Steel sheet for cans and method for producing same

Abstract

Provided are: a steel sheet for cans having excellent ductility, bead workability, and can body pressure resistance strength, while having corrosion resistance required for a steel sheet for cans; and a method for producing the same. The steel sheet for cans has a specific component composition, has a metal structure in which the N content in the form of solid solution N among N is 0.0025 mass% or less, the ferrite phase is contained in an area ratio of 90% or more, and the proportion of un-recrystallized ferrite is 3% or less in the area ratio with respect to the whole structure, and has a yield strength of 450 MPa or more and a total elongation of 15% or more.

Description

Steel Sheet for Can and Method for Producing the Same

[0001] The present invention relates to a steel sheet for cans having a yield strength (YP) of 450 MPa or more and an elongation of 15% or more, which is excellent in the workability of the can body, and a method for producing the same.

[0002] In recent years, in order to reduce costs in steel sheets for cans, the thickness reduction of steel sheets due to high strength has been promoted. Particularly in the case of a can body with a large area, in order to improve the pressure resistance, the thickness reduction due to high strength has been promoted. Specifically, the application of a high-strength thin steel sheet with a YP of 450 MPa or more to cans has been studied. When such high strength is required, instead of the SR (Single Reduce) material that undergoes rolling only once, which has been conventionally used for steel sheets for cans, a DR (Double Reduce) material that undergoes rolling once again after annealing may be used. However, although the DR material can achieve high strength by performing rolling twice, its ductility decreases, so it cannot be used for cans such as those in which bead processing is performed on the can body. In addition, the DR material is more costly than the SR material because it undergoes rolling twice.

[0003] In response to such requirements, for example, in Patent Document 1, as a component composition, in mass%, C: 0.085% or more and 0.130% or less, Si: 0.04% or less, Mn: 0.10% or more and 0.60% or less, P: 0.02% or less, S: more than 0.010% and less than 0.020%, Al: 0.02% or more and 0.10% or less, N: 0.0005% or more and 0.0040% or less, Nb: 0.007% or more and 0.030% or less, B: 0.0010% or more and 0.0050% or less are contained, and B / N, which is the ratio of the content of B (mass%) to the content of N (mass%), is 0.80 or more, and the balance consists of iron and unavoidable impurities, and a ferrite structure containing pearlite with an area ratio of 1.0% or more, and a steel sheet for cans having high strength and excellent workability, with a yield strength of 500 MPa or more, a tensile strength of 550 MPa or more, a uniform elongation of 10% or more, and a yield elongation of 5.0% or less, is disclosed.

[0004] Patent Document 2 discloses a steel sheet for cans, having a composition in mass percent of C: 0.03 to 0.13%, Si: 0.03% or less, Mn: 0.3 to 0.6%, P: 0.02% or less, Al: 0.1% or less, N: 0.012% or less, and further containing one or more of Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, and B: 0.0005 to 0.005%, with the remainder being iron and unavoidable impurities, and having a ferrite structure with a cementite content of 0.5% or more, an average ferrite grain size of 7 μm or less, a tensile strength of 450 to 550 MPa after paint baking treatment, a total elongation of 20% or more, and a yield elongation of 5% or less.

[0005] Patent Document 3 discloses a steel sheet for cans containing, by mass ratio, C: 0.020 to 0.150%, Si: 0.05% or less, Mn: 1.00%, P: 0.050% or less, S: 0.010% or less, N: 0.0100% or less, Al: 0.100% or less, and Nb: 0.005 to 0.025%, with the remainder being iron and unavoidable impurities, having a substantially single-phase ferrite structure, a yield strength of 40 kgf / mm2 or more, an average grain size of 10 μm or less, and a sheet thickness of 0.300 mm or less.

[0006] Patent Document 4 describes a material containing, by mass%, C: 0.0010 to 0.10%, Si: 0.04% or less, Mn: 0.10 to 0.80%, P: 0.007 to 0.100%, S: 0.10% or less, Al: 0.001 to 0.100%, and N: 0.0010 to 0.0250%, with the remainder being Fe and unavoidable impurities. The difference between the dislocation density at the outermost layer and the dislocation density at a depth of 1 / 4 of the plate thickness from the surface is 1.94 × 10¹⁴ m². -2 The following high-strength steel sheet for containers is disclosed, having a tensile strength of 400 MPa or more and a fracture elongation of 10% or more.

[0007] International Publication No. 2020 / 105406, Japanese Patent Publication No. 2008-274332, Japanese Patent Publication No. Hei 8-325670, International Publication No. 2015 / 166653

[0008] However, the steel sheet for cans described in Patent Document 1 has a lower limit of uniform elongation of 10%, and it is unclear whether the bead workability is superior or inferior at such a ductility level as the lower limit.

[0009] While Patent Document 2 mentions strength and elongation, it does not evaluate bead workability, and therefore the relative superiority or inferiority of bead workability remains unclear.

[0010] Although Patent Document 3 proposes a steel with a balanced combination of high strength and ductility achieved by optimizing the steel structure, it does not consider the bead processing of the can body at all. Therefore, it is difficult to obtain a steel sheet that satisfies the workability required for the bead portion of the can body using the steel sheet for cans and the manufacturing method thereof described in Patent Document 3.

