Steel sheet for cans and method for manufacturing the same

Abstract

The present invention aims to provide a steel sheet for cans that has high strength, high ductility, and excellent deep drawing processability, as well as a method for manufacturing the same. A steel sheet for cans having a specific component composition, wherein the carbon content as solid-solution carbon among the carbon is 0.003% or more and 0.030% or less, containing ferrite, the average particle size of the ferrite is 2.0 μm or more and 7.0 μm or less, the dislocation density is 3.0 × 10¹⁴ m⁻² or more and 1.0 × 10¹⁵ m⁻² or less, and the average plastic strain ratio r- is 1.0 or more.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet for cans that has high strength, high ductility, and excellent deep drawing processability, and a method for manufacturing the same. [Background technology]

[0002] In recent years, there has been a demand to reduce the weight of cans by reducing the gauge of the steel sheets used for cans, in order to reduce CO2 emissions during can transport. However, since reducing the gauge of the steel sheets lowers the strength of the cans, increasing the strength of the steel sheets is essential to achieve both weight reduction and maintenance of can strength. 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 and annealing, followed by cold rolling again. However, DR material has the problem of poor workability due to its low elongation. To solve this problem, improving the ductility of the steel sheet for cans by reducing the rolling ratio during the secondary cold rolling is effective, but there is a concern that reducing the rolling ratio will decrease the strength. In order to achieve both high strength and high ductility in steel sheets for cans, it is important to optimize the steel composition and manufacturing conditions.

[0003] Patent Document 1 proposes a steel sheet for cans having a composition by mass%, containing C: 0.010% to 0.080%, Si: 0.05% or less, Mn: 0.10% to 0.70%, P: 0.03% or less, S: 0.020% or less, Al: 0.005% to 0.020%, and N: 0.0120% to 0.0180%, with the remainder being Fe and unavoidable impurities, an in-plane anisotropy Δr of r value of -0.3 or more and a tensile strength in the rolling direction after aging treatment of 650 MPa or more.

[0004] Patent Document 2 describes a material with a composition of the following components by mass%, containing C: 0.010% to 0.080%, Si: 0.05% or less, Mn: 0.10% to 0.70%, P: 0.03% or less, S: 0.020% or less, Al: 0.005% to 0.070%, N: 0.0120% to 0.0180%, with the remainder being Fe and unavoidable impurities. Of the N contained, the N content as solid-solution N is 0.0100% or more, the average ferrite particle size is 7.0 μm or less, and the dislocation density at a depth of 1 / 4 of the plate thickness from the surface is 4.0 × 10⁻⁶. 14 m -2 The above 2.0 × 10 15 m -2 The following high-strength steel sheet has been proposed, characterized by a tensile strength of 530 MPa or more and an elongation of 7% or more in the direction perpendicular to the rolling direction after aging treatment under the conditions of 210°C for 10 minutes. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2020 / 203052 [Patent Document 2] International Publication No. 2015 / 166646 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The above-mentioned conventional technology has the following problems. In the technology described in Patent Document 1, secondary cold rolling is required at a rolling ratio of more than 20% but less than or equal to 40% in order to obtain the target properties, which raises concerns about a decrease in the ductility of the steel sheet. While the technology described in Patent Document 2 yields high-strength and high-ductility steel sheets, there are no specifications regarding the Lankford value (r value), raising concerns about fracture during the drawing process of the steel sheets.

[0007] The present invention aims to provide a steel sheet for cans that has high strength, high ductility, and excellent deep drawing processability, and a method for manufacturing the same, which solves the aforementioned problems.

[0008] In addition, the high strength referred to in the present invention means that a yield stress of 500 MPa or more can be obtained. Further, the high ductility referred to in the present invention means that a total elongation of 7.0% or more can be obtained.

Means for Solving the Problems

[0009] The present invention has been made to solve the above problems, and the gist is as follows. [1] In mass%, C: 0.010% or more and 0.060% or less, Si: 0.05% 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 0.100% or less, Nb: 0.003% or more and 0.030% or less containing, and the balance consisting of Fe and inevitable impurities, having a component composition, wherein the C content as dissolved C among the C is 0.003% or more and 0.030% or less, having ferrite, wherein the average ferrite grain size is 2.0 μm or more and 7.0 μm or less, and the dislocation density is 3.0×10 14 m -2 or more and 1.0×10 15 m -2 or less, a steel sheet for cans having an average plastic strain ratio r- of 1.0 or more. [2] The component composition further contains, in mass%, Cu: 0.30% or less, Sn: 0.030% or less, Ni: 0.15% or less, Mo: 0.10% or less selected from one or more of the above, the steel sheet for cans according to [1]. [3] A method for manufacturing steel sheets for cans according to [1] or [2] above, A steel material having the above-mentioned component composition is heated to 1150°C or higher. Hot-rolled at a finishing temperature of 820°C to 950°C. The cooling is performed at an average cooling rate of 20°C / s or more from 820°C to a winding temperature of 500°C to 650°C. A hot rolling process in which the ball is wound at the aforementioned winding temperature, A primary cold rolling process is performed on the hot-rolled sheet obtained after the hot-rolling process, under conditions of a rolling ratio of 85% or more. After the cold-rolled sheet obtained in the primary cold-rolling process is held at an annealing temperature of 670°C to 730°C for 5 to 90 seconds, Cool down to a cooling stop temperature between 350°C and 600°C. An annealing process in which the temperature is maintained at 250°C or higher and below the cooling stop temperature for 30 seconds or more and 250 seconds or less, A method for manufacturing steel sheets for cans, comprising: a secondary cold rolling step in which the annealed sheet obtained after the aforementioned annealing step is cold-rolled under conditions of a rolling ratio of 5% to 20%. [Effects of the Invention]

