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

Steel plate and its manufacturing method

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

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

[0003] DR (Double Reduce) material is known as a high-strength steel sheet for cans. DR material is a steel sheet for cans that has been strengthened by cold rolling, annealing, and then cold rolling again. However, DR material has a problem of low formability due to its small elongation. To solve this problem, it is effective to reduce the rolling reduction ratio during secondary cold rolling after annealing, but there is a concern that reducing the rolling reduction ratio will reduce the strength of the steel sheet. In order to achieve both high strength and high ductility in steel sheets for cans, it is important to increase the strength of the steel sheet by utilizing strengthening mechanisms such as solid solution strengthening and precipitation strengthening, and to optimize the rolling reduction ratio during secondary cold rolling.

[0004] Patent Document 1 proposes a steel sheet for cans 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, and N: 0.0120% or more and 0.0180% or less, with the balance being Fe and unavoidable impurities, and having an in-plane anisotropy Δr of r-value of -0.3 or more and 0.3 or less, and having a tensile strength in the rolling direction after aging treatment of 650 MPa or more.

[0005] Patent Document 2 proposes a steel sheet for cans which contains, 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%, with the balance being iron and unavoidable impurities, has a substantially ferrite single-phase structure, has an average ferrite grain size of 7 μm or less, and has, after paint baking treatment, a yield strength of 500 MPa or more, a yield ratio of 0.9 or more, a total elongation of 10% or more, and a Δr of −0.50 to 0.

[0006] International Publication No. 2020 / 203052 Japanese Patent Application Laid-Open No. 2008-214658

[0007] The above-mentioned conventional techniques have the following problems: In the technique described in Patent Document 1, secondary cold rolling must be performed at a rolling ratio of more than 20% and not more than 40% in order to obtain the target properties, which raises concerns about a decrease in the ductility of the steel sheet.

[0008] In the technology described in Patent Document 2, the adjustment rate after continuous annealing needs to be 1.5% or less in order to obtain the target properties, and therefore there is a concern that the Rockwell superficial hardness of the steel sheet may be insufficient.

[0009] In order to improve the strength of a can body when the steel sheet is used for a can body or a can end, it is necessary to ensure a certain level of Rockwell superficial hardness by optimizing the rolling ratio when the steel sheet is subjected to secondary cold rolling. However, neither Patent Document 1 nor Patent Document 2 describes hardness.

[0010] An object of the present invention is to provide a high-strength, high-ductility steel sheet that solves the above-mentioned problems, and a method for manufacturing the same.

[0011] The present invention has been made to solve the above-mentioned problems, and is summarized as follows: [1] A steel sheet containing, by mass%, C: 0.010% to 0.060%, Si: 0.10% or less, Mn: 0.10% to 0.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.100% or less, N: 0.0050% to 0.0120% or less, Cr: 0.010% to 0.150% or less, Nb: 0.003% to 0.030% or less, with the balance being Fe and unavoidable impurities, and having a structure mainly composed of ferrite, wherein the amount of N present as AlN and the total amount of N satisfy the following formula (1), and the amount of Nb present as Nb precipitates and the total amount of Nb satisfy the following formula (2), A steel sheet having a yield strength of 500 MPa or more, an HR30T of 66 or more, and a total elongation of 7.0% or more. (Amount of N present as AlN) / (Total N)≦0.40 (1) The total N amount refers to the total amount of N contained in the steel sheet. 0.10≦(Amount of Nb present as Nb precipitates) / (Total Nb)≦0.80 (2) The total Nb amount refers to the total amount of Nb contained in the steel sheet. [2] The steel sheet according to [1], wherein the chemical composition further contains, in mass%, one or more 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 according to [1] or [2], wherein the average grain size of the ferrite is 8.0 μm or less.[4] A method for producing a steel sheet according to any one of [1] to [3], comprising: a heating step of heating a steel material having the chemical composition at 1150°C or higher; a hot rolling step of hot rolling the steel material after the heating step at a finishing temperature of 800°C or higher and 950°C or lower, cooling from 800°C to a coiling temperature of 450°C or higher and 700°C or lower at an average cooling rate of 20°C / s or higher, and coiling at the coiling temperature; a first cold rolling step of cold rolling the hot rolled sheet obtained after the hot rolling step under conditions of a rolling ratio of 80% or higher; and an annealing step of holding the cold rolled sheet obtained after the first cold rolling step at an annealing temperature of 660°C or higher and 850°C or lower for 5 seconds or higher and 90 seconds or lower, and then cooling to a cooling stop temperature range of 600°C or lower at an average cooling rate of 15°C / s or higher. a secondary cold rolling step of cold rolling the annealed sheet obtained after the annealing step at a rolling reduction of 5% or more and 20% or less.