[0011] Patent Document 4 describes evaluating the pressure resistance by forming a can lid using a high-strength steel plate for containers that has a tensile strength of 400 MPa or more and an elongation at break of 10% or more. However, Patent Document 4 does not consider the shape of the bead portion of the can body at all, and it is difficult to obtain a good bead portion of the can body using the technology described in Patent Document 4.

[0012] Furthermore, none of the Patent Documents 1 to 4 mention the pressure resistance strength of the can body, so the relative strengths and weaknesses of the pressure resistance strengths remain unclear.

[0013] The present invention was made to solve the problems of the prior art described above, and aims to provide a steel sheet for cans and a method for manufacturing the same, which has excellent ductility, bead workability, and can shell pressure resistance, while possessing the corrosion resistance necessary for steel sheets for cans. In this invention, excellent ductility refers to a total elongation (EL) of 15% or more.

[0014] The inventors diligently conducted research to solve the aforementioned problems. As a result, they found that by adjusting the steel composition, the amount of dissolved nitrogen, and the proportion of ferrite phase and unrecrystallized material in the metal structure, they could obtain a high-strength steel sheet with excellent ductility and bead workability, and a YP of 450 MPa or higher. In particular, they found that controlling the amount of dissolved nitrogen could suppress the occurrence of wrinkles in the bead area, that is, improve bead workability.

[0015] Furthermore, we found that precisely controlling the heating temperature, finishing temperature, and winding temperature in the hot rolling process, as well as the heating rate, holding time, cooling rate after annealing, and cooling stop temperature in the annealing process, is important for adjusting the amount of dissolved nitrogen, the proportion of the ferrite phase, and the amount of unrecrystallized material.

[0016] This invention was completed as a result of further investigation based on the above-mentioned findings by the inventors, and its gist is as follows.

[0017] [1] In mass percent, C: 0.05% to 0.13%, Si: 0.01% to 0.04%, Mn: 0.1% to 0.6%, P: 0.02% or less, S: 0.02% or less, Al: 0.01% to 0.10%, N: 0.0005% to 0.0040%, Ti: 0.005% to 0.030%, Nb: 0.005% to 0.030%, Mo: 0.01% to 0.05%, B: 0.0005% to 0.00 A steel sheet for cans having a composition comprising 50% or less of Cu: 0.080% or less, Cr: 0.080% or less, with the remainder being iron and unavoidable impurities, with a N content of 0.0025% by mass or less as solid-solution N, containing a ferrite phase of 90% or more by area, having a metallic structure in which the proportion of unrecrystallized ferrite is 3% or less by area relative to the total structure, having a yield strength of 450 MPa or more, and a total elongation of 15% or more.

[0018] Furthermore, "the amount of N as solid-solution N among the N is 0.0025% by mass or less" means that the amount of N as solid-solution N is 0.0025% by mass or less relative to the total mass of all components.

[0019] [2] The steel sheet for cans according to [1], further comprising, in mass percent, one or more of the following: V: 0.005% or more and 0.100% or less, and Ni: 0.005% or more and 0.150% or less.

[0020] [3] A method for manufacturing steel sheets for cans as described in [1] or [2], comprising: a hot rolling step in which a steel slab having the above-mentioned component composition is heated to 1200°C or higher and hot-rolled under conditions such that the finishing temperature is 800°C or higher and 950°C or lower and the winding temperature is 550°C or higher and 750°C or lower; a cold rolling step in which the hot-rolled sheet having undergone the hot-rolling step is cold-rolled to a reduction ratio of 85% or higher; and an average lifting step in which the cold-rolled sheet having undergone the cold-rolling step is raised A method for manufacturing steel sheets for cans, comprising: an annealing step of raising the temperature to an annealing temperature of 640°C to 760°C at a heating rate of 5 to 40°C / second, holding the temperature at the annealing temperature for 10 to 90 seconds, and then cooling to a cooling stop temperature of 250°C to 450°C at an average cooling rate of 15°C to 150°C / second; and a temper rolling step of subjecting the annealed sheet obtained through the annealing step to temper rolling with a temper rolling ratio of 0.5% to 5.0%.

[0021] According to the present invention, it is possible to provide a steel sheet that possesses the corrosion resistance necessary for can steel sheets, has a yield strength of 450 MPa or more, and has excellent bead workability, ductility, and compressive strength. Because the can steel sheet of the present invention has excellent ductility, it is suitable for steel sheets for cans that are formed into complex shapes, such as steel sheets for three-piece cans that undergo bead processing. Furthermore, by applying steel sheets manufactured by the manufacturing method of the can steel sheet of the present invention to cans, further increases in strength and weight reduction can be achieved, which will greatly contribute to the development of industry.

[0022] <Steel Sheet for Cans> A steel sheet for cans according to one embodiment of the present invention will be described below. [Component Composition] First, the appropriate range and reasons for limiting the component composition of the steel sheet for cans of this embodiment will be explained. In addition, the "%" expressed in the component composition below means "mass %" unless otherwise specified. Also, when both ductility and bead workability are excellent, it will be simply referred to as having excellent workability. ・C: 0.05% or more and 0.13% or less C is an element that contributes to strength and has the effect of increasing the strength of steel by being dissolved in solid solution or precipitated as carbide in steel. In order to obtain the desired yield strength by utