Hot-rolled steel sheet and method for producing same
Patent Information
- Application Number
- AU2025260666
- Authority / Receiving Office
- AU · AU
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2024-04-23
- Filing Date
- 2025-04-18
- Publication Date
- 2026-09-17
Abstract
Description
Title of Invention: HOT ROLLED STEEL SHEET AND METHOD FOR PRODUCING SAME Technical Field
[0001] The present invention particularly relates to a hot rolled steel sheet that is suitable as a base material of a high-strength, high-ductility, and high-r-value steel sheet for cans, and to a method for producing the same. Background Art
[0002] In recent years, downgauging of the steel sheets for cans has been pursued to reduce the weight of cans. Since downgauging decreases the can strength, it is essential to increase the strength of the steel sheet. Double reduced (DR) steel is known as high-strength steel sheets for cans. DR steel is a steel sheet for cans that has been strengthened by performing cold rolling again after previous cold rolling and annealing. However, the DR steel has a problem of low elongation and thus low workability. In order to solve this problem, it is effective to decrease the rolling reduction during the second cold rolling after the annealing; however, the concern is that decreasing the rolling reduction may decrease the strength of the steel sheet. In order to achieve both high strength and high ductility in a steel sheet for cans, the strength of the base material needs to be increased by utilizing the strengthening mechanisms such as solid solution strengthening and grain refinement strengthening.
[0003] Patent Literature 1 proposes a hot rolled steel sheet for use as a base material for a steel sheet for cans, the hot rolled steel sheet having a composition containing, 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%, and the balance Fe and inevitable impurities, and having a microstructure that is substantially a ferrite single phase, in which the ferrite average crystal grain size is 6 pm or more, and, after a bake coating process, the yield strength is 500 MPa or more, the yield ratio is 0.9 or more, the total elongation is 10% or more, and Ar is -0.50 to 0. Citation List Patent Literature
[0004] PTL 1: Japanese Unexamined Patent Application Publication No. 2013-32596 Summary of Invention Technical Problem
[0005] The aforementioned related art has the following problems. According to the features set forth in Patent Literature 1, a hot rolled steel sheet for use as a base material for a high-strength and high-ductility steel sheet for cans can be produced; however, the r value of the steel sheet for cans is not specified. In drawing a steel sheet for cans, rupture during drawing needs to be suppressed, and one way of achieving this is to increase the r value of the steel sheet for cans. It is critical to control the microstructure of the hot rolled steel sheet in order to increase the r value of the steel sheet for cans; however, the features set forth in Patent Literature 1 do not consider this.
[0006] The present invention has been made to address the problems described above, and an object thereof is to provide a hot rolled steel sheet that can be used as a raw material for a high-strength, high-ductility, and high-rvalue steel sheet for cans, and a method for producing the same. Solution to Problem
[0007] It has been found that a steel sheet for cans produced by using a hot rolled steel sheet in which ferrite grains have been refined can achieve high strength, high ductility, and a high r value. It has also been found that, for refinement of ferrite grains in the hot rolled steel sheet, it is critical to control the temperature of the steel sheet during hot rolling.
[0008] The present invention has been made to solve the aforementioned problems and the gist thereof is as follows. [1] A hot rolled steel sheet which comprises a chemical composition containing, in mass%: C: 0.010% or more and 0.150% or less, Si: 0.10% or less, Mn: 0.10% or more and 1.20% or less, P: 0.025% or less, S: 0.040% or less, Al: 0.100% or less, N: 0.0050% or more and 0.0200% or less, Cr: 0.010% or more and 0.150% or less, and the balance being Fe and inevitable impurities, and a microstructure composed mainly of ferrite in which a ferrite area ratio is 70% or more. The ferrite has an average crystal grain size of 15 pm or less in equivalent circular diameter, and an amount of N present as nitrides and a total amount of N satisfy formula (1) below: (amount of N present as nitrides) / (total amount of N) < 0.40 ••• (1) where the total amount of N is a total amount of N contained in the hot rolled steel sheet. [2] The hot rolled steel sheet described in [1], in which the chemical composition further includes, in mass%: Nb: 0.003% or more and 0.030% or less, and an amount of Nb present as Nb precipitates and a total amount of Nb satisfy formula (2) below: 0.10 < (amount of Nb present as Nb precipitates) / (total amount of Nb) < 0.80 ••• (2) where the total amount of Nb is a total amount of Nb contained in the hot rolled steel sheet. [3] The hot rolled steel sheet described in [1] or [2], in which the chemical composition further contains, in mass%, at least one 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 the hot rolled steel sheet described in any one of [1] to [3] which comprises: a heating step of heating a steel material having the chemical composition at 1150°C or more; and a hot rolling step, after the heating step, of hot rolling the steel material at a finishing temperature of 800°C or more and 950°C or less, cooling at an average cooling rate of 20°C / s or more from 800°C to a coiling temperature of 450°C or more and 700°C or less, and coiling at the coiling temperature. Advantageous Effects of Invention