[0012] The present invention has made it possible to manufacture high-strength and high-ductility steel sheets that are particularly suitable for use as steel sheets for cans. The present invention makes it possible to reduce the gauge of steel sheets for cans, thereby reducing the CO2 emissions during can body transportation due to the reduction in can body weight. ２ Furthermore, by optimizing the reduction ratio during secondary cold rolling, high ductility of the steel sheet has been ensured, making it possible to apply more complex processing to the steel sheet when it is used for can bodies and can ends.

[0013] The chemical composition, structure, mechanical properties, and manufacturing conditions of the steel sheet of the present invention will be described. In the description of the chemical composition, % means mass %. In the present invention, a steel sheet having a yield strength of 500 MPa or more and a HR30T of 66 or more is referred to as high strength. A steel sheet having a total elongation of 7.0% or more is referred to as high ductility.

[0014] C: 0.010% or more and 0.060% or less C is an element that contributes to improving strength. If the C content is less than 0.010%, the strength decreases due to coarsening of ferrite grain size and a decrease in the amount of solute C. Therefore, the C content must be 0.010% or more. The C content is preferably 0.015% or more. The C content is more preferably 0.017% or more, even more preferably 0.019% or more, and most preferably 0.020% or more. On the other hand, if the C content exceeds 0.060%, the ductility of the steel sheet decreases. Therefore, the C content must be 0.060% or less. In order to achieve both high strength and high ductility of the steel sheet, the C content is preferably 0.050% or less. The C content is more preferably 0.045% or less, even more preferably 0.040% or less, and most preferably 0.039% or less.

[0015] Si: 0.10% or less While Si is an element that contributes to improving strength, excessive Si content reduces the ductility and corrosion resistance of the steel sheet. Therefore, the Si content must be 0.10% or less, and the Si content is preferably 0.08% or less. The Si content is more preferably 0.05% or less, even more preferably 0.03% or less, and most preferably 0.02% or less. There is no need to particularly limit the lower limit, but in order to improve the strength of the steel sheet, the Si content is preferably 0.01% or more.

[0016] Mn: 0.10% or more and 0.80% or less Mn is an element that improves hardenability and promotes the solid solution of C in ferrite. Furthermore, it is known that Mn contributes to improving strength by solid solution strengthening of Mn itself and by refining ferrite grain size due to increased Mn content. Since the strength of the steel sheet is insufficient when the Mn content is less than 0.10%, the Mn content is set to 0.10% or more. The Mn content is preferably set to 0.20% or more. The Mn content is more preferably set to 0.25% or more, even more preferably set to 0.30% or more, and most preferably set to 0.35% or more. On the other hand, the ductility of the steel sheet decreases when the Mn content exceeds 0.80%, so the Mn content is set to 0.80% or less. The Mn content is preferably set to 0.70% or less. The Mn content is more preferably set to 0.65% or less, even more preferably set to 0.63% or less, and most preferably set to 0.60% or less.

[0017] P: 0.025% or less Since P reduces the ductility and corrosion resistance of the steel sheet, the P content is set to 0.025% or less. The P content is preferably set to 0.023% or less. The P content is more preferably set to 0.021% or less, even more preferably set to 0.020% or less, and most preferably set to 0.018% or less. On the other hand, since P contributes to improving the strength of the steel sheet, it is preferable to contain 0.001% or more. There is no need to particularly set a lower limit, but in order to improve the strength of the steel sheet, the P content is more preferably set to 0.005% or more. The P content is even more preferably set to 0.006% or more.

[0018] S: 0.035% or less S forms MnS in steel and reduces the amount of Mn, which contributes to improving strength, so the S content is set to 0.035% or less. The S content is preferably set to 0.030% or less. The S content is more preferably set to 0.028% or less, even more preferably set to 0.025% or less, and most preferably set to 0.020% or less. There is no need to particularly set a lower limit, but in order to reduce the production load in the desulfurization treatment, the S content is preferably set to 0.003% or more. It is more preferably set to 0.005% or more, and even more preferably set to 0.006% or more.

