Hot-rolled steel sheet and method for producing same

Abstract

The purpose of the present invention is to provide: a hot-rolled steel sheet which serves as a starting material of a steel sheet for a can having high strength, high ductility and a high r value; and a method for producing the same.　Disclosed is a hot-rolled steel sheet which has a specific component composition and a ferrite-based structure that contains ferrite in an area ratio of 70% or more, wherein: the average crystal grain size of the ferrite is 15 µm or less in terms of equivalent circle diameter; and the amount of N that is present in the form of a nitride and the total amount of N satisfy formula (1). (1): (Amount of N that is present in form of nitride) / (Total amount of N) ≤ 0.40 The total amount of N is the amount of all N included in the hot-rolled steel sheet.

Description

Hot-rolled steel sheet and manufacturing method thereof

[0001] The present invention relates to a hot-rolled steel sheet suitable as a base material for can steel sheets having high strength, high ductility and a high r-value, and a method for producing the same.

[0002] In recent years, there has been a demand for gauge reduction of steel sheets for cans to reduce the weight of can bodies. Since gauge reduction reduces the strength of can bodies, it is essential to increase the strength of steel sheets. 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, reducing the reduction ratio during secondary cold rolling after annealing is effective, but there is a concern that reducing the reduction ratio will reduce the strength of the steel sheet. To achieve both high strength and high ductility in steel sheets for cans, it is necessary to increase the strength of the base material by utilizing strengthening mechanisms such as solid solution strengthening and grain refinement strengthening.

[0003] Patent Document 1 proposes a hot-rolled steel sheet to be used as a base material for cans, which contains, in mass %, C: 0.02 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, and which has a substantially ferrite single-phase structure, an average ferrite grain size of 6 μm or more, and has 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 after paint baking treatment.

[0004] JP 2013-32596 A

[0005] The above-mentioned conventional techniques have the following problems. The technique described in Patent Document 1 makes it possible to manufacture a hot-rolled steel sheet used as a base material for a steel sheet for cans that has high strength and high ductility, but does not specify the r-value of the steel sheet for cans. In the drawing of steel sheets for cans, it is necessary to suppress fracture during the drawing, and one example of this is to increase the r-value of the steel sheet for cans. In order to increase the r-value of steel sheets for cans, it is important to control the structure of the hot-rolled steel sheet, but this is not considered in the technique described in Patent Document 1.

[0006] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a hot-rolled steel sheet that can be used as a raw material for steel sheets for cans having high strength, high ductility and a high r-value, and a manufacturing method thereof.

[0007] It has been found that by refining the ferrite grains in a hot-rolled steel sheet, a steel sheet for cans manufactured using such a hot-rolled steel sheet can have a high r-value in addition to high strength and high ductility. It has also been found that controlling the temperature of the steel sheet during hot rolling is important for refining the ferrite grains in the hot-rolled steel sheet.

[0008] The present invention has been made to solve the above-mentioned problems, and is summarized as follows: [1] A hot-rolled steel sheet having a chemical composition containing, by mass%, C: 0.010% to 0.150%, Si: 0.10% or less, Mn: 0.10% to 1.20%, P: 0.025% or less, S: 0.040% or less, Al: 0.100% or less, N: 0.0050% to 0.0200%, Cr: 0.010% to 0.150%, with the balance being Fe and unavoidable impurities, and having a structure mainly composed of ferrite with an area ratio of ferrite of 70% or more, wherein the average grain size of the ferrite is 15 μm or less in terms of circle equivalent diameter, and the amount of N present as nitrides and the total amount of N satisfy the following formula (1): (Amount of N present as nitrides) / (Total amount of N)≦0.40 (1) The total amount of N refers to the amount of all N contained in the hot-rolled steel sheet. [2] The hot-rolled steel sheet according to [1], wherein the chemical composition further contains, in mass%, Nb: 0.003% or more and 0.030% or less, and the amount of Nb present as Nb precipitates and the total amount of Nb satisfy 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 refers to the amount of all Nb contained in the hot-rolled steel sheet. [3] The hot-rolled steel sheet according to [1] or [2], wherein the chemical composition further contains, in mass%, one or more selected from Cu: 0.40% or less, Sn: 0.04% or less, Ni: 0.15% or less, and Mo: 0.10% or less. [4] A method for producing a hot-rolled steel sheet according to any one of [1] to [3] above, comprising: a heating step of heating a steel material having the chemical composition at 1150°C or higher; and a hot rolling step of, after the heating step, hot-rolling the steel material 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.