[0009] The present invention has made it possible to produce a hot rolled steel sheet that is suitable as a base material of a high-strength, high-ductility, and high-r-value steel sheet for cans. Since the present invention makes downgauging of the steel sheet for cans possible, weight reduction of the cans can be achieved. Furthermore, since the workability of the steel sheet for cans is improved, can bodies and can lids can be subjected to a more complicated process. Description of Embodiments
[0010] The chemical composition, the microstructure, and the production conditions of the hot rolled steel sheet according to the present invention will now be described. As described below, a steel sheet for cans is produced by performing cold rolling, annealing, and temper rolling on a hot rolled steel sheet. First, the chemical composition of the hot rolled steel sheet is 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 the strength. At a C content of less than 0.010%, the strength of the hot rolled steel sheet (base material) decreases due to coarsening of the ferrite grain size and the decrease in amount of solute C; thus, the C content needs to be 0.010% or more. The C content is preferably 0.012% or more. The C content is more preferably 0.014% or more, yet more preferably 0.016% or more, and most preferably 0.018% or more. At a C content exceeding 0.150%, the ductility and the r value of the base material and the steel sheet for cans decrease. Thus, the C content needs to be 0.150% or less. For the steel sheet for cans to achieve high strength, high ductility, and a high r value, the C content is preferably 0.135% or less, more preferably 0.100% or less, yet more preferably 0.080% or less, and most preferably 0.070% or less.
[0012] Si: 0.10% or less Si is an element that contributes to improving the strength, but excessive Si degrades the corrosion resistance of the hot rolled steel sheet and the steel sheet for cans. Thus, 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, yet more preferably 0.05% or less, and most preferably 0.03% or less. Although there is no particular need to limit the lower limit, the Si content is preferably 0.01% or more and more preferably 0.02% or more to improve the strength of the base material.
[0013] Mn: 0.10% or more and 1.20% or less Mn is an element that improves hardenability and promotes C to solidly dissolve into ferrite. Furthermore, Mn is known to contribute to improving the strength through the solid solution strengthening of Mn itself; also known is that the ferrite grain size becomes finer as the Mn content increases. At a Mn content of less than 0.10%, a base material having sufficient strength is not obtained; thus, the Mn content is to be 0.10% or more. The Mn content is preferably 0.20% or more. The Mn content is more preferably 0.30% or more, yet more preferably 0.40% or more, and most preferably 0.50% or more. Meanwhile, at a Mn content exceeding 1.20%, the ductility and the r value of the hot rolled steel sheet and the steel sheet for cans decrease; thus, the Mn content is to be 1.20% or less. The Mn content is preferably 1.00% or less and more preferably 0.80% or less. The Mn content is yet more preferably 0.70% or less and most preferably 0.65% or less.
[0014] P: 0.025% or less P decreases the ductility and corrosion resistance of the hot rolled steel sheet and the steel sheet for cans, and thus the P content is to be 0.025% or less. The P content is preferably 0.023% or less. The P content is more preferably 0.020% or less, yet more preferably 0.018% or less, and most preferably 0.016% or less. Meanwhile, P contributes to improving the strength of the hot rolled steel sheet, and thus a P content of 0.001% or more is preferable. Although the lower limit is not particularly limited, the P content is preferably 0.005% or more and more preferably 0.007% or more to increase the strength of the hot rolled steel sheet.
[0015] S: 0.040% or less S forms MnS in the steel and decreases the amount of Mn contributing to improving the strength of the hot rolled steel sheet; thus, the S content is to be 0.040% or less. The S content is preferably 0.030% or less. The S content is more preferably 0.025% or less, yet more preferably 0.023% or less, and most preferably 0.020% or less. Although the lower limit is not particularly limited, the S content is preferably 0.005% or more and more preferably 0.007% or more to reduce the production load such as the desulfurization process.
[0016] Al: 0.100% or less Al is an element contained to remove oxygen in the steel. However, since AlN is formed in the steel, the amount of precipitated N increases, and the amount of solute N decreases; thus, the strength of the hot rolled steel sheet decreases. Thus, the Al content is to be 0.100% or less. The Al content is preferably 0.080% or less. The Al content is more preferably 0.070% or less, yet more preferably 0.055% or less, and most preferably 0.040% or less. Although the lower limit is not limited, the Al content is preferably 0.010% or more and more preferably 0.015% or more from the viewpoint of reducing the production load such as processing of Al-based inclusions.