[0019] Al: 0.100% or less Al is an element contained in steel to remove oxygen. However, it is known that the formation of AlN in steel reduces the amount of solute N, which contributes to improving the strength of steel sheets. Therefore, the Al content is set to 0.100% or less. The Al content is preferably set to 0.080% or less. The Al content is more preferably set to 0.075% or less, even more preferably set to 0.070% or less, and most preferably set to 0.065% or less. There is no particular need to set a lower limit, but from the viewpoint of reducing the manufacturing load in treating Al-based inclusions, it is preferably set to 0.010% or more, more preferably set to 0.012% or more, and even more preferably set to 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 steel sheet through solid solution strengthening. Therefore, the N content is set to 0.0050% or more. The N content is preferably set to 0.0060% or more. The N content is more preferably set to 0.0065% or more, even more preferably set to 0.0070% or more, and most preferably set to 0.0071% or more. On the other hand, if the N content exceeds 0.0120%, the ductility of the steel sheet decreases. Therefore, the N content is set to 0.0120% or less. In order to achieve both high strength and high ductility of the steel sheet, the N content is preferably set to 0.0110% or less. The N content is more preferably set to 0.0108% or less, even more preferably set to 0.0105% or less, and most preferably set to 0.0101% or less.

[0021] Cr: 0.010% or more and 0.150% or less Cr improves hardenability, promoting the solid solution of C in ferrite and improving the strength of the steel sheet. Therefore, the Cr content is set to 0.010% or more. The Cr content is preferably set to 0.020% or more. The Cr content is more preferably set to 0.025% or more, even more preferably set to 0.030% or more, and most preferably set to 0.031% or more. On the other hand, if the Cr content exceeds 0.150%, the amount of precipitation of Cr nitrides increases, and the amount of solute N that contributes to improving strength decreases, thereby reducing the strength of the steel sheet. Therefore, the Cr content is set to 0.150% or less. The Cr content is preferably set to 0.120% or less. The Cr content is more preferably set to 0.100% or less, even more preferably set to 0.080% or less, and most preferably set to 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 by forming fine NbC in steel. To ensure sufficient steel sheet strength, the Nb content is set to 0.003% or more. The Nb content is preferably set to 0.005% or more. The Nb content is more preferably set to 0.007% or more, even more preferably set to 0.009% or more, and most preferably set to 0.011% or more. On the other hand, if the Nb content exceeds 0.030%, the ductility of the steel sheet decreases due to an increase in the recrystallization temperature after cold rolling. Therefore, the Nb content is set to 0.030% or less. To achieve both high strength and high ductility in the steel sheet, the Nb content is preferably set to 0.025% or less. The Nb content is more preferably set to 0.023% or less, even more preferably set to 0.021% or less, and most preferably set to 0.019% or less.

[0023] In the present invention, in addition to the above-mentioned component composition, one or more elements selected from the following may be contained.

[0024] Cu: 0.30% or less, Sn: 0.03% or less, Ni: 0.15% or less, Mo: 0.10% or less Cu, Sn, Ni, and Mo improve the strength of the steel sheet by solid solution strengthening. On the other hand, excessive content of these elements reduces the ductility of the steel sheet. Therefore, when Cu, Sn, Ni, and Mo are contained, the Cu content is set to 0.30% or less, the Sn content is set to 0.03% or less, the Ni content is set to 0.15% or less, and the Mo content is set to 0.10% or less. To ensure sufficient ductility, the Cu content is preferably set to 0.25% or less, the Sn content is preferably set to 0.02% or less, the Ni content is preferably set to 0.12% or less, and the Mo content is preferably set to 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 even more preferably 0.20% or less, the Ni content is even more preferably 0.08% or less, and the Mo content is even more preferably 0.04% or less. On the other hand, although there are no particular lower limits, 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 one embodiment of the present invention has a composition containing the above-mentioned components, with the balance being Fe and unavoidable impurities, such as Ca, O, H, Ti, Co, W, Zn, Pb, As, Sb, and Bi.

[0026] The structure and mechanical properties of the steel sheet of the present invention will be described.