[0009] The present invention has made it possible to manufacture hot-rolled steel sheets suitable as base materials for can steel sheets, which have high strength, high ductility, and a high r-value. The present invention makes it possible to reduce the gauge of can steel sheets, thereby enabling the weight of can bodies to be reduced. Furthermore, the improvement in the formability of can steel sheets makes it possible to perform more complex processing on can bodies and can ends.

[0010] The chemical composition, structure, and manufacturing conditions of the hot-rolled steel sheet of the present invention will be described below. As will be described later, the hot-rolled steel sheet is subjected to cold rolling, annealing, and temper rolling to manufacture a steel sheet for cans. First, the chemical composition of the hot-rolled steel sheet will be described. In the description of the chemical composition, % means mass %.

[0011] C: 0.010% or more and 0.150% or less C is an element that contributes to improving strength. If the C content is less than 0.010%, the strength of the hot-rolled steel sheet (base material) will decrease 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 0.012% or more. The C content is more preferably 0.014% or more, even more preferably 0.016% or more, and most preferably 0.018% or more. On the other hand, if the C content exceeds 0.150%, the ductility and r-value of the base material and the steel sheet for cans will decrease. Therefore, the C content must be 0.150% or less. In order to make the steel sheet for cans have high strength, high ductility and a high r-value, the C content is preferably 0.135% or less, more preferably 0.100% or less, further preferably 0.080% or less, and most preferably 0.070% or less.

[0012] Si: 0.10% or less While Si is an element that contributes to improving strength, excessive Si content reduces the corrosion resistance of hot-rolled steel sheets and steel sheets for cans. Therefore, the Si content needs to be 0.10% or less. The Si content is preferably 0.08% or less, more preferably 0.06% or less, even more preferably 0.05% or less, and most preferably 0.03% or less. There is no particular need to set the lower limit, but in order to improve the strength of the base metal, the Si content is preferably 0.01% or more, more preferably 0.02% or more.

[0013] Mn: 0.10% or more and 1.20% or less Mn is an element that improves hardenability and promotes the solid solution of C in ferrite. Furthermore, it is known that Mn itself contributes to improving strength through solid solution strengthening, and that increasing the Mn content refines the ferrite grain size. Since a base metal with sufficient strength cannot be obtained 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 0.30% or more, even more preferably set to 0.40% or more, and most preferably set to 0.50% or more. On the other hand, since the ductility and r-value of hot-rolled steel sheets and steel sheets for cans decrease when the Mn content exceeds 1.20%, the Mn content is set to 1.20% or less. The Mn content is preferably set to 1.00% or less, and more preferably set to 0.80% or less. The Mn content is more preferably 0.70% or less, and most preferably 0.65% or less.

[0014] P: 0.025% or less Since P reduces the ductility and corrosion resistance of hot-rolled steel sheets and steel sheets for cans, 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 0.020% or less, even more preferably set to 0.018% or less, and most preferably set to 0.016% or less. On the other hand, since P contributes to improving the strength of hot-rolled steel sheets, it is preferable to include 0.001% or more. The lower limit is not particularly limited, but in order to increase the strength of hot-rolled steel sheets, it is preferably set to 0.005% or more, and more preferably set to 0.007% or more.

