aforementioned steel sheets and analyzing the extraction residues. The amount of N present as AlN was measured by performing Br methanol extraction, H2SO4 + K2SO4 decomposition, and alkali water vapor distillation, and then performing bis-pyrazolone spectrophotometry. The amount of Nb present as Nb precipitates was determined by performing AA electrolytic extraction, filter collection, and mixed acid decomposition and then performing ICP-AES measurement. Table 3 shows the calculation results of formula (1) and formula (2) below. (Amount of N present as AlN) / (total amount of N) ^ 0.40 ••• (1) Here, the total amount of N is the total amount of N contained in the steel sheet. 0.10 ^ (Amount of Nb present as Nb precipitates) / (total amount of Nb) ^ 0.80 ••• (2) Here, the total amount of Nb is the total amount of Nb contained in the steel sheet. The amount of solute N was determined by subtracting the amount of N present as AlN from the total amount of N.

[0049] Microstructure observation The steel sheet microstructure was observed by the following procedure. After a test specimen was taken from the steel sheet, a cross section parallel to the rolling direction was polished and was etched with Nital to reveal the microstructure, and then a sample for microstructure observation was taken. A position at 1 / 2 in the sheet thickness direction was observed with a scanning electron microscope (SEM) at an acceleration voltage of 15 kV and a magnification of 1500x, and the microstructure was photographed in three fields of view selected at random. Table 3 shows the ferrite area ratio in a SEM image measured with image processing software, Image-J. The area ratio shown in Table 3 was an average value of three fields of view. In the SEM image, a region that can be observed as a black lump was identified as ferrite.

[0050] The average crystal grain size of ferrite was measured by the following procedure. After a test specimen was taken from the center of the steel sheet in the sheet width direction, a cross section parallel to the rolling direction and parallel to the sheet thickness direction was polished so that an observation surface was prepared at the position 1 / 2 of the sheet thickness of the steel sheet, and then was etched with Nital to reveal the microstructure, thereby preparing a sample for microstructure observation. The aforementioned subject surface was observed with an optical microscope at a magnification of 200x to 500x, and the microstructure was photographed in three fields of view selected at random. The ferrite average crystal grain size was determined by the intercept method set forth in JIS G 0551 and was an average of three fields of view.

[0051] In all Examples in Table 3, the yield strength was 500 MPa or more, HR30T was 66 or more, and the total elongation was 7.0% or more. Thus, Examples are considered to be steel sheets that have high strength and high ductility and that are suitable for use as a material for cans.

[0052] In contrast, in Comparative Examples, at least one of the yield strength, HR30T, and the total elongation was outside the scope of the invention.

[0053] Steel grade No. Chemical composition (mass%) Note C Si Mn P S Al N Cr Nb Cu Sn Ni Mo 1 0.026 0.02 0.57 0.015 0.007 0.016 0.0095 0.045 0.015 - - - - Example steel 2 0.018 0.01 0.49 0.014 0.008 0.017 0.0087 0.043 0.012 - - - 0.09 Example steel 3 0.022 0.03 0.56 0.021 0.012 0.032 0.0098 0.062 0.018 0.28 - - - Example steel 4 0.058 0.01 0.55 0.014 0.009 0.095 0.0052 0.147 0.029 - 0.02 - - Example steel 5 0.011 0.03 0.77 0.013 0.010 0.018 0.0103 0.023 0.018 - - 0.14 - Example steel 6 0.031 0.08 0.11 0.024 0.033 0.010 0.0118 0.012 0.009 0.13 0.01 0.06 0.03 Example steel 7 0.062 0.11 0.83 0.018 0.011 0.025 0.0111 0.038 0.026 0.31 0.04 - - Comparative steel 8 0.007 0.02 0.08 0.011 0.036 0.051 0.0066 0.044 0.002 0.08 - 0.01 - Comparative steel 9 0.054 0.06 0.44 0.027 0.014 0.022 0.0079 0.033 0.032 - - 0.17 0.12 Comparative steel 10 0.049 0.02 0.35 0.013 0.013 0.103 0.0122 0.151 0.014 0.02 - - 0.02 Comparative steel 11 0.024 0.01 0.22 0.011 0.020 0.083 0.0047 0.008 0.006 0.03 - 0.03 - Comparative steel 12 0.031 0.02 0.61 0.011 0.007 0.058 0.0108 0.036 0.004 - 0.01 0.11 0.05 Example steel 13 0.032 0.01 0.44 0.011 0.012 0.095 0.0100 0.131 0.018 0.05 - 0.02 0.01 Example steel 14 0.005 0.01 0.55 0.009 0.008 0.102 0.0149 0.154 0.001 - - - - Comparative steel 15 0.032 0.01 0.44 0.011 0.009 0.092 0.0110 0.142 0.011 - - 0.01 0.02 Example steel 16 0.057 0.03 0.61 0.021 0.011 0.024 0.0099 0.051 0.027 - 0.01 0.02 - Example steel 17 0.016 0.02 0.33 0.006 0.013 0.038 0.0086 0.021 0.008 - - - - Example steel Underlines: Items that are outside the scope of the present invention