[0017] N: 0.0050% or more and 0.0200% or less N is an element that contributes to improving the strength of the hot rolled steel sheet through solid solution strengthening. Thus, the N content is to be 0.0050% or more. The N content is preferably 0.0060% or more. The N content is more preferably 0.0065% or more, yet more preferably 0.0068% or more, and most preferably 0.0070% or more. At a N content exceeding 0.0200%, the amount of N present as nitrides increases, and the r value decreases. Thus, the N content is to be 0.0200% or less. In order to achieve both high strength and a high r value in a steel sheet for cans, the N content is preferably 0.0180% or less. The N content is more preferably 0.0160% or less, yet more preferably 0.0140% or less, and most preferably 0.0120% or less.
[0018] Cr: 0.010% or more and 0.150% or less Cr promotes C to solidly dissolve into ferrite by improving the hardenability, and improves the strength of the hot rolled steel sheet; thus, the Cr content is to be 0.010% or more. The Cr content is preferably 0.020% or more. The Cr content is more preferably 0.025% or more, yet more preferably 0.028% or more, and most preferably 0.030% or more. Meanwhile, at a Cr content exceeding 0.150%, the amount of Cr nitrides becomes excessive, and the amount of solute N decreases; thus, the strength of the hot rolled steel sheet decreases. Thus, the Cr content is to be 0.150% or less. The Cr content is preferably 0.100% or less. The Cr content is more preferably 0.080% or less, yet more preferably 0.070% or less, and most preferably 0.060% or less.
[0019] In the present invention, the aforementioned components are essential components, and, in addition to the aforementioned component composition, the following elements are more preferably contained.
[0020] Nb: 0.003% or more and 0.030% or less Nb is an element that contributes to precipitation strengthening and grain refinement strengthening through formation of fine NbC in the steel. Thus, when Nb is contained, the Nb content is to be 0.003% or more. The Nb content is preferably 0.005% or more. Furthermore, in order to achieve both high strength and high ductility in a steel sheet for cans, the Nb content is more preferably 0.010% or more. The Nb content is yet more preferably 0.012% or more and most preferably 0.014% or more. Meanwhile, at a Nb content exceeding 0.030%, the ductility of the steel sheet for cans decreases due to the increase in recrystallization temperature after the cold rolling. Thus, when Nb is contained, the Nb content is to be 0.030% or less. In order to achieve both high strength and high ductility in a steel sheet for cans, the Nb content is preferably 0.025% or less. The Nb content is more preferably 0.022% or less, yet more preferably 0.021% or less, and most preferably 0.020% or less.
[0021] In the present invention, at least one selected from the following elements may be contained in addition to the chemical composition described above.
[0022] Cu: 0.40% or less, Sn: 0.04% or less, Ni: 0.15% or less, and Mo: 0.10% or less Cu, Sn, Ni, and Mo improve the strength of the base material through solid solution strengthening. However, when these elements are excessively contained, the ductility decreases. Thus, when Cu, Sn, Ni, and / or Mo is to be contained, the Cu content is 0.40% or less, the Sn content is 0.04% or less, the Ni content is 0.15% or less, and the Mo content is 0.10% or less. In order to obtain the ductility of the steel sheet for cans, the Cu content is preferably 0.30% or less, the Sn content is preferably 0.03% or less, the Ni content is preferably 0.12% or less, and the Mo content is preferably 0.08% or less. The Cu content is more preferably 0.25% 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. Although the lower limit is not particularly limited, 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 an embodiment of the present invention has a chemical composition that contains the aforementioned components and the balance Fe and inevitable impurities. Examples of the inevitable impurities include H, Ca, O, Co, W, Zn, Pb, As, Sb, and Bi that mix in from the raw materials or during the production process.
[0024] The microstructure of the hot rolled steel sheet according to the present invention will now be described.
[0025] Microstructure composed mainly of ferrite microstructure The hot rolled steel sheet according to the present invention has a microstructure composed mainly of ferrite. Here, a microstructure is considered to be composed mainly of ferrite when the area ratio of ferrite is 70% or more. Thus, the ferrite area ratio is 70% or more. The ferrite area ratio is more preferably 80% or more, yet more preferably 85% or more, still more preferably 90% or more, and most preferably 92% or more. The upper limit is not particularly limited and may be 100%. The ferrite area ratio is more preferably 99% or less. The remaining portion other than ferrite may include cementite, pearlite, bainite, martensite, and retained austenite. The requirement of the present invention is considered satisfied as long as the area ratio of the remaining portion is 30% or less, and the area ratio of the remaining portion may be 0%.