[0015] S: 0.040% or less S forms MnS in steel and reduces the amount of Mn, which contributes to improving the strength of the hot-rolled steel sheet, so the S content is set to 0.040% or less. The S content is preferably set to 0.030% or less. The S content is more preferably set to 0.025% or less, even more preferably set to 0.023% or less, and most preferably set to 0.020% or less. There is no particular lower limit, but in order to reduce the manufacturing load, such as desulfurization treatment, it is preferably set to 0.005% or more, and more preferably set to 0.007% or more.

[0016] Al: 0.100% or less Al is an element contained to remove oxygen from steel. However, the formation of AlN in steel increases the amount of precipitated N and decreases the amount of dissolved N, thereby reducing the strength of the hot-rolled steel sheet. Therefore, the Al content is set to 0.100% or less. It is preferably set to 0.080% or less. The Al content is more preferably set to 0.070% or less, even more preferably set to 0.055% or less, and most preferably set to 0.040% or less. There is no lower limit, but from the viewpoint of reducing the manufacturing load, such as treating Al-based inclusions, it is preferably set to 0.010% or more, and more preferably set to 0.015% or more.

[0017] N: 0.0050% or more and 0.0200% or less N is an element that contributes to improving the strength of hot-rolled steel sheets 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 0.0068% or more, and most preferably set to 0.0070% or more. On the other hand, if the N content exceeds 0.0200%, the amount of N present as nitrides increases, and the r-value decreases. Therefore, the N content is set to 0.0200% or less. In order to achieve both high strength and a high r-value in can steel sheets, the N content is preferably set to 0.0180% or less. The N content is more preferably set to 0.0160% or less, even more preferably set to 0.0140% or less, and most preferably set to 0.0120% or less.

[0018] Cr: 0.010% or more and 0.150% or less Cr improves hardenability, thereby promoting the solid solution of C in ferrite and improving the strength of the hot-rolled steel sheet. Therefore, the Cr content is set to 0.010% or more. Cr is preferably contained in an amount of 0.020% or more. The Cr content is more preferably 0.025% or more, even more preferably 0.028% or more, and most preferably 0.030% or more. On the other hand, if the Cr content exceeds 0.150%, Cr nitrides become excessive, reducing the amount of solute N, thereby reducing the strength of the hot-rolled steel sheet. Therefore, the Cr content is set to 0.150% or less. The Cr content is preferably set to 0.100% or less. The Cr content is more preferably 0.080% or less, even more preferably 0.070% or less, and most preferably 0.060% or less.

[0019] In the present invention, the above components are essential components, and it is more preferable to contain the following elements in addition to the above component composition.

[0020] 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. Therefore, when Nb is contained, the Nb content is set to 0.003% or more. The Nb content is preferably set to 0.005% or more. Furthermore, in order to achieve both high strength and high ductility in the steel sheet for cans, the Nb content is more preferably set to 0.010% or more. The Nb content is further preferably set to 0.012% or more, and most preferably set to 0.014% or more. On the other hand, if the Nb content exceeds 0.030%, the ductility of the steel sheet for cans decreases due to an increase in the recrystallization temperature after cold rolling. Therefore, when Nb is contained, the Nb content is set to 0.030% or less. In order to achieve both high strength and high ductility in the steel sheet for cans, the Nb content is preferably set to 0.025% or less. The Nb content is more preferably 0.022% or less, further preferably 0.021% or less, and most preferably 0.020% or less.

[0021] In the present invention, in addition to the above-mentioned component composition, one or more elements selected from the following may be contained.