[0027] Ferrite-based structure The steel sheet in the present invention has a ferrite-based structure. Here, a structure in which the area fraction (area rate) of ferrite is 70% or more is considered to be a ferrite-based structure. The area fraction of ferrite is preferably 70% or more. Furthermore, the area fraction of ferrite is more preferably 80% or more, even more preferably 85% or more, and most preferably 90% or more. The upper limit is not particularly limited, but may be 100%. The remainder other than ferrite may include cementite, pearlite, bainite, martensite, retained austenite, etc. The requirements of the present invention are met when the area fraction of the remainder is 30% or less, and the area fraction of the remainder may be 0%.

[0028] As will be described later, it is preferable to recrystallize the ferrite during annealing, and it is preferable to obtain recrystallized ferrite. Furthermore, since obtaining fine ferrite facilitates an increase in strength, it is preferable that the average crystal grain size of the ferrite is 8.0 μm or less. Furthermore, the average crystal grain size of the ferrite is more preferably 7.5 μm or less, and even more preferably 7.0 μm or less. Although there is no particular lower limit, the average crystal grain size of the ferrite is preferably 3.0 μm or more, more preferably 3.5 μm or more, and even more preferably 4.0 μm or more.

[0029] The amount of N present as AlN and the total amount of N satisfy the formula (1): (Amount of N present as AlN) / (Total amount of N)≦0.40 (1). The total amount of N refers to the total amount of N contained in the steel sheet. To increase the strength of the steel sheet, it is important to suppress the ratio of the amount of N present as AlN (NasAlN) to the total amount of N and ensure the amount of solute N that contributes to strength. The amount of N present as AlN refers to the amount of N contained in AlN. If formula (1) is not satisfied, the amount of solute N will be insufficient, resulting in a decrease in the strength of the steel sheet. Therefore, (NasAlN) / (Total amount of N) is set to 0.40 or less. To increase the strength of the steel sheet, the ratio (Amount of N present as AlN) / (Total amount of N) is preferably set to 0.35 or less. It is more preferably set to 0.32 or less, even more preferably set to 0.30 or less, most preferably set to 0.28 or less, and even most preferably set to 0.25 or less. The lower limit does not need to be particularly limited, but is preferably 0.01 or more to improve the ductility of the steel sheet. It is more preferably 0.02 or more, and even more preferably 0.03 or more. As will be described later, the precipitation of AlN is affected by the coiling temperature and the cooling rate from 800°C to the coiling temperature. Therefore, formula (1) is within the range of the present invention by controlling the coiling temperature and the average cooling rate from 800°C to the coiling temperature. The amount of solute N is the amount of N obtained by subtracting the amount of N contained in AlN from the total amount of N. Therefore, the amount of solute N is preferably (amount of N present as solute N) / (total amount of N)≧0.60. The ratio (amount of N present as solute N) / (total amount of N) is more preferably 0.65 or more, even more preferably 0.68 or more, and most preferably 0.70 or more. The upper limit is not particularly limited, but is preferably 0.99 or less, more preferably 0.98 or less, and even more preferably 0.97 or less. The value of (amount of N present as solid solution N) / (total amount of N) tends to increase as the average cooling rate from 800° C. to the coiling temperature increases.

[0030] The amount of Nb present as Nb precipitates satisfies the following 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 steel sheets by utilizing precipitation strengthening due to Nb addition, it is important 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. If the value of (amount of Nb present as Nb precipitates) / (total amount of Nb) is less than 0.10, the amount of Nb contributing to precipitation strengthening is small, resulting in insufficient strength of the steel sheet. Therefore, the ratio (amount of Nb present as Nb precipitates) / (total amount of Nb) is set to 0.10 or more. Preferably, it is set to 0.15 or more. It is more preferably 0.17 or more, even more preferably 0.20 or more, and most preferably 0.22 or more. On the other hand, if the value of (amount of Nb present as Nb precipitates) / (total amount of Nb) exceeds 0.80, the ductility of the steel sheet decreases. Therefore, (amount present as Nb precipitates) / (total amount of Nb) is set to 0.80 or less. It is preferably set to 0.75 or less. It is more preferably set to 0.73 or less, even more preferably to 0.70 or less, most preferably to 0.68 or less, and even most preferably to 0.60 or less. Note that Nb precipitates include all of Nb carbides, Nb nitrides, and Nb-containing carbonitrides. As will be described later, the coiling temperature and the average cooling rate from 800°C to the coiling temperature affect the precipitation of Nb, so by controlling the coiling temperature and the average cooling rate from 800°C to the coiling temperature, formula (2) is within the range of the present invention.