[0054] Steel sheet No. Production condition No. Steel grade No. Hot rolling step First cold rolling step Annealing step Second cold rolling step Note Heating temperature (°C) Finishing temperature (°C) Average cooling rate to coiling temperature (°C / s) Coiling temperature (°C) Rolling reduction in first cold rolling (%) Annealing temperature (°C) Holding time (s) Average cooling rate to cooling stop temperature (°C / s) Cooling stop temperature (°C) Rolling reduction in second cold rolling (%) 1 1 1 1170 900 20 600 89 720 21 55 570 10 Example 2 2 2 1200 940 40 580 91 680 22 40 600 12 Example 3 3 3 1180 880 30 610 85 740 25 40 570 6 Example 4 4 4 1250 890 80 560 81 840 15 85 520 5 Example 5 5 5 1230 870 20 630 88 710 60 15 580 19 Example 6 6 6 1280 920 30 460 90 700 20 60 590 8 Example 7 7 7 1300 880 45 590 85 660 10 70 550 18 Comparative Example 8 8 8 1280 930 25 680 82 840 88 20 580 6 Comparative Example 9 9 9 1200 880 90 600 90 690 18 30 570 11 Comparative Example 10 10 10 1160 820 30 570 82 720 21 40 560 8 Comparative Example 11 11 11 1290 870 25 470 90 820 45 20 580 7 Comparative Example 12 12 12 1200 820 25 680 88 700 22 30 570 9 Example 13 13 13 1140 790 15 430 79 700 91 50 620 8 Comparative Example 14 14 13 1250 880 75 600 89 740 20 100 560 9 Example 15 15 14 1160 830 40 470 85 710 20 40 560 7 Comparative Example 16 16 15 1170 960 30 720 88 860 21 10 580 4 Comparative Example 17 17 16 1200 830 60 600 85 640 4 60 500 21 Comparative Example 18 18 17 1180 910 35 640 90 710 60 35 580 7 Example Underlines: Items that are outside the scope of the present invention