[0026] Ferrite average crystal grain size of hot rolled steel sheet is 15 pm or less in equivalent circular diameter. In order to achieve both high strength and high ductility in a steel sheet for cans, refinement of ferrite grains in a hot rolled steel sheet to be used as a base material of a steel sheet for cans is critical, and thus it is important to increase the strength of the hot rolled steel sheet through refinement of ferrite grains in the hot size of the hot rolled steel sheet exceeds 15 pm, the rolling reduction during second cold rolling must be increased in order to obtain a steel sheet for cans having sufficient strength, and it becomes difficult to achieve both high strength and high ductility in a steel sheet for cans. Furthermore, the most critical feature is that, through refinement of ferrite grains in the hot rolled steel sheet, the ferrite texture of the steel sheet for cans obtained by using this hot rolled steel sheet can be controlled, and the r value of the steel sheet for cans can be increased. The reason for this is presumably that, while the {111} texture of the steel sheet for cans needs to be developed in order to improve the r value of the steel sheet for cans, refinement of ferrite grains in the hot rolled steel sheet promotes development of the {111} texture of the steel sheet for cans. Thus, the ferrite average crystal grain size of the hot rolled steel sheet is to be 15 pm or less in equivalent circular diameter. The ferrite average crystal grain size is preferably 12 pm or less. The ferrite average crystal grain size is more preferably 11 pm or less, yet more preferably 10 pm or less, and most preferably 9 pm or less. Although the lower limit is not limited, the ferrite average crystal grain size is preferably 3 pm or more in equivalent circular diameter from the viewpoint of reducing the production load. The ferrite average crystal grain size is more preferably 4 pm or more and yet more preferably 5 pm or more. It should be noted that, as described below, in order to obtain a predetermined ferrite average crystal grain size, it is critical to control the finishing temperature in the hot rolling, the average cooling rate from 800°C to the coiling temperature, and the coiling temperature.
[0027] Amount of N present as nitrides and total amount of N satisfy formula (1) (Amount of N present as nitrides) / (total amount of N) ^ 0.40 ••• (1) where the total amount of N is a 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 critical to suppress the ratio of the amount of N present as nitrides (N as nitride) in the hot rolled steel sheet to the total amount of N in the hot rolled steel sheet. When formula (1) is not satisfied, the amount of solute N becomes deficient, and the strength of the hot rolled steel sheet decreases. Thus, (amount of N present as nitrides) / (total amount of N) is to be 0.40 or less. Furthermore, when the amount of N present as nitrides in the hot rolled steel sheet is excessive, the r value of the steel sheet for cans produced by using this hot rolled steel sheet may decrease. Thus, in order to further increase the r value of the steel sheet for cans, the value of (amount of N present as nitrides) / (total amount of N) in the hot rolled steel sheet is preferably 0.30 or less. The value is more preferably 0.28 or less, more preferably 0.25 or less, yet more preferably 0.23 or less, and most preferably 0.20 or less. Although the lower limit is not particularly limited, the value is preferably 0.01 or more and more preferably 0.03 or more. It should be noted that the coiling temperature and the cooling rate from 800°C to the coiling temperature have significant influence on formula (1); thus, in order to satisfy formula (1), it is critical to control the coiling temperature and the average cooling rate from 800°C to the coiling temperature to be within predetermined ranges.
[0028] Amount of Nb present as Nb precipitates and total amount of Nb satisfy formula (2) below 0.10 < (Amount of Nb present as Nb precipitates) / (total amount of Nb) < 0.80 ••• (2) Here, the total amount of Nb is a total amount of Nb contained in the hot rolled steel sheet. When Nb is contained, it is critical 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. When the value of (amount of Nb present as Nb precipitates) / (total amount of Nb) is less than 0.10, there is not enough Nb contributing to the precipitation strengthening, and a further increase in strength 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 steel cannot be expected. Thus, (amount of Nb present as Nb precipitates) / (total amount of Nb) is preferably 0.10 or more. This value is more preferably 0.15 or more. The value is yet more preferably 0.17 or more and most preferably 0.20 or more. When the value of (amount of Nb present as Nb precipitates) / (total amount of Nb) exceeds 0.80, the ductility of the steel sheet for cans decreases due to the increase in recrystallization temperature after the cold rolling. Thus, (amount of Nb present as Nb precipitates) / (total amount of Nb) is preferably 0.80 or less. This value is more preferably 0.70 or less. The value is more preferably 0.69 or less, yet more preferably 0.68 or less, and most preferably 0.60 or less. Here, the Nb precipitates include all of Nb carbides, Nb nitrides, and Nb-containing carbonitrides.
[0029] The method for producing the hot rolled steel sheet according to the present invention will now be described.