[0010] This invention enables the production of high-strength, high-ductility, and excellent deep-drawing properties in steel sheets for cans by optimizing the steel composition and manufacturing conditions. Because this invention allows for gauge reduction of the steel sheet for cans, it enables a reduction in CO2 emissions during can transport due to the lighter weight of the can body. Furthermore, its superior processability compared to conventional technologies makes it possible to manufacture can bodies and lids with more complex processing. [Modes for carrying out the invention]

[0011] Embodiments of the steel sheet for cans of the present invention will now be described. In the tensile test described later, if a yield stress of 500 MPa or more is obtained, it is referred to as high strength, and if a total elongation of 7.0% or more is obtained, it is referred to as high ductility. Furthermore, in the Rankford test described later, if the average plastic strain ratio r- is 1.0 or more, it is referred to as a high r value.

[0012] First, the component composition of the steel sheet for cans according to the present invention will be described. In the description of the component composition, % refers to mass %.

[0013] C: 0.010% to 0.060%, C content as solid-solution C: 0.003% to 0.030% Carbon (C) is an element that contributes to strength improvement through solid solution strengthening. If the C content is less than 0.010%, the strength of the steel sheet for cans decreases due to the coarsening of the average ferrite grain size and the decrease in the amount of solid solution C; therefore, the C content must be 0.010% or more. A C content of 0.012% or more is preferable. A C content of 0.014% or more is more preferable, 0.016% or more is even preferable, and 0.018% or more is most preferable. On the other hand, if the C content exceeds 0.060%, the amount of solid solution C becomes excessive, and the ductility and r value of the steel sheet for cans decreases. Therefore, the C content must be 0.060% or less. In order to make the steel sheet for cans high strength, high ductility, and high r value, a C content of 0.055% or less is preferable, 0.050% or less is even preferable, and 0.040% or less is most preferable. By controlling the carbon content as solid-solution carbon (solid-solution carbon amount) to an optimal range, high-strength, high-ductility, and high-r-value steel sheets for cans can be obtained. In particular, controlling the solid-solution carbon amount is an important technique for increasing the r-value. Although the detailed mechanism is unknown, it is presumed that reducing the solid-solution carbon amount promotes the formation and development of the {111} texture, which contributes to improving the r-value. Furthermore, in order to ensure sufficient strength through solid-solution strengthening, the solid-solution carbon amount should be 0.003% or more. Preferably, the solid-solution carbon amount should be 0.004% or more, more preferably 0.005% or more, even more preferably 0.007% or more, and most preferably 0.009% or more. On the other hand, in order to ensure sufficient ductility and r-value, the solid-solution carbon amount should be 0.030% or less. Preferably, the solid-solution carbon amount should be 0.028% or less, more preferably 0.026% or less, even more preferably 0.024% or less, and most preferably 0.022% or less.

[0014] Si: 0.05% or less While silicon (Si) contributes to improving strength, excessive Si content reduces the ductility and corrosion resistance of steel sheets for cans. Therefore, the Si content should be 0.05% or less. A Si content of 0.04% or less is more preferable, and 0.03% or less is even more preferable. There is no particular lower limit, but to improve the strength of the steel sheet, a Si content of 0.01% or more is preferable.

[0015] Mn: 0.10% or more and 0.80% or less Mn is an element that improves hardenability and promotes the solid solution of C into ferrite. Furthermore, it is known that Mn contributes to increasing the strength of can steel sheets through solid solution strengthening by Mn itself and the refinement of ferrite grain size by increasing the Mn content. If the Mn content is less than 0.10%, the strength of the can steel sheet becomes insufficient, so the Mn content should be 0.10% or more. A Mn content of 0.20% or more is preferable. A Mn content of 0.25% or more is more preferable, 0.30% or more is even more preferable, and 0.40% or more is most preferable. On the other hand, if the Mn content exceeds 0.80%, the ductility and r value of the can steel sheet decrease, so the Mn content should be 0.80% or less. A Mn content of 0.75% or less is preferable. A Mn content of 0.70% or less is more preferable, 0.65% or less is even more preferable, and 0.60% or less is most preferable.