[0055] Steel sheet No. Steel grade No. Microstructure Ferrite area ratio (%) Ferrite average crystal grain size (pm) (Amount of N present as AIN) / (total amount ofN) <0.40 formula (1)* 0.10 < (Amount of Nb present as Nb precipitates) / (total amount of Nb) < 0.80 formula (2)** (Amount of solute N) / (total amount of N) Yield strength (MPa) HR30T Total elongation (%) Note 1 1 a+0 98 5.4 0.05 0.30 0.95 551 72 10.8 Example 2 2 a+0 96 6.7 0.04 0.30 0.96 532 68 12.9 Example 3 3 a+P+0 94 7.1 0.08 0.34 0.92 521 69 13.1 Example 4 4 a+P+0 88 4.5 0.10 0.42 0.90 536 70 12.7 Example 5 5 a 100 6.3 0.21 0.71 0.79 588 73 7.2 Example 6 6 a+0 98 6.3 0.10 0.19 0.90 565 71 9.6 Example 7 7 a+0 79 2.9 0.14 0.85 0.86 692 77 3.1 Comparative Example 8 8 a+0 99 8.3 0.41 0.06 0.59 454 63 26.0 Comparative Example 9 9 a+0 96 3.7 0.06 0.82 0.94 601 76 5.3 Comparative Example 10 10 a+0 97 4.2 0.69 0.52 0.31 469 64 12.1 Comparative Example 11 11 a+0 92 6.8 0.11 0.09 0.89 478 65 13.7 Comparative Example 12 12 a+0 98 6.7 0.36 0.35 0.64 529 71 9.2 Example 13 13 a+0 90 8.5 0.65 0.22 0.35 462 64 14.6 Comparative Example 14 13 a+P+0 94 5.2 0.24 0.36 0.76 518 71 9.6 Example 15 14 a+0 99 8.1 0.72 0.05 0.28 450 63 14.1 Comparative Example 16 15 a+P+0 93 8.2 0.75 0.48 0.25 466 63 19.2 Comparative Example 17 16 a+0 90 2.8 0.12 0.83 0.88 688 77 1.2 Comparative Example 18 17 a+0 98 8.3 0.36 0.13 0.64 504 67 13.2 Example Underlines: Items that are outside the scope of the present invention a: ferrite, 0: cementite, P: pearlite *: (Amount of N present as AIN) / (total amount of N) in formula (1) **: (Amount of Nb present as Nb precipitates) / (total amount of Nb) in formula (2)

Claims

1. A steel sheet comprising a chemical composition containing, in mass%:C: 0.010% or more and 0.060% or less,Si: 0.10% or less,Mn: 0.10% or more and 0.80% or less,P: 0.025% or less,S: 0.035% or less,Al: 0.100% or less,N: 0.0050% or more and 0.0120% or less,Cr: 0.010% or more and 0.150% or less,Nb: 0.003% or more and 0.030% or less, andthe balance being Fe and inevitable impurities, anda microstructure composed mainly of ferrite, wherein:an amount of N present as AlN and a total amount of Nsatisfy formula (1) below,an amount of Nb present as Nb precipitates and a totalamount of Nb satisfy formula (2) below, andthe steel sheet has a yield strength of 500 MPa or more, HR30T of 66 or more, and a total elongation of 7.0% ormore:(amount of N present as AlN) / (total amount of N) ^ 0.40  •••  (1)where the total amount of N is an total amount of N contained in the steel sheet, and0.10 ^ (amount of Nb present as Nbprecipitates) / (total amount of Nb) ^ 0.80 ••• (2)where the total amount of Nb is an total amount of Nb contained in the steel sheet.

2. The steel sheet according to Claim 1, wherein the chemical composition further contains, in mass%, at least one selected from:Cu: 0.30% or less,Sn: 0.03% or less,Ni: 0.15% or less, andMo: 0.10% or less.

3. The steel sheet according to Claim 1 or 2, wherein the ferrite has an average crystal grain size of 8.0 pm or less.

4. A method for producing the steel sheet according to any one of Claims 1 to 3, the method comprising:a heating step of heating a steel material having the chemical composition at 1150°C or more;a hot rolling step, after the heating step, of hotrolling the steel material at a finishing temperature of800°C or more and 950°C or less, cooling at an averagecooling rate of 20°C / s or more from 800°C to a coiling temperature of 450°C or more and 700°C or less, and coiling at the coiling temperature;a first cold rolling step of cold rolling a hot rolled sheet obtained by the hot rolling step under a condition ofa rolling reduction of 80% or more;an annealing step of holding a cold rolled sheet obtained by the first cold rolling step at an annealingtemperature of 660°C or more and 850°C or less for 5 s ormore and 90 s or less and then cooling at an average coolingrate of 15°C / s or more to a cooling stop temperature rangeof 600°C or less; anda second cold rolling step of cold rolling an annealedsheet obtained by the annealing step under a condition of arolling reduction of 5% or more and 20% or less.