[0022] Cu: 0.40% or less, Sn: 0.04% or less, Ni: 0.15% or less, Mo: 0.10% or less Cu, Sn, Ni, and Mo improve the strength of the base material through solid solution strengthening. However, excessive content of these elements reduces the ductility of the hot-rolled steel sheet and the steel sheet for cans. Therefore, when Cu, Sn, Ni, and Mo are contained, the Cu content is set to 0.40% or less, the Sn content is set to 0.04% 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 the ductility of the steel sheet for cans, the Cu content is preferably set to 0.30% or less, the Sn content is preferably set to 0.03% 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 0.25% or less, the Sn content is more preferably 0.02% or less, the Ni content is more preferably 0.10% or less, and the Mo content is more preferably 0.06% or less. There are no particular lower limits, but the Cu content is preferably 0.01% or more, and more preferably 0.03% or more. The Sn content is preferably 0.01% or more. The Ni content is preferably 0.01% or more, and more preferably 0.02% or more. The Mo content is preferably 0.01% or more, and more preferably 0.02% or more.

[0023] A hot-rolled 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 H, Ca, O, Co, W, Zn, Pb, As, Sb, and Bi, which are mixed in from raw materials or during the manufacturing process.

[0024] The structure of the hot-rolled steel sheet in the present invention will be described.

[0025] Ferrite-based structure The hot-rolled steel sheet of the present invention has a ferrite-based structure. Here, a ferrite-based structure is defined as one in which the area ratio (area fraction) of ferrite is 70% or more. Therefore, the area fraction of ferrite is 70% or more. Furthermore, the area fraction of ferrite is preferably 80% or more, more preferably 85% or more, even more preferably 90% or more, and most preferably 92% or more. The upper limit is not particularly limited, but may be 100%. 99% or less is more preferable. The remainder other than ferrite may include cementite, pearlite, bainite, martensite, retained austenite, etc. If the area fraction of the remainder is 30% or less, the requirements of the present invention are met, and the area fraction of the remainder may be 0%.

[0026] Average ferrite grain size of hot-rolled steel sheet is 15 μm or less in equivalent circle diameter. In order to achieve both high strength and high ductility in steel sheets for cans, it is important to refine the ferrite grains in the hot-rolled steel sheet that serves as the base material for the steel sheet for cans. Therefore, it is important to increase the strength of the hot-rolled steel sheet by refining the ferrite grains in the hot-rolled steel sheet. If the average ferrite grain size of the hot-rolled steel sheet exceeds 15 μm, the reduction ratio during secondary cold rolling must be increased to obtain a steel sheet for cans with sufficient strength, making it difficult to achieve both high strength and high ductility in the steel sheet for cans. Furthermore, the most important technology is as follows: by refining the ferrite grains in the hot-rolled steel sheet, the ferrite texture of the steel sheet for cans obtained using the hot-rolled steel sheet can be controlled, enabling the steel sheet for cans to have a high r-value. The reason for this is presumably that, to improve the r-value of a steel sheet for cans, it is necessary to develop the {111} texture of the steel sheet for cans, and the refinement of ferrite grains in the hot-rolled steel sheet promotes the development of the {111} texture of the steel sheet for cans. Therefore, the average ferrite grain size of the hot-rolled steel sheet is set to 15 μm or less in equivalent circle diameter. Preferably, it is set to 12 μm or less. More preferably, it is set to 11 μm or less, even more preferably, it is set to 10 μm or less, and most preferably, it is set to 9 μm or less. Although there is no lower limit, from the viewpoint of reducing the manufacturing load, the average ferrite grain size is preferably set to 3 μm or more in equivalent circle diameter. More preferably, it is set to 4 μm or more, and even more preferably, it is set to 5 μm or more. As will be described later, in order to obtain a predetermined average ferrite grain size, it is important to control the finishing temperature in hot rolling, the average cooling rate from 800°C to the coiling temperature, and the coiling temperature.