[0031] Yield strength of 500 MPa or more, HR30T of 66 or more, and total elongation of 7.0% or more In order to ensure sufficient can body strength when the steel sheet is gauged down, it is necessary to make the steel sheet have a yield strength of 500 MPa or more and an HR30T of 66 or more. Preferably, the yield strength is 520 MPa or more and an HR30T of 67 or more. More preferably, the yield strength is 530 MPa or more and an HR30T of 68 or more. Even more preferably, the yield strength is 540 MPa or more and an HR30T of 69 or more, and most preferably, the yield strength is 550 MPa or more and an HR30T of 70 or more. There is no particular need to set the upper limits, but it is preferable that the yield strength is 700 MPa or less and the HR30T is 80 or less. Furthermore, in order to ensure sufficient formability, the total elongation of the steel sheet is 7.0% or more. Preferably, the total elongation is 7.5% or more. The total elongation is more preferably 8.0% or more, even more preferably 8.5% or more, and most preferably 9.0% or more. There is no particular upper limit, but the total elongation is preferably 25.0% or less.

[0032] The above-mentioned yield strength, total elongation, and HR30T were determined by the test methods explained in the Examples, and are the properties after the obtained steel sheets were subjected to aging heat treatment at 210°C for 10 minutes.

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

[0034] The method for producing a steel sheet according to the present invention includes a heating step of heating a steel material having the above-described chemical composition to 1150°C or higher, a hot rolling step of hot rolling the steel material after the heating step at a finishing temperature of 800°C or higher and 950°C or lower, cooling from 800°C to a coiling temperature of 450°C or higher and 700°C or lower at an average cooling rate of 20°C / s or higher, and coiling under conditions of 450°C or higher and 700°C or lower, and a hot-rolled sheet obtained after the hot rolling step. The cold-rolled steel sheet is characterized by comprising: a first cold-rolling step in which cold-rolling is performed on the steel sheet at a rolling ratio of 80% or more; an annealing step in which the cold-rolled sheet obtained after the first cold-rolling step is held at an annealing temperature of 660°C to 850°C for 5 seconds to 90 seconds 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 cold-rolling is performed on the annealed sheet obtained after the annealing step at a rolling ratio of 5% to 20%. Unless otherwise specified, the steel sheet temperature described in the present invention refers to the temperature of the steel sheet surface, and the temperature is measured with a radiation thermometer.

[0035] Hot Rolling Process Heating Temperature: 1150°C or Higher If the heating temperature in the heating process 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. Therefore, the heating temperature is set to 1150°C or higher. The heating temperature is preferably set to 1170°C or higher, more preferably 1180°C or higher, even more preferably 1190°C or higher, and most preferably 1200°C or higher. There is no particular need to limit the upper limit of the heating temperature, but from the viewpoint of production costs, it is preferably set to 1300°C or lower, more preferably 1290°C or lower, even more preferably 1280°C or lower, and most preferably 1270°C or lower.

[0036] Finishing temperature: 800°C or higher and 950°C or lower If the finishing temperature in the hot rolling process exceeds 950°C, the ferrite grain size will coarsen, resulting in a decrease in the strength of the steel sheet. Therefore, the finishing temperature is set to 950°C or lower. The finishing temperature is preferably set to 940°C or lower. The finishing temperature is more preferably set to 930°C or lower, even more preferably set to 920°C or lower, most preferably set to 910°C or lower, and even more preferably set to 900°C or lower. On the other hand, if the finishing temperature is lower than 800°C, coarse ferrite grains will be generated during rolling, and coarse Nb(C,N) will precipitate during hot rolling. The coarse Nb(C,N) will not contribute to improving the strength of the steel sheet, but will reduce the amount of solute C, solute N, and fine Nb precipitates that improve the strength of the steel sheet, thereby causing a decrease in the strength of the steel sheet. Therefore, the finishing temperature in the hot rolling process is set to 800°C or higher. Note that the finishing temperature is preferably set to 830°C or higher. The finishing temperature is more preferably 840°C or higher, even more preferably 845°C or higher, and most preferably 850°C or higher.