[0016] P:0.025% or less Since phosphorus (P) reduces the ductility and corrosion resistance of steel sheets for cans, the P content should be 0.025% or less. Preferably, the P content should be 0.024% or less, more preferably 0.022% or less. Even more preferably, the P content should be 0.021% or less, and most preferably 0.020% or less. On the other hand, since P contributes to increasing the strength of steel sheets for cans, it is preferable to include 0.001% or more. There is no particular lower limit, but to improve the strength of steel sheets for cans, it is more preferable that the P content be 0.002% or more. Even more preferably, the P content should be 0.003% or more, and most preferably 0.005% or more.

[0017] S: 0.035% or less S forms MnS in steel, reducing the amount of Mn that contributes to strength improvement, therefore the S content should be 0.035% or less. Preferably, the S content should be 0.033% or less, more preferably 0.030% or less. Even more preferably, the S content should be 0.028% or less, and most preferably 0.026% or less. There is no particular need to limit the lower limit, but in order to reduce the manufacturing burden in the desulfurization treatment, preferably, the S content should be 0.001% or more, more preferably 0.003% or more, even more preferably 0.004% or more, and most preferably 0.005% or more.

[0018] Al: 0.100% or less Al is an element included to remove oxygen from steel. However, it is known that forming AlN in steel reduces the amount of solid-solution nitrogen, which contributes to improving the strength of steel sheets for cans. Therefore, the Al content should be 0.100% or less. Preferably, the Al content should be 0.090% or less. More preferably, the Al content should be 0.085% or less, even more preferably 0.080% or less, and most preferably 0.070% or less. There is no particular need to limit the lower limit, but from the viewpoint of reducing the manufacturing load in Al-based inclusion processing, the Al content should be 0.005% or more, more preferably 0.007% or more, and even more preferably 0.010% or more.

[0019] N: 0.0050% or more and 0.0120% or less N is an element that contributes to improving the strength of steel sheets for cans through solid solution strengthening. Therefore, the N content should be 0.0050% or more. Preferably, the N content should be 0.0055% or more. More preferably, the N content should be 0.0060% or more, even more preferably 0.0065% or more, and most preferably 0.0070% or more. On the other hand, if the N content exceeds 0.0120%, the ductility and r value of the steel sheet for cans will decrease. Therefore, the N content should be 0.0120% or less. In order to make the steel sheet for cans high strength, high ductility and high r value, it is preferable that the N content be 0.0115% or less. More preferably, the N content should be 0.0110% or less. Even more preferably, the N content should be 0.0108% or less, and most preferably 0.0105% or less.

[0020] Cr: 0.010% or more and 0.100% or less Cr improves hardenability, thereby promoting the solid solution of carbon (C) into ferrite and improving the strength of can steel sheets; therefore, the Cr content should be 0.010% or more. Preferably, the Cr content should be 0.015% or more. More preferably, the Cr content should be 0.020% or more, and even more preferably, 0.025% or more. On the other hand, if the Cr content exceeds 0.100%, the amount of Cr nitride precipitated increases, and the amount of solid-solution nitrogen (N) that contributes to strength improvement decreases, thus reducing the strength of the can steel sheet. Therefore, the Cr content should be 0.100% or less. Preferably, the Cr content should be 0.090% or less. More preferably, the Cr content should be 0.080% or less, even more preferably, 0.070% or less, and most preferably, 0.060% or less.

[0021] Nb: 0.003% or more and 0.030% or less Nb is an element that contributes to precipitation strengthening and fine-grain strengthening by forming fine NbC in steel. To ensure sufficient strength, the Nb content should be 0.003% or more. Preferably, the Nb content should be 0.005% or more. More preferably, the Nb content should be 0.007% or more, even more preferably 0.009% or more, and most preferably 0.012% or more. On the other hand, if the Nb content exceeds 0.030%, the ductility of the steel sheet for cans decreases due to the rise in the recrystallization temperature after cold rolling. Therefore, the Nb content should be 0.030% or less. To achieve both high strength and high ductility in the steel sheet for cans, it is preferable that the Nb content be 0.028% or less. More preferably, the Nb content should be 0.026% or less, even more preferably 0.024% or less, and most preferably 0.022% or less.