[0027] The amount of N present as nitrides and the total amount of N satisfy the following formula (1): (Amount of N present as nitrides) / (Total amount of N)≦0.40 (1). Note that the total amount of N refers to the total amount of N contained in the hot-rolled steel sheet. In order to increase the strength of the hot-rolled steel sheet, it is important to suppress the ratio of the amount of N present as nitrides in the hot-rolled steel sheet (NasNitride) to the total amount of N in the hot-rolled steel sheet. If formula (1) is not satisfied, the amount of dissolved N becomes insufficient, resulting in a decrease in the strength of the hot-rolled steel sheet. Therefore, the ratio (Amount of N present as nitrides) / (Total amount of N) is set to 0.40 or less. Furthermore, if the amount of N present as nitrides in the hot-rolled steel sheet becomes excessive, there is a concern that the r-value of the steel sheet for cans manufactured using the hot-rolled steel sheet will decrease. Therefore, in order to further increase the r-value of the steel sheet for cans, the ratio (Amount of N present as nitrides) / (Total amount of N) in the hot-rolled steel sheet is preferably set to 0.30 or less. It is more preferably 0.28 or less, even more preferably 0.25 or less, most preferably 0.23 or less, and most preferably 0.20 or less. There are no particular restrictions on the lower limit, but it is preferably 0.01 or more, more preferably 0.03 or more. Note that, since the coiling temperature and the cooling rate from 800°C to the coiling temperature have a significant effect on formula (1), it is important to control the coiling temperature and the average cooling rate from 800°C to the coiling temperature within a predetermined range in order to obtain formula (1).

[0028] The amount of Nb present as Nb precipitates and the total amount of Nb satisfy the following formula (2): 0.10≦(amount of Nb present as Nb precipitates) / (total Nb)≦0.80 (2). The total amount of Nb refers to the total amount of Nb contained in the hot-rolled steel sheet. When Nb is contained, it is important to increase the strength of the hot-rolled steel sheet by increasing the ratio of the amount of Nb present as Nb precipitates in the hot-rolled steel sheet to the total amount of Nb in the hot-rolled steel sheet. If the value of (amount of Nb present as Nb precipitates) / (total Nb) is less than 0.10, the amount of Nb contributing to precipitation strengthening is small, and further strengthening of the hot-rolled steel sheet cannot be expected. Furthermore, the ferrite grains in the hot-rolled steel sheet are not refined, and further improvement in the r-value of the can steel sheet obtained using the hot-rolled steel sheet cannot be expected. Therefore, it is preferable that the ratio of (amount of Nb present as Nb precipitates) / (total Nb) is 0.10 or more. It is more preferably 0.15 or more. It is even more preferably 0.17 or more, and most preferably 0.20 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 for cans decreases due to an increase in the recrystallization temperature after cold rolling. Therefore, it is preferable that (amount of Nb present as Nb precipitates) / (total amount of Nb) is 0.80 or less. It is more preferably 0.70 or less. It is even more preferably 0.69 or less, most preferably 0.68 or less, and even most preferably 0.60 or less. Note that Nb precipitates include all of Nb carbides, Nb nitrides, and Nb-containing carbonitrides.

[0029] The method for producing a hot-rolled steel sheet according to the present invention will be described.

[0030] The method for producing a hot-rolled steel sheet according to the present invention is characterized by comprising a heating step of heating a steel material having the above-described chemical composition to 1150°C or higher, and 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. Unless otherwise specified, the steel sheet temperature described in the present invention refers to the temperature of the surface of the steel sheet, and this temperature is measured with a radiation thermometer.

[0031] Heating temperature: 1150°C or higher If the heating temperature in the heating step is low, coarse nitrides such as AlN are formed, and the ratio of the amount of N present as nitrides to the total amount of N in the hot-rolled steel sheet increases, resulting in a decrease in the strength of the base material and the r-value of the steel sheet for cans. 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 1200°C or higher, and most preferably 1220°C or higher. There is no upper limit to the heating temperature, but from the viewpoint of production costs, it is preferably set to 1300°C or lower, more preferably 1280°C or lower.