[0030] A method for producing a hot rolled steel sheet according to the present invention includes a heating step in which a steel material having the chemical composition described above is heated at 1150°C or more; and a hot rolling step in which the steel material after the heating step is hot-rolled at a finishing temperature of 800°C or more and 950°C or less, cooled at an average cooling rate of 20°C / s or more from 800°C to a coiling temperature of 450°C or more and 700°C or less, and coiled under a condition of 450°C or more and 700°C or less. Note that the steel sheet temperature described in the present invention refers to the temperature at the steel sheet surface unless otherwise noted, and is measured with a radiation thermometer.
[0031] Heating temperature: 1150°C or more When the heating temperature in the heating step is low, coarse nitrides such as AlN are formed, and the ratio of the amount of N present as nitrides to the total amount of N in the hot rolled steel sheet increases; thus the strength of the base material and the r value of the steel sheet for cans decrease. Thus the heating temperature is to be 1150°C or more. The heating temperature is preferably 1170°C or more, more preferably 1180°C or more, yet more preferably 1200°C or more, and most preferably 1220°C or more. Although the upper limit of the heating temperature is not limited, from the production cost viewpoint, the heating temperature is preferably 1300°C or less and more preferably 1280°C or less.
[0032] Finishing temperature: 800°C or more and 950°C or less When the finishing temperature in the hot rolling step exceeds 950°C, ferrite grains coarsen, the strength of the hot rolled steel sheet decreases, and the r value of the steel sheet for cans produced by using the hot rolled steel sheet decreases; thus, the finishing temperature is to be 950°C or less. The finishing temperature is preferably 930°C or less. The finishing temperature is more preferably 920°C or less and most preferably 900°C or less. Meanwhile, when the finishing temperature is less than 800°C, coarse ferrite grains occur during hot rolling, and this not only decreases the strength of the hot rolled steel sheet but also decreases the r value of the steel sheet for cans produced by using this hot rolled steel sheet. Thus, the finishing temperature is to be 800°C or more. Furthermore, when Nb is contained, coarse Nb (C,N) precipitates during hot rolling at a finishing temperature of less than 800°C. Coarse Nb(C,N) does not contribute to improving the strength of the hot rolled steel sheet and decreases the strength of the hot rolled steel sheet since coarse Nb(C,N) decreases the amounts of solute C and solute N. Thus, the finishing temperature in the hot rolling step is 800°C or more. Regardless of whether Nb is contained or not, the finishing temperature is preferably 820°C or more, more preferably 830°C or more, yet more preferably 840°C or more, and most preferably 850°C or more.
[0033] Average cooling rate from 800°C to coiling temperature: 20°C / s or more When the average cooling rate from 800°C to the coiling temperature is less than 20°C / s, the ferrite grains after the hot rolling step coarsen, and the strength of the hot rolled steel sheet decreases. Thus, the average cooling rate from 800°C to the coiling temperature is to be 20°C / s or more. The average cooling rate is preferably 25°C / s or more and more preferably 30°C / s or more to increase the strength of the hot rolled steel sheet. The average cooling rate is yet more preferably 35°C / s or more and most preferably 40°C / s or more. Although the upper limit of the average cooling rate to the coiling temperature is not limited, from the viewpoint of reducing the production load, the average cooling rate is preferably 80°C / s or less, more preferably 75°C / s or less, and yet more preferably 70°C / s or less. Here, the average cooling rate can be determined by dividing the temperature difference between the cooling start temperature (800°C in this case) and the cooling stop temperature (coiling temperature in this case) by the cooling time taken for cooling.
[0034] Coiling temperature: 450°C or more and 700°C or less When the coiling temperature exceeds 700°C, ferrite grains coarsen, and the strength of the hot rolled steel sheet decreases. Furthermore, since formation of coarse alloy carbides and nitrides is accelerated, the amount of solute C and the amount of solute N decrease, and this causes the strength of the hot rolled steel sheet to decrease. Thus, the coiling temperature is to be 700°C or less. The coiling temperature is preferably 650°C or less. The coiling temperature is more preferably 640°C or less, yet more preferably 630°C or less, and most preferably 620°C or less. Meanwhile, when the coiling temperature is less than 450°C, the precipitation amount of alloy carbides, such as Nb carbides, decreases, and thus the strength of the hot rolled steel sheet decreases. Thus, the coiling temperature is to be 450°C or more. The coiling temperature is preferably 500°C or more. The coiling temperature is more preferably 520°C or more, more preferably 530°C or more, yet more preferably 540°C or more, and most preferably 550°C or more. After coiling, pickling may be performed with an aqueous solution such as H2SO4, HCl, or H3PO4 to remove scales.