[0022] The above is the basic component composition of the present invention. In addition to the above component composition, the present invention may contain one or more elements selected from the following elements. Cu: 0.30% or less, Sn: 0.030% or less, Ni: 0.15% or less, Mo: 0.10% or less Cu, Sn, Ni, and Mo improve the strength of can steel sheets through solid solution strengthening. However, excessive amounts of these elements reduce the ductility and r-value of the can steel sheets. Therefore, when Cu, Sn, Ni, and Mo are included, the Cu content should be 0.30% or less, the Sn content 0.030% or less, the Ni content 0.15% or less, and the Mo content 0.10% or less. To ensure sufficient ductility and r-value, it is preferable that the Cu content be 0.25% or less, the Sn content 0.020% or less, the Ni content 0.12% or less, and the Mo content 0.08% or less. It is more preferable that the Cu content be 0.22% or less, the Sn content 0.016% or less, the Ni content 0.10% or less, and the Mo content 0.06% or less. Furthermore, the Cu content is more preferably 0.20% or less, the Ni content is more preferably 0.08% or less, and the Mo content is more preferably 0.04% or less. While there is no particular lower limit, it is preferable that the Cu content be 0.01% or more, the Sn content be 0.001% or more, the Ni content be 0.01% or more, and the Mo content be 0.01% or more. Furthermore, it is more preferable that the Cu content be 0.03% or more, the Sn content be 0.002% or more, the Ni content be 0.03% or more, and the Mo content be 0.02% or more. Even more preferable that the Cu content be 0.04% or more, the Sn content be 0.003% or more, the Ni content be 0.04% or more, and the Mo content be 0.03% or more.

[0023] A steel sheet for cans according to one embodiment of the present invention has a composition containing the above-mentioned components, with the remainder being Fe and unavoidable impurities. Examples of unavoidable impurities include Ca, O, H, Ti, Co, W, Zn, Pb, As, Sb, Bi, etc.

[0024] The microstructure, average ferrite grain size, dislocation density, and average plastic strain ratio r- of the steel sheet for cans in this invention will be described.

[0025] organization The microstructure of the steel sheet for cans according to the present invention has ferrite. In addition to ferrite, it may also have a second phase structure such as cementite or pearlite. Within the scope of the present invention, the influence of the area fraction of the second phase on the effect of the invention is small.

[0026] Ferrite average particle size: 2.0 μm to 7.0 μm To increase the strength of steel sheets for cans, refining ferrite grains is effective. By setting the average ferrite grain size to 7.0 μm or less, in addition to an increase in the amount of grain boundary strengthening, the amount of dislocation strengthening increases due to the promotion of dislocation introduction in the secondary cold rolling process. The average ferrite grain size is preferably 6.7 μm or less, more preferably 6.5 μm or less. The average ferrite grain size is even more preferably 6.3 μm or less, and most preferably 6.0 μm or less. On the other hand, when the average ferrite grain size is less than 2.0 μm, the amount of grain boundary strengthening and the amount of dislocation strengthening become excessive, and the ductility of the steel sheet for cans decreases. Therefore, the average ferrite grain size is set to 2.0 μm or more. To improve the ductility of the steel sheet for cans, the average ferrite grain size is preferably 2.3 μm or more, more preferably 2.5 μm or more. The average ferrite grain size is even more preferably 2.6 μm or more, and most preferably 2.7 μm or more. The average ferrite grain size is measured based on the method described in the examples. Also, to obtain a predetermined average ferrite grain size, as described later, controlling the annealing temperature and holding time is important. In particular, by setting the annealing temperature to 670°C or higher and 730°C or lower, a predetermined average ferrite grain size can be obtained.

[0027] Dislocation density: 3.0×10 14 m -2 or more 1.0×10 15 m -2 or less To achieve both high strength and high ductility of the steel sheet for cans, controlling the dislocation density is important. When the dislocation density is less than 3.0×10 14 m -2 , the strength of the steel sheet for cans decreases. Therefore, the dislocation density is set to 3.0×10 14 m -2 or more. To improve the strength of the steel sheet for cans, the dislocation density is preferably 3.3×10 14 m -2 or more, more preferably 3.6×10 14 m - 2 or more, even more preferably 4.0×10 14 m -2 or more. Also, when the dislocation density is 1.0×1015 m -2 If it exceeds this value, the ductility of steel sheets for cans decreases. Therefore, the dislocation density should be 1.0 × 10⁻⁶. 15 m -2 The following conditions apply: To improve the ductility of steel sheets for cans, the dislocation density should be 9.7 × 10⁻⁶. 14 m -2 The following is preferable: 9.5 × 10 14 m -2 The following is more preferable: 9.3 × 10 14 m -2 It is even more preferable to have the following: 9.0 × 10 14 m -2 The following is most preferable. The dislocation density is measured based on the method described in the examples. Furthermore, in order to obtain a predetermined dislocation density, as will be described later, it is important to control the rolling ratio in secondary cold rolling, and in particular, the predetermined dislocation density can be obtained by setting the rolling ratio in secondary cold rolling to 5% or more and 20% or less.