[0032] Finishing temperature: 800°C or higher and 950°C or lower If the finishing temperature in the hot rolling process exceeds 950°C, ferrite grains become coarse, not only reducing the strength of the hot-rolled steel sheet but also reducing the r-value of the steel sheet for cans manufactured using the hot-rolled steel sheet. Therefore, the finishing temperature is set to 950°C or lower. The finishing temperature is preferably set to 930°C or lower. The finishing temperature is more preferably set to 920°C or lower, and most preferably set to 900°C or lower. On the other hand, if the finishing temperature is lower than 800°C, coarse ferrite grains are generated during hot rolling, not only reducing the strength of the hot-rolled steel sheet but also reducing the r-value of the steel sheet for cans manufactured using the hot-rolled steel sheet. Therefore, the finishing temperature is set to 800°C or higher. Furthermore, if Nb is contained, coarse Nb(C,N) precipitates during hot rolling if the finishing temperature is lower than 800°C. Coarse Nb(C,N) does not contribute to improving the strength of the hot-rolled steel sheet, and also reduces the amount of solute C and N, thereby lowering the strength of the hot-rolled steel sheet. Therefore, the finishing temperature in the hot rolling process is set to 800°C or higher. Regardless of whether Nb is contained or not, the finishing temperature is preferably set to 820°C or higher, more preferably 830°C or higher, even more preferably 840°C or higher, and most preferably 850°C or higher.

[0033] Average cooling rate from 800°C to the coiling temperature: 20°C / s or more If the average cooling rate from 800°C to the coiling temperature is less than 20°C / s, ferrite grains after the hot rolling process will become coarse, and the strength of the hot-rolled steel sheet will decrease. Therefore, the average cooling rate from 800°C to the coiling temperature is set to 20°C / s or more. In order to increase the strength of the hot-rolled steel sheet, the average cooling rate is preferably 25°C / s or more, more preferably 30°C / s or more. It is further preferably 35°C / s or more, and most preferably 40°C / s or more. There is no upper limit to the average cooling rate to the coiling temperature, but from the viewpoint of reducing the production load, it is preferably 80°C / s or less, more preferably 75°C / s or less, and even more preferably 70°C / s or less. The average cooling rate can be calculated by dividing the temperature difference between the cooling start temperature (here, 800°C) and the cooling stop temperature (here, the coiling temperature) by the cooling time required for cooling.

[0034] Coiling temperature: 450°C or higher and 700°C or lower When the coiling temperature exceeds 700°C, ferrite grains become coarse, and the strength of the hot-rolled steel sheet decreases. Furthermore, the formation of coarse alloy carbides and nitrides is promoted, which reduces the amount of solute C and solute N, causing a decrease in the strength of the hot-rolled 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 640°C or lower, even more preferably set to 630°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 alloy carbides such as Nb carbide decreases, causing a decrease in the strength of the hot-rolled 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 520°C or higher, even more preferably 530°C or higher, most preferably 540°C or higher, and most preferably 550°C or higher. After coiling, the wire is heated by H ２ SO ４ , HCl, H ３ P.O. ４ Alternatively, pickling may be carried out using an aqueous solution of the above.

[0035] The cooling rate after coiling is not particularly limited, but particularly when the coiling temperature is high (650°C or higher and 700°C or lower), the average ferrite grain size and the sizes of nitrides and Nb precipitates are affected. Therefore, the average cooling rate after coiling is preferably 1°C / s or higher, more preferably 2°C / s or higher, and even more preferably 3°C / s or higher. Furthermore, the average cooling rate after coiling is preferably 100°C / s or lower, more preferably 95°C / s or lower, and even more preferably 90°C / s or lower. The average cooling rate after coiling refers to the average cooling rate from the coiling temperature to 300°C, and is calculated by dividing the temperature difference between the coiling temperature and 300°C by the cooling time (the time required for cooling during this period).