[0035] Although the cooling rate after the coiling is not particularly limited, particularly when the coiling temperature is high (650°C or more and 700°C or less), the ferrite average grain size and the size of nitrides and Nb precipitates are affected; thus, the average cooling rate after the coiling is preferably 1°C / s or more, more preferably 2°C / s or more, and yet more preferably 3°C / s or more. The average cooling rate after the coiling is preferably 100°C / s or less, more preferably 95°C / s or less, and yet more preferably 90°C / s or less. The average cooling rate after the coiling refers to the average cooling rate from the coiling temperature to 300°C, and is determined by dividing the temperature difference between the coiling temperature and 300°C by the cooling time (the time taken for this cooling).
[0036] A high-strength, high-ductility, and high-r-value steel sheet for cans can be obtained by performing, under typical conditions for producing steel sheets for cans, cold rolling, annealing, and temper rolling on a hot rolled steel sheet obtained as described above. Although cold rolling, annealing, and temper rolling are not particularly limited, cold rolling is preferably performed at a rolling reduction of 70% or more and more preferably 75% or more. The rolling reduction is preferably 95% or less and more preferably 93% or less. For example, annealing is preferably performed at 680°C or more and more preferably 700°C or more. Annealing is preferably performed at 850°C or less and more preferably 800°C or less. In annealing, the aforementioned temperature is preferably held for 5 s or more and more preferably 10 s or more. The temperature is preferably held for 90 s or less and more preferably 85 s or less. After the aforementioned holding time, cooling is preferably performed at an average cooling rate of 10°C / s or more and more preferably 30°C / s or more to a cooling stop temperature of 600°C or less. Cooling is preferably performed at 150°C / s or less and more preferably 130°C / s or less. Temper rolling is preferably performed at a rolling reduction of 0.1% or more and more preferably a rolling reduction of 0.3% or more. The rolling reduction is preferably 5.0% or less and more preferably 3.0% or less. Here, the average cooling rate to a cooling stop temperature of 600°C or less is determined by dividing the temperature difference between the annealing temperature and the cooling stop temperature by the cooling time taken for this cooling.
[0037] The chemical composition of the steel sheet for cans is preferably the same as the chemical composition of the aforementioned hot rolled steel sheet, and the reasons therefor are the same as those described for the hot rolled steel sheet. The microstructure of the steel sheet for cans is preferably composed mainly of ferrite.
[0038] By using the above-described 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. The upper limits of these properties of the steel sheets for cans are not particularly limited, but the upper yield stress is preferably 700 MPa or less, the total elongation is preferably 50% or less, and the r value is preferably 2.5 or less.
[0039] The aforementioned upper yield stress, total elongation, and r value are determined by testing methods described in Examples, and are the properties after the obtained steel sheet is subjected to aging heat treatment at 210°C for 10 minutes. EXAMPLES
[0040] Examples of the present invention are described below. The present invention is not limited to the examples below.
[0041] Steels containing the components of Steel Grade Nos. 1 to 16 shown in Table 1 with the balance Fe and inevitable impurities were melted and cast to obtain steel materials (steel slabs). The obtained steel slabs were heated and hot-rolled under the conditions shown in Table 2 to obtain hot rolled steel sheets of Nos. 1 to 17.
[0042] The hot rolled steel sheet microstructure was observed by the following procedure. After a test piece was taken from the hot rolled steel sheet such that the observation surface was at the sheet width center, a cross section (L section) parallel to the rolling direction and the sheet thickness direction and at a 1 / 2 position in the sheet thickness direction was polished and etched with Nital to reveal the microstructure, thereby preparing a sample for microstructure observation. The aforementioned subject surface was observed with an optical microscope at a magnification of 400x, and the microstructure was photographed in three fields of view selected at random. The ferrite grain size was determined by the intercept method set forth in JIS G 0551, and the average of the three fields of view was assumed to be the ferrite average crystal grain size.
[0043] The amount of N present as nitrides and the amount of Nb present as Nb precipitates were measured by pickling samples taken from the aforementioned hot rolled steel sheets to remove scales and performing the following measurement procedures. The amount of N present as nitrides was measured by performing Br methanol extraction, H2SO4 + K2SO4 decomposition, and alkali water vapor distillation, and then performing bis-pyrazolone spectrophotometry. The amount of Nb present as Nb precipitates was determined by performing AA electrolytic extraction, filter collection, and mixed acid decomposition and then performing ICP-AES measurement.
[0044] Table 3 shows the calculation results of formula (1) and formula (2) below. (Amount of N present as nitrides) / (total amount of N) < 0.40 ••• (1) where the total amount of N is a 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) where the total amount of Nb is a total amount of Nb contained in the hot rolled steel sheet.
[0045] In addition, JIS No. 5 test pieces for a tensile test were taken from the obtained hot rolled steel sheets with the tensile direction being the direction along the rolling direction, and were subjected to a tensile test in accordance with JIS Z 2241 to evaluate the tensile strength. Hot rolled steel sheets that had a tensile strength of 350 MPa were evaluated as having excellent tensile strength.