[0028] Average plastic strain ratio r-: 1.0 or higher To improve the drawability of steel sheets for cans, it is important to increase the r value. In other words, it is necessary to increase the r value in all directions, and if the average plastic strain ratio r- is less than 1.0, there is a concern that the steel sheets for cans will break during the draw process. For this reason, the average plastic strain ratio r- should be 1.0 or higher. It is more preferable that the average plastic strain ratio r- be 1.1 or higher. It is even more preferable that the average plastic strain ratio r- be 1.2 or higher, and most preferably 1.3 or higher. There is no particular upper limit, but it is preferable that the average plastic strain ratio r- be 3.5 or lower, more preferably 3.3 or lower, even more preferably 3.0 or lower, and most preferably 2.8 or lower. The average plastic strain ratio r- can be calculated using the following formula. r - = (r0 + 2 × r 45 +r 90 ) / 4 Here, r0 is the r value in the direction parallel to the rolling direction, r 45 The r value is the r value in the direction 45° with respect to the rolling direction, r 90This is the r value in the direction perpendicular to the rolling direction. Note that all of the above r values ​​were measured after aging heat treatment at 210°C for 10 minutes. The average plastic strain ratio r- is measured based on the method described in the examples. Furthermore, the predetermined average plastic strain ratio r - To obtain this, as will be described later, it is important to control the winding temperature after hot rolling and the rolling ratio in primary cold rolling, in particular the predetermined average plastic strain ratio r - To ensure this is achieved, it is necessary to strictly adhere to keeping the coiling temperature after hot rolling at 500°C or higher.

[0029] The method for manufacturing steel sheets for cans according to the present invention described above will be explained below. The present invention provides a method for manufacturing steel sheets for cans, comprising: a hot rolling step in which a steel material having the above-described component composition is heated to 1150°C or higher, hot-rolled at a finishing temperature of 820°C to 950°C, cooled from 820°C to a winding temperature of 500°C to 650°C at an average cooling rate of 20°C / s or higher, and wound at a winding temperature of 500°C to 650°C; and cold-rolling the hot-rolled sheet obtained after the hot-rolling step with a rolling ratio of 85% or higher. The method is characterized by comprising: a primary cold rolling step; an annealing step in which the cold-rolled sheet obtained after the primary cold rolling step is held at an annealing temperature of 670°C to 730°C for 5 to 90 seconds, then cooled to a cooling stop temperature of 350°C to 600°C, and held at a temperature range of 250°C to the cooling stop temperature for 30 to 250 seconds; and a secondary cold rolling step in which the annealed sheet obtained after the annealing step is cold-rolled under conditions of a rolling ratio of 5% to 20%.

[0030] Hot rolling process Heating temperature: 1150℃ or more If the heating temperature in the heating process is low, coarse nitrides such as AlN are formed, reducing the amount of dissolved nitrogen that contributes to improving the strength of the steel sheet for cans. Therefore, the heating temperature should be 1150°C or higher. Preferably, the heating temperature should be 1170°C or higher. More preferably, the heating temperature should be 1180°C or higher, even more preferably 1190°C or higher, and most preferably 1200°C or higher. There is no particular upper limit to the heating temperature, but from the viewpoint of manufacturing cost, it is preferable to set it at 1300°C or lower, more preferably 1280°C or lower, even more preferably 1260°C or lower, and most preferably 1240°C or lower.

[0031] Finishing temperature: 820°C to 950°C If the finishing temperature in the hot rolling process exceeds 950°C, the ferrite grain size becomes coarser, reducing the strength of the steel sheet for cans. Therefore, the finishing temperature should be 950°C or lower. Preferably, the finishing temperature should be 945°C or lower. More preferably, the finishing temperature should be 940°C or lower, even more preferably 930°C or lower, and most preferably 920°C or lower. On the other hand, if the finishing temperature is below 820°C, in addition to the formation of coarse ferrite grains during rolling, coarse Nb(C,N) precipitates during hot rolling. Coarse Nb(C,N) does not contribute to improving the strength of the steel sheet for cans and reduces the solid-solution C, solid-solution N, and fine Nb precipitates that improve the strength of the steel sheet for cans, thus contributing to a decrease in strength. Therefore, the finishing temperature in the hot rolling process should be 820°C or higher. Preferably, the finishing temperature should be 825°C or higher. The finishing temperature is more preferably 830°C or higher, even more preferably 840°C or higher, and most preferably 850°C or higher. The finishing temperature refers to the temperature at the end of the finishing rolling process.