[0037] Average cooling rate from 800°C to a coiling temperature of 450°C or higher and 700°C or lower: 20°C / s or higher If the average cooling rate from 800°C to a coiling temperature of 450°C or higher and 700°C or lower is less than 20°C / s, the amount of AlN precipitation increases, the amount of Nb present as Nb precipitates decreases, and ferrite grains become coarse, resulting in a decrease in the strength of the steel sheet. Therefore, the average cooling rate to the coiling temperature is set to 20°C / s or higher. To increase the strength of the steel sheet, the average cooling rate is preferably set to 25°C / s or higher. The average cooling rate is more preferably set to 30°C / s or higher, even more preferably set to 35°C / s or higher, and most preferably set to 40°C / s or higher. There is no particular need to limit the upper limit of the average cooling rate to the coiling temperature, but from the viewpoint of reducing the production load, it is preferably set to 80°C / s or lower, more preferably set to 75°C / s or lower, and even more preferably set to 70°C / s or lower. The average cooling rate can be determined by dividing the temperature difference between the cooling start temperature (here, 800°C) and the cooling end temperature (here, the coiling temperature) by the cooling time required for cooling to the temperature difference.

[0038] Coiling temperature: 450°C or higher and 700°C or lower When the coiling temperature exceeds 700°C, the ferrite grains of the steel sheet become coarse, and the amount of solute C decreases due to the promotion of the formation of coarse alloy carbides, and the amount of solute N decreases due to the promotion of the formation of coarse alloy nitrides, resulting in a decrease in the strength of the steel sheet. Therefore, the coiling temperature is set to 700°C or lower. The coiling temperature is preferably set to 650°C or lower. The coiling temperature is more preferably set to 630°C or lower, even more preferably set to 625°C or lower, and most preferably set to 620°C or lower. On the other hand, when the coiling temperature is lower than 450°C, the amount of precipitation of fine alloy carbides such as Nb carbide decreases, resulting in a decrease in the strength of the steel sheet. Therefore, the coiling temperature is set to 450°C or higher. The coiling temperature is preferably set to 500°C or higher. The coiling temperature is more preferably set to 530°C or higher, even more preferably set to 540°C or higher, and most preferably set to 550°C or higher. After winding, in order to remove scale, ２ SO ４ , HCl, H ３ P.O. ４ Alternatively, pickling may be carried out using an aqueous solution of the above.

[0039] First cold rolling process: Reduction ratio in cold rolling: 80% or more After the hot rolling process, cold rolling (first cold rolling) is performed. The strain accumulated by this cold rolling process acts as a driving force to promote ferrite recrystallization in the subsequent annealing process, thereby refining the ferrite grains and increasing the strength of the steel sheet. To ensure sufficient strength, the rolling ratio is set to 80% or more. The rolling ratio is preferably set to 85% or more. The rolling ratio is more preferably set to 86% or more, even more preferably set to 87% or more, and most preferably set to 88% or more. There is no particular need to limit the upper limit of the rolling ratio, but to ensure sufficient ductility, it is preferably set to 95% or less, more preferably set to 93% or less, even more preferably set to 92% or less, and most preferably set to 91% or less.

[0040] Annealing Process Annealing temperature: 660°C or higher and 850°C or lower, holding time: 5 seconds or higher and 90 seconds or lower, cooling stop temperature: 600°C or lower, average cooling rate to cooling stop temperature: 15°C / s or higher. Annealing is performed after the cold rolling process. In order to ensure sufficient ductility by promoting ferrite recrystallization, the annealing temperature is set to 660°C or higher. The annealing temperature is preferably set to 680°C or higher. The annealing temperature is more preferably set to 700°C or higher, even more preferably set to 710°C or higher, and most preferably set to 720°C or higher. On the other hand, if the annealing temperature exceeds 850°C, the strength of the steel sheet decreases due to coarsening of ferrite grains and fine precipitates that contribute to precipitation strengthening. Therefore, the annealing temperature is set to 850°C or lower. The annealing temperature is preferably set to 830°C or lower, more preferably set to 800°C or lower, even more preferably set to 780°C or lower, and most preferably set to 760°C or lower.