[0046] The hot rolled steel sheets of Nos. 1 to 17 were pickled and then cold-rolled at a rolling reduction 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. From each of the steel sheets for cans, test specimens to be subjected to a tensile test and a Lankford test were taken and were thermally aged at 210°C for 10 minutes in a thermostat. Subsequently, a tensile test and a Lankford test were carried out as follows.
[0047] For a tensile test, a JIS No. 5 test piece for a tensile test in which the tensile direction was the direction along the rolling direction was used. A tensile test according to JIS Z 2241 was performed to evaluate the upper yield stress and the total elongation.
[0048] The Lankford test was performed by a natural frequency method (JSS YR-05058). The resonance frequency was measured in the rolling direction, a direction 45° with respect to the rolling direction, and a direction perpendicular to the rolling direction to determine the Young's modulus of 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 indicates the microstructure of the hot rolled steel sheet, evaluation results of the tensile strength, and evaluation results for the upper yield stress, total elongation, and r value of the steel sheet for cans. In Examples shown in Table 3, all of the hot rolled steel sheets had a ferrite area ratio of 70% or more and a ferrite average crystal grain size of 15 gm or less, satisfied formula (1) below, and exhibited a tensile strength of 350 MPa or more. Steel sheets for cans produced by coldrolling and annealing the hot rolled steel sheets of Examples described above all had 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] Thus, it can be said that the hot rolled steel sheets of Examples are suitable as the base material of a high- strength, high-ductility, and high-r-value steel sheet for cans. In contrast, in Comparative Examples in which the chemical composition or the production conditions were outside the scope of the invention, at least one of the ferrite area ratio, the ferrite average crystal grain size, and formula (1) was outside the scope of the present invention, and at least one of the tensile strength of the hot rolled steel sheet and the upper yield stress, the total elongation, and the r value of the steel sheet for cans did not achieve the targeted property.
[0051] [Table 1] Steel grade No. Chemical composition (mass%) Note C Si Mn P S Al N Cr Nb Cu Sn Ni Mo 1 0.026 0.02 0.57 0.015 0.007 0.016 0.0122 0.045 - - - - - Example steel 2 0.031 0.01 0.58 0.016 0.010 0.018 0.0094 0.046 0.017 - - - - Example steel 3 0.083 0.03 0.28 0.012 0.008 0.013 0.0053 0.080 - - - - 0.08 Example steel 4 0.148 0.05 0.13 0.013 0.012 0.022 0.0088 0.051 0.028 - - 0.14 - Example steel 5 0.012 0.09 1.17 0.016 0.038 0.011 0.0189 0.023 0.005 - 0.03 - - Example steel 6 0.131 0.04 0.49 0.024 0.008 0.096 0.0057 0.013 - 0.37 - - - Example steel 7 0.068 0.01 0.45 0.012 0.011 0.040 0.0077 0.147 - 0.28 0.01 0.11 0.06 Example steel 8 0.029 0.12 1.21 0.018 0.024 0.071 0.0066 0.039 0.025 0.16 - 0.17 0.11 Comparative Steel 9 0.013 0.01 0.09 0.011 0.043 0.014 0.0081 0.061 - 0.12 - 0.06 - Comparative Steel 10 0.008 0.02 0.22 0.008 0.031 0.103 0.0202 0.152 0.002 - - 0.05 0.04 Comparative Steel 11 0.041 0.03 0.16 0.013 0.014 0.089 0.0047 0.008 0.012 - 0.01 - 0.05 Comparative Steel 12 0.153 0.06 0.84 0.027 0.007 0.017 0.0145 0.031 0.026 0.42 0.05 - - Comparative Steel 13 0.013 0.01 0.76 0.016 0.009 0.012 0.0104 0.023 0.004 0.08 - - 0.02 Example steel 14 0.016 0.02 0.64 0.018 0.010 0.014 0.0151 0.028 0.005 0.04 - 0.02 0.01 Example steel 15 0.024 0.01 0.54 0.012 0.011 0.067 0.0158 0.069 0.005 - - - - Example steel 16 0.136 0.02 0.48 0.011 0.009 0.044 0.0102 0.024 - - - - - Example steel Underlines: Items that are outside the scope of the present invention -: No intentional addition