[0036] The hot-rolled steel sheet obtained as described above can be subjected to cold rolling, annealing, and temper rolling, which are the usual manufacturing conditions for can steel sheets, to obtain a can steel sheet having high strength, high ductility, and a high r-value. The cold rolling, annealing, and temper rolling are not particularly limited. For example, cold rolling is preferably performed at a rolling reduction of 70% or more, more preferably 75% or more. Furthermore, it is preferably performed at a rolling reduction of 95% or less, more preferably 93% or less. For example, annealing is preferably performed at 680°C or more, more preferably 700°C or more. Furthermore, annealing is preferably performed at 850°C or less, more preferably 800°C or less. The annealing is preferably performed at the above temperature for 5 seconds or more, more preferably 10 seconds or more. Furthermore, it is preferably performed at the above temperature for 90 seconds or less, more preferably 85 seconds or less. After holding for the above time, the steel sheet is preferably cooled at an average cooling rate of 10°C / s or more, more preferably 30°C / s or more, to a cooling stop temperature of 600°C or less. Furthermore, the steel sheet is preferably cooled at a rate of 150°C / s or less, more preferably 130°C / s or less. The temper rolling is preferably carried out at a reduction rate of 0.1% or more, more preferably 0.3% or more. The reduction rate is preferably 5.0% or less, more preferably 3.0% or less. The average cooling rate to the cooling stop temperature of 600°C or less is calculated by dividing the temperature difference between the annealing temperature and the cooling stop temperature by the cooling time required for cooling during this period.

[0037] The chemical composition of the steel sheet for cans is preferably the same as that of the hot-rolled steel sheet described above, for the reasons explained in the hot-rolled steel sheet. The microstructure of the steel sheet for cans is preferably a structure mainly composed of ferrite.

[0038] By using the above-mentioned hot-rolled steel sheet, a steel sheet for cans having an upper yield stress of 380 MPa or more, a total elongation of 20% or more, and an r-value of 0.9 or more was obtained. Although there are no particular upper limits for the properties of the above-mentioned steel sheet for cans, it is preferable that the upper yield stress is 700 MPa or less, the total elongation is 50% or less, and the r-value is 2.5 or less.

[0039] The above-mentioned upper yield stress, total elongation, and r-value 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.

[0040] Examples of the present invention are shown below, but the present invention is not limited to the examples shown here.

[0041] Steel materials (steel slabs) were obtained by melting and casting steels containing the components of steel types No. 1 to 16 shown in Table 1, with the balance being Fe and unavoidable impurities. The steel slabs thus obtained were heated and hot-rolled under the conditions shown in Table 2, to obtain hot-rolled steel sheets No. 1 to 17.

[0042] The observation of the hot-rolled steel sheet structure was carried out by the following procedure. A test piece was taken from the hot-rolled steel sheet so that the center of the sheet width was the observation surface, and then a cross section (L cross section) parallel to the rolling direction and the sheet thickness direction at a position halfway in the sheet thickness direction was polished and subjected to nital etching to reveal the structure, thereby obtaining a sample for structure observation. The target surface was observed at a magnification of 400 times using an optical microscope, and the structure of three randomly selected fields was photographed. The ferrite grain size was determined by the cutting method described in JIS G 0551, and the average value of the three fields was taken as the average ferrite grain size.

[0043] The amount of N present as nitrides and the amount of Nb present as Nb precipitates were measured by pickling a sample taken from the hot-rolled steel sheet to remove scale, and then measuring the amount of N present as nitrides using the following method. ２ SO ４ +K ２ SO ４ The amount of Nb present as Nb precipitates was determined by ICP-AES measurement after AA electrolytic extraction, filter collection, and mixed acid decomposition.

[0044] Table 3 shows the calculation results of the following formulas (1) and (2): (Amount of N present as nitrides) / (Total amount of N)≦0.40 (1) The total amount of N is the total amount of N contained in the hot-rolled steel sheet. 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 hot-rolled steel sheet.

[0045] Furthermore, JIS No. 5 tensile test pieces were taken from the hot-rolled steel sheets obtained above, with the tensile direction being along the rolling direction, and tensile strength was evaluated by carrying out a tensile test in accordance with JIS Z 2241. Hot-rolled steel sheets having a tensile strength of 350 MPa or more were determined to have excellent tensile strength.