[0032] Average cooling rate from 820°C to the winding temperature between 500°C and 650°C: 20°C / s or higher If the average cooling rate from 820°C to the winding temperature of 500°C to 650°C is less than 20°C / s, the amount of Nb present as Nb precipitates decreases, and the ferrite grains become coarser, resulting in a decrease in the strength of the steel sheet for cans. Therefore, the average cooling rate to the winding temperature should be 20°C / s or higher. To increase the strength of the steel sheet for cans, it is preferable that the average cooling rate be 25°C / s or higher. It is more preferable that the average cooling rate be 30°C / s or higher, even more preferable that it be 35°C / s or higher, and most preferable that it be 40°C / s or higher. There is no particular upper limit to the average cooling rate to the winding temperature, but from the viewpoint of reducing the manufacturing load, it is preferable that it be 80°C / s or lower, more preferable that it be 75°C / s or lower, even more preferable that it be 70°C / s or lower, and most preferable that it be 65°C / s or lower. The average cooling rate can be determined by dividing the difference between the cooling start temperature and the cooling stop temperature by the cooling time.

[0033] Winding temperature: 500℃ to 650℃ If the winding temperature exceeds 650°C, the ferrite grains in the can steel sheet become coarser, and the amount of dissolved carbon decreases due to the promotion of the formation of coarse alloy carbides, and the amount of dissolved nitrogen decreases due to the promotion of the formation of coarse alloy nitrides, thus reducing the strength of the can steel sheet. Therefore, the winding temperature should be 650°C or lower. Preferably, the winding temperature should be 645°C or lower. More preferably, the winding temperature should be 640°C or lower, even more preferably 630°C or lower, and most preferably 620°C or lower. On the other hand, if the winding temperature is below 500°C, the amount of dissolved carbon becomes excessive, and the r value of the can steel sheet decreases. Therefore, the winding temperature should be 500°C or higher. Preferably, the winding temperature should be 510°C or higher, more preferably 520°C or higher, even more preferably 530°C or higher, and most preferably 540°C or higher. After winding, pickling may be performed with an aqueous solution of H2SO4, HCl, H3PO4, etc., for the purpose of removing scale.

[0034] Primary cold rolling process Rolling ratio in cold rolling: 85% or more Cold rolling (primary cold rolling) is performed on the hot-rolled sheet obtained after the hot-rolling process described above. This cold-rolling process refines the ferrite grains, improving the strength and r-value of the steel sheet for cans. To ensure sufficient strength and r-value, the rolling ratio is set to 85% or higher. Preferably, the rolling ratio is 86% or higher. More preferably, the rolling ratio is 87% or higher, even more preferably 88% or higher, and most preferably 89% or higher. There is no particular upper limit to the rolling ratio, but to ensure sufficient ductility, it is preferably 95% or lower, preferably 94% or lower, more preferably 93% or lower, even more preferably 92% or lower, and most preferably 91% or lower.

[0035] Annealing process Annealing temperature: 670°C to 730°C, holding time: 5s to 90s, cooling stop temperature: 350°C to 600°C, holding for 30s to 250s in a temperature range of 250°C to the aforementioned cooling stop temperature. Annealing is performed after the cold rolling process described above. The annealing temperature is set to 670°C or higher in order to ensure sufficient ductility by promoting the recrystallization of ferrite. Preferably, the annealing temperature is 675°C or higher, and more preferably 680°C or higher. Even more preferably 685°C or higher, and most preferably 690°C or higher. On the other hand, if the annealing temperature exceeds 730°C, in addition to the coarsening of ferrite grains, the amount of dissolved carbon decreases due to the precipitation of coarse carbides, and the amount of dissolved nitrogen decreases due to the precipitation of coarse nitrides, thus reducing the strength of the steel sheet for cans. For this reason, the annealing temperature is set to 730°C or lower. Preferably, the annealing temperature is 725°C or lower, and more preferably 722°C or lower. Even more preferably 720°C or lower, and most preferably 710°C or lower. 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 should be 5 seconds or longer. Preferably, the holding time should be 8 seconds or longer, and more preferably 10 seconds or longer. Even more preferably, the holding time should be 12 seconds or longer, and most preferably 14 seconds or longer. On the other hand, if the holding time is longer than 90 seconds, the ferrite grain size will coarseen, and the strength of the steel sheet will decrease. Therefore, the holding time at the annealing temperature should be 90 seconds or less. Preferably, the holding time should be 85 seconds or less, and more preferably 80 seconds or less. Even more preferably, the holding time should be 75 seconds or less, and most preferably 70 seconds or less.