[0041] If the holding time at the annealing temperature is less than 5 seconds, ferrite recrystallization will be insufficient and sufficient ductility will not be obtained. Therefore, the holding time is set to 5 seconds or more. The holding time is preferably set to 8 seconds or more, more preferably 10 seconds or more, even more preferably 12 seconds or more, and most preferably 14 seconds or more. On the other hand, if the holding time is longer than 90 seconds, the ferrite grain size will coarsen and the strength of the steel sheet will decrease. Therefore, the holding time at the annealing temperature is set to 90 seconds or less. The holding time is preferably set to 85 seconds or less, more preferably 80 seconds or less, even more preferably 75 seconds or less, most preferably 70 seconds or less, and even most preferably 65 seconds or less.

[0042] If the cooling stop temperature after annealing exceeds 600°C, the amount of cementite precipitated becomes excessive, and the amount of solute C that contributes to improving the strength of the steel sheet becomes insufficient. Therefore, the cooling stop temperature is set to 600°C or lower. The cooling stop temperature is preferably set to 590°C or lower, more preferably 580°C or lower, even more preferably 575°C or lower, and most preferably 570°C or lower. There is no need to particularly limit the lower limit of the cooling stop temperature, but from the viewpoint of manufacturing costs, the cooling stop temperature is preferably set to 150°C or higher. It is more preferably set to 300°C or higher, even more preferably 320°C or higher, and most preferably 350°C or higher. Furthermore, in order to improve the ductility of the steel sheet, the steel sheet may be held in a temperature range of 150°C or higher and 600°C or lower after cooling is stopped. In order to ensure a sufficient amount of solute C and achieve both strength and ductility of the steel sheet, the holding time in the temperature range of 150°C or higher and 600°C or lower after cooling is stopped is preferably 300 seconds or shorter, more preferably 280 seconds or shorter, even more preferably 260 seconds or shorter, most preferably 240 seconds or shorter, and even most preferably 180 seconds or shorter. The lower limit of the holding time is not particularly limited, and the holding time may be 0 seconds, but in order to improve the ductility of the steel sheet, holding is preferably performed for 5 seconds or longer, more preferably 10 seconds or longer, even more preferably 15 seconds or longer, most preferably 20 seconds or longer, and even most preferably 30 seconds or longer.

[0043] If the average cooling rate to the cooling stop temperature is less than 15°C / s, the amount of solute C becomes insufficient, resulting in a decrease in the strength of the steel sheet. Therefore, the average cooling rate after annealing is set to 15°C / s or more. The average cooling rate is preferably set to 20°C / s or more. The average cooling rate is more preferably set to 25°C / s or more, even more preferably set to 30°C / s or more, most preferably set to 35°C / s or more, and even most preferably set to 40°C / s or more. There is no particular upper limit to the average cooling rate, but in order to reduce the manufacturing load, the average cooling rate is preferably set to 200°C / s or less, more preferably set to 180°C / s or less, even more preferably set to 160°C / s or less, and most preferably set to 120°C / s or less. The average cooling rate can be calculated by dividing the temperature difference between the cooling start temperature (here, the annealing temperature) and the cooling stop temperature by the cooling time required for cooling to the temperature difference.

[0044] Secondary cold rolling process Rolling reduction ratio in secondary cold rolling: 5% or more and 20% or less Secondary cold rolling is performed after the annealing process. If the rolling reduction ratio in secondary cold rolling is less than 5%, the strength of the steel sheet will decrease. Therefore, the rolling reduction ratio is set to 5% or more. The rolling reduction ratio is preferably set to 6% or more. The rolling reduction ratio is more preferably set to 7% or more, and most preferably set to 8% or more. On the other hand, if the rolling reduction ratio in secondary cold rolling exceeds 20%, the ductility of the steel sheet will decrease. Therefore, the rolling reduction ratio is set to 20% or less. The rolling reduction ratio is preferably set to 18% or less. The rolling reduction ratio is more preferably set to 15% or less, even more preferably set to 12% or less, most preferably set to 11% or less, and even most preferably set to 10% or less.

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

[0046] Steel slabs were obtained by melting and casting steels containing the components of steel types No. 1 to 17 shown in Table 1, with the balance being Fe and unavoidable impurities. The steel slabs thus obtained were heated, hot rolled, first cold rolled, annealed, and second cold rolled under the conditions shown in Table 2, to obtain steel plates No. 1 to 18.