[0046] In addition, hot-rolled steel sheets No. 1 to 17 were pickled, then cold-rolled at a reduction ratio of 85% or more and 90% or less, and then annealed at a soaking temperature of 720°C for 21 seconds to produce steel sheets for cans. Test specimens to be subjected to tensile tests and Lankford tests were taken from the steel sheets for cans and subjected to aging heat treatment at 210°C for 10 minutes in an incubator. Thereafter, the tensile tests and Lankford tests were carried out as described below.

[0047] For the tensile test, a JIS No. 5 tensile test piece was used, with the tensile direction aligned with the rolling direction. The tensile test was carried out in accordance with JIS Z 2241, and the upper yield stress and total elongation were evaluated.

[0048] The Lankford test was carried out by the natural frequency method (JSS YR-05058). The resonance frequencies in the rolling direction, the direction at 45° to the rolling direction, and the direction perpendicular to the rolling direction were measured to obtain the Young's modulus in each direction, and the average plastic strain ratio r- was calculated and evaluated as the r-value of the steel sheet for cans.

[0049] Table 3 shows the evaluation results of the structure and tensile strength of the hot-rolled steel sheets, and the evaluation results of the upper yield stress, total elongation, and r-value of the steel sheets for cans. In all of the invention examples in Table 3, the hot-rolled steel sheets had a ferrite area ratio of 70% or more, an average ferrite grain size of 15 μm or less, satisfied the following formula (1), and obtained a tensile strength of 350 MPa or more. All of the steel sheets for cans produced by cold rolling and annealing the hot-rolled steel sheets of the invention examples described above obtained an upper yield stress of 380 MPa or more, a total elongation of 20% or more, and an r-value of 0.9 or more.

[0050] Therefore, it can be said that the examples of the present invention are hot-rolled steel sheets suitable as base materials for steel sheets for cans, which have high strength, high ductility, and a high r-value. On the other hand, in the comparative examples, whose chemical compositions or manufacturing conditions are outside the scope of the present invention, any one of the ferrite fraction, the average ferrite grain size, and the formula (1) is outside the scope of the present invention, and therefore, the tensile strength of the hot-rolled steel sheet, the upper yield stress of the steel sheet for cans, the total elongation, and the r-value did not achieve the target properties.

[0051]

[0052]

[0053]

Claims

1. A hot-rolled steel sheet containing, by mass%, the following: C: 0.010% to 0.150%, Si: 0.10% or less, Mn: 0.10% to 1.20%, P: 0.025% or less, S: 0.040% or less, Al: 0.100% or less, N: 0.0050% to 0.0200% or less, Cr: 0.010% to 0.150% or less, with the balance being Fe and unavoidable impurities, and having a structure mainly composed of ferrite with an area ratio of ferrite of 70% or more, the average grain size of the ferrite being 15 μm or less in equivalent circle diameter, and the amounts of N present as nitrides and total N satisfying the following formula (1): (Amount of N present as nitrides) / (Total N)≦0.40 (1) The total N amount is the amount of all N contained in the hot-rolled steel sheet.

2. The hot-rolled steel sheet according to claim 1, wherein the chemical composition further contains, in mass%, Nb: 0.003% to 0.030%, and the amount of Nb present as Nb precipitates and the total amount of Nb satisfy the following formula (2): 0.10≦(amount of Nb present as Nb precipitates) / (total amount of Nb)≦0.80 (2). Note that the total amount of Nb is the amount of all Nb contained in the hot-rolled steel sheet.

3. The hot-rolled steel sheet according to claim 1 or 2, wherein the chemical composition further contains, in mass%, one or more elements selected from Cu: 0.40% or less, Sn: 0.04% or less, Ni: 0.15% or less, and Mo: 0.10% or less.

4. A method for producing hot-rolled 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; and a hot rolling step of, after the heating step, hot-rolling the steel material 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.

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