[0036] To produce a steel sheet for cans with high strength, high ductility, and excellent deep drawing processability by controlling the amount of solid-solution carbon within an optimal range, the sheet is cooled to a cooling stop temperature of 350°C to 600°C after annealing and soaking, and then held in a temperature range of 250°C to below the aforementioned cooling stop temperature for 30 to 250 seconds. If the cooling stop temperature after annealing exceeds 600°C, the amount of cementite precipitation becomes excessive, resulting in insufficient solid solution carbon, which contributes to improving the strength of the steel sheet for cans. Furthermore, the introduction of dislocations is suppressed, reducing the amount of dislocation strengthening in the steel sheet for cans. Therefore, the cooling stop temperature should be 600°C or lower. Preferably, the cooling stop temperature should be 590°C or lower, and more preferably 580°C or lower. Even more preferably, the cooling stop temperature should be 575°C or lower, and most preferably 570°C or lower. If the cooling stop temperature is below 350°C, the amount of solid solution strengthening due to solid solution carbon and the amount of dislocation strengthening due to the promotion of dislocation introduction become excessive, resulting in a decrease in the ductility and r value of the steel sheet for cans. Therefore, the cooling stop temperature should be 350°C or higher. Preferably, the cooling stop temperature should be 360°C or higher, and more preferably 370°C or higher. Even more preferably, the cooling stop temperature should be 380°C or higher, and most preferably 400°C or higher. Furthermore, if the holding time in the temperature range of 250°C or higher but below the cooling stop temperature exceeds 250 s, the amount of cementite precipitation becomes excessive, resulting in insufficient solid solution carbon, which contributes to improving the strength of the can steel sheet. In addition, the introduction of dislocations is suppressed, reducing the amount of dislocation strengthening in the can steel sheet. Therefore, the holding time in the temperature range of 250°C or higher but below the cooling stop temperature should be 250 s or less. Preferably, the holding time in the temperature range of 250°C or higher but below the cooling stop temperature should be 245 s or less, more preferably 230 s or less, even more preferably 215 s or less, and most preferably 200 s or less. If the holding time in the temperature range of 250°C or higher but below the cooling stop temperature is less than 30 s, the amount of solid solution strengthening due to solid solution carbon and the amount of dislocation strengthening due to the promotion of dislocation introduction become excessive, resulting in a decrease in the ductility and r value of the can steel sheet. Therefore, the holding time in the temperature range of 250°C or higher but below the cooling stop temperature should be 30 s or more. The holding time between 250°C and the cooling stop temperature is preferably 35 seconds or more, more preferably 40 seconds or more, even more preferably 50 seconds or more, and most preferably 60 seconds or more.

[0037] Secondary cold rolling process Rolling ratio in secondary cold rolling: 5% to 20% Secondary cold rolling is performed after the aforementioned annealing process. If the rolling ratio in secondary cold rolling is less than 5%, the dislocation density of the steel sheet for cans decreases, resulting in insufficient strength. Therefore, the rolling ratio should be 5% or more. Preferably, the rolling ratio should be 6% or more, and more preferably 7% or more. Even more preferably, the rolling ratio should be 8% or more, and most preferably 9% or more. On the other hand, if the rolling ratio in secondary cold rolling exceeds 20%, the dislocation density of the steel sheet for cans becomes excessive, and the ductility decreases. Therefore, the rolling ratio should be 20% or less. Preferably, the rolling ratio should be 18% or less, and more preferably 15% or less. Even more preferably, the rolling ratio should be 13% or less, and most preferably 12% or less.

[0038] The steel sheet for cans of the present invention is obtained as described above. From the viewpoint of improving corrosion resistance, the steel sheet for cans obtained here may be subjected to surface treatment such as plating. However, when measuring the dislocation density as described later, the plating or other surface treatment should be removed beforehand before performing the XRD measurement of the steel sheet surface. [Examples]

[0039] Examples of the present invention are shown below. The present invention is not limited to the examples shown herein. Steel slabs were obtained by melting and casting steel containing the components of steel grades No. 1 to 31 shown in Table 1, with the remainder being Fe and unavoidable impurities. These steel slabs were then subjected to hot rolling, primary cold rolling, annealing, and secondary cold rolling under the conditions shown in Table 2 to obtain steel plates No. 1 to 49.

[0040] From the aforementioned steel plates, specimens were taken for analysis of the extraction residue, measurement of the average ferrite particle size, measurement of dislocation density, Lankford test, deep drawing test, and JIS No. 5 tensile test with the direction aligned with the rolling direction as the tensile direction. The samples used for the Lankford test, deep drawing test, and tensile test were subjected to aging heat treatment at 210°C for 10 minutes in a constant temperature oven.

[0041] The amount of dissolved carbon was calculated by analyzing the extraction residue using the following procedure. Samples taken from the aforementioned steel plate were subjected to 10% AA electrolytic extraction, filtration and collection, and mixed acid decomposition, after which the amount of precipitated Fe and precipitated Nb was measured by ICP-AES. From the amount of precipitated Fe and precipitated Nb, the amount of carbon precipitated as Fe3C and NbC (precipitated carbon) was calculated, and the difference between the total amount of carbon and the amount of precipitated carbon was defined as the amount of dissolved carbon.

[0042] The average ferrite grain size was measured using the following procedure. After taking a specimen so that the center of the plate width was the observation surface, the cross section parallel to the rolling direction and at the 1 / 2 position in the plate thickness direction was polished, and the microstructure was exposed by nital etching to prepare a sample for microstructure observation. The above target surface was observed using an optical microscope at a magnification of 200 to 500 times, and the microstructure of three randomly selected fields of view was photographed. The average ferrite grain size was determined by the sectioning method described in JIS G 0551, and the average value of the three fields of view was used.