[0052] [Table 2] Hot rolled steel sheet No. Production condition No. Steel grade No. Heating temperature (°C) Finishing temperature (°C) Coiling temperature (°C) Average cooling rate from 800°C to coiling temperature (°C / s) Average cooling rate from coiling temperature to 300°C (°C / s) Note 1 1 1 1250 880 580 70 25 Example 2 2 2 1190 870 600 30 70 Example 3 3 3 1170 820 560 60 30 Example 4 4 4 1260 880 580 70 105 Example 5 5 5 1300 900 610 50 35 Example 6 6 6 1180 890 690 20 50 Example 7 7 7 1290 880 600 50 25 Example 8 8 8 1200 920 620 80 95 Comparative Example 9 9 9 1250 930 680 25 5 Comparative Example 10 10 10 1160 870 460 20 7 Comparative Example 11 11 11 1170 790 440 25 10 Comparative Example 12 12 12 1280 860 600 70 95 Comparative Example 13 13 13 1200 940 470 100 3 Example 14 14 14 1250 910 500 70 1 Example 15 15 15 1140 970 710 15 5 Comparative Example 16 16 15 1270 850 590 45 40 Example 17 17 16 1200 940 490 80 20 Example Underlines: Items that are outside the scope of the present invention
[0053] Hot rolled steel sheet No. Steel grade No. Hot rolled steel sheet Properties after being formed into steel sheet for cans Note Ferrite area ratio (%) Ferrite average crystal grain size (pm) (Amount of N present in nitrides) / (total amount of N) < 0.40 formula (1)* 0.10 s (Amount of Nb present in Nb precipitates) / (total amount of Nb) s 0.80 formula (2)** Tensile strength (MPa) Upper yield stress of steel sheet for cans (MPa) Total elongation of steel sheet for cans (%) rvalue of steel sheet for cans 1 1 97 12 0.11 - 420 435 35.2 1.1 Example 2 2 94 7 0.13 0.35 463 482 28.1 1.4 Example 3 3 98 11 0.38 - 400 441 34.9 1.0 Example 4 4 92 4 0.10 0.78 491 511 24.9 1.5 Example 5 5 96 13 0.09 0.12 448 454 26.6 1.3 Example 6 6 87 8 0.18 - 452 478 29.2 1.1 Example 7 7 98 10 0.20 - 455 496 23.7 1.1 Example 8 8 90 2 0.14 0.81 522 582 17.1 1.6 Comparative Example 9 9 95 18 0.25 - 334 363 40.6 0.6 Comparative Example 10 10 100 17 0.43 0.06 316 346 38.7 0.5 Comparative Example 11 11 96 11 0.51 0.14 338 352 37.2 0.9 Comparative Example 12 12 69 4 0.19 0.88 544 597 16.3 1.0 Comparative Example 13 13 100 13 0.24 0.08 367 391 29.6 0.9 Example 14 14 99 14 0.27 0.09 383 405 28.5 0.9 Example 15 15 97 16 0.68 0.16 345 355 36.9 0.6 Comparative Example 16 15 98 9 0.15 0.49 451 463 32.5 1.3 Example 17 16 95 12 0.11 - 463 476 29.8 0.9 Example Underlines: Items outside the scope or target range of the present invention -: Addition of Nb is not intended *: (Amount of N present in nitrides) / (total amount of N) in formula (1) **: (Amount of Nb present in Nb precipitates) / (total amount of Nb) in formula (2)
Claims
1. A hot rolled steel sheet comprising a chemicalcomposition containing, in mass%:C: 0.010% or more and 0.150% or less,Si: 0.10% or less,Mn: 0.10% or more and 1.20% or less,P: 0.025% or less,S: 0.040% or less,Al: 0.100% or less,N: 0.0050% or more and 0.0200% or less,Cr: 0.010% or more and 0.150% or less, andthe balance being Fe and inevitable impurities, anda microstructure composed mainly of ferrite in which aferrite area ratio is 70% or more, wherein:the ferrite has an average crystal grain size of 15 pmor less in equivalent circular diameter, andan amount of N present as nitrides and a total amountof N satisfy formula (1) below:(amount of N present as nitrides) / (total amount of N) < 0.40 ••• (1)where the total amount of N is a total amount of 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% or more and 0.030% or less, andan amount of Nb present as Nb precipitates and a totalamount of Nb satisfy formula (2) below:0.10 ^ (amount of Nb present as Nb precipitates) / (totalamount of Nb) ^ 0.80 ••• (2)where the total amount of Nb is a total amount of Nbcontained 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%,at least one selected from:Cu: 0.40% or less,Sn: 0.04% or less,Ni: 0.15% or less, andMo: 0.10% or less.
4. A method for producing the hot rolled steel sheet according to any one of Claims 1 to 3, the method comprising:a heating step of heating a steel material having thechemical composition at 1150°C or more; anda hot rolling step, after the heating step, of hot rolling the steel material at a finishing temperature of800°C or more and 950°C or less, cooling at an averagecooling rate of 20°C / s or more from 800°C to a coiling temperature of 450°C or more and 700°C or less, and coiling at the coiling temperature.