[0047] From the steel sheet, JIS No. 5 tensile test specimens with the tensile direction along the rolling direction and 30 mm square test specimens used for Rockwell superficial hardness measurement were taken and subjected to aging heat treatment at 210 °C for 10 minutes in an incubator. Tensile tests in accordance with JIS Z 2241 were conducted on the tensile test specimens to evaluate yield strength and total elongation. HR30T was determined by measuring the Rockwell superficial hardness of the sheet surface at HR15T and converting it using the conversion table in JIS G 3303 (2017). Table 3 shows the evaluation results for yield strength, HR30T, and total elongation.

[0048] The amount of N present as AlN (Nas AlN amount) and the amount of Nb present as Nb precipitates were measured by taking samples from the steel sheet and analyzing the extraction residue. ２ SO ４ +K ２ SO ４ The Nb content was measured by bis-pyrazolone absorptiometry after decomposition and alkaline steam distillation. The amount of Nb present as Nb precipitates was determined by ICP-AES measurement after AA electrolytic extraction, filter collection, and mixed acid decomposition. Table 3 shows the calculation results of the following formulas (1) and (2). (Amount of N present as AlN) / (Total N)≦0.40 (1) The total N content is the total amount of N contained in the steel sheet. 0.10≦(Amount of Nb present as Nb precipitates) / (Total Nb)≦0.80 (2) The total N content is the total amount of Nb contained in the steel sheet. The amount of dissolved N was determined by subtracting the amount of N present as AlN from the total N content.

[0049] Structural Observation Observation of the steel sheet structure was carried out according to the following procedure. After taking a test piece from the steel sheet, the cross section parallel to the rolling direction was polished and subjected to nital etching to reveal the structure, and a sample for structural observation was taken. Using a scanning electron microscope (SEM), a position halfway in the sheet thickness direction was observed at an acceleration voltage of 15 kV and a magnification of 1500x, 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. Note that the area that can be observed as black lumps in the SEM photograph was determined to be ferrite.

[0050] The average ferrite grain size was measured using the following procedure. A test piece was taken from the center of the steel plate width, polished so that the observation surface was at half the plate thickness of the steel plate in a cross section parallel to the rolling direction and the plate thickness direction, and then subjected to nital etching to reveal the structure, which was used as a sample for structure observation. The target surface was observed using an optical microscope at a magnification of 200 to 500 times, and the structure of three randomly selected fields was photographed. The average ferrite grain size was determined using the cutting method described in JIS G 0551 and was taken as the average value of the three fields.

[0051] All of the invention examples in Table 3 have a yield strength of 500 MPa or more, an HR30T of 66 or more, and a total elongation of 7.0% or more. Therefore, it can be said that the invention examples are high-strength, high-ductility steel sheets suitable for use as can materials.

[0052] On the other hand, in the comparative examples, any of the yield strength, HR30T, and total elongation was outside the range of the present invention.

[0053]

[0054]

[0055]

Claims

1. A steel sheet comprising, by 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, with the balance being Fe and unavoidable impurities, and having a structure mainly composed of ferrite, wherein the amount of N present as AlN and the total amount of N satisfy the following formula (1), and the amount of Nb present as Nb precipitates and the total amount of Nb satisfy the following formula (2), A steel sheet having a yield strength of 500 MPa or more, an 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) The total amount of N refers to 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) The total amount of Nb refers to the 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%, one or more 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 according to claim 1 or 2, wherein the average grain size of the ferrite is 8.0 μm or less.

4. A method for producing a steel sheet according to any one of claims 1 to 3, comprising: a heating step of heating a steel material having the above-mentioned chemical composition to 1150°C or higher; a hot rolling step of hot rolling the steel material after the heating step at a finishing temperature of 800°C to 950°C, cooling from 800°C to a coiling temperature of 450°C to 700°C at an average cooling rate of 20°C / s or higher, and coiling the steel at the coiling temperature; a first cold rolling step of cold rolling the hot rolled sheet obtained after the hot rolling step at a rolling ratio of 80% or higher; and an annealing step of holding the cold rolled sheet obtained after the first cold rolling step at an annealing temperature of 660°C to 850°C for 5 seconds to 90 seconds, and then cooling the cold rolled sheet to a cooling stop temperature range of 600°C or lower at an average cooling rate of 15°C / s or higher. a secondary cold rolling step of cold rolling the annealed sheet obtained after the annealing step at a rolling reduction of 5% or more and 20% or less.

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