[0043] Dislocation density was measured using the Williamson-Hall method. XRD was used to measure the αFe(110), (211), (220), and (200) diffraction lines on the steel plate surface using a Co source. The full width at half maximum (FWHM) of the diffraction peaks was corrected using the FWHM obtained from measurements of annealed pure iron (α-Fe) with known dislocation density. The strain ε was then calculated, resulting in ρ = 14.4ε. 2 / (0.25×10 ‐ 9 ) 2 Dislocation density ρ(m -2 ) was calculated.

[0044] The Rankford test was performed using the natural vibration method (JIS Z 2254:2008). The resonance frequencies were measured in the extension direction, at a 45° angle to the rolling direction, and perpendicular to the rolling direction to determine the Young's modulus in each direction. The average plastic strain ratio r- was then calculated and evaluated as the r-value for can steel sheets.

[0045] Deep drawing tests were conducted using cylindrical drawing tests, and the drawability was evaluated by the limiting drawing ratio. A cylindrical punch with a punch diameter of 53.22 mm was used for the test, and a die with a die diameter of 53.84 mm was used. The drawing process was performed under a wrinkle-suppressing force of 2 tons. In addition, to suppress the influence of the sliding state of the steel plate surface on the test results, a polyethylene sheet was placed between the test piece and the die, and the test was conducted under high lubrication conditions. The blank diameter was varied from 80 mm to 120 mm, and the ratio D / d of the maximum blank diameter D that did not break during drawing to the punch diameter d was defined as the limiting drawing ratio. In this invention, a limiting drawing ratio of 2.0 or higher was judged to be excellent in terms of drawability.

[0046] Tensile tests were conducted according to the method specified in JIS Z 2241, and the yield stress and total elongation were evaluated.

[0047] Table 3 shows the evaluation results for solid solution carbon content, ferrite grain size, dislocation density, mean plastic strain ratio r-, critical reduction of area ratio, yield stress, and total elongation. All of the inventive examples in Table 3 have a yield stress of 500 MPa or higher, a total elongation of 7.0% or higher, and a critical reduction of area ratio of 2.0 or higher. Therefore, the inventive examples can be said to be steel sheets for cans with high strength, high ductility, and excellent deep drawing processability. On the other hand, in comparative examples where either the component composition or manufacturing conditions were outside the scope of the invention, one or more of the solid solution carbon content, ferrite particle size, dislocation density, or average plastic strain ratio r- were outside the scope of the invention, and one or more of the limiting reduction ratio, yield stress, or total elongation were inferior to the target values.

[0048] [Table 1]

[0049] [Table 2]

[0050] [Table 3]

Claims

1. In mass percent, C: 0.010% or more and 0.060% or less, Si: 0.05% 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 0.100% or less, Nb: 0.003% or more and 0.030% or less It contains, with the remainder consisting of Fe and unavoidable impurities, The C content of the aforementioned C as solid-solution C is 0.003% or more and 0.030% or less. It has ferrite, The average particle size of the ferrite is 2.0 μm or more and 7.0 μm or less, and Dislocation density is 3.0 × 10⁻⁶ １４ I understand －２ The above 1.0 x 10 １５ I understand －2 The following: Steel sheet for cans, having an average plastic strain ratio r- of 1.0 or higher.

2. The aforementioned component composition is further expressed in mass%, Cu: 0.30% or less, Sn: 0.030% or less, Ni: 0.15% or less, Mo: 0.10% or less The steel sheet for cans according to claim 1, comprising one or more selected from the following.

3. A method for manufacturing steel sheets for cans according to claim 1 or 2, A steel material having the above-mentioned component composition is heated to 1150°C or higher. Hot-rolled at a finishing temperature of 820°C to 950°C, The cooling is performed at an average cooling rate of 20°C / s or more from 820°C to a winding temperature of 500°C to 650°C. A hot rolling process in which the ball is wound at the aforementioned winding temperature, A primary cold rolling process is performed on the hot-rolled sheet obtained after the hot-rolling process, under conditions of a rolling ratio of 85% or more. After the cold-rolled sheet obtained after the primary cold-rolling process is held at an annealing temperature of 670°C to 730°C for 5 to 90 seconds, Cool down to a cooling stop temperature of 350°C to 600°C. An annealing process in which the temperature is maintained at 250°C or higher and below the cooling stop temperature for 30 seconds or more and 250 seconds or less, A method for manufacturing steel sheets for cans, comprising: a secondary cold rolling step in which the annealed sheet obtained after the annealing step is cold-rolled under conditions of a rolling ratio of 5% to 20%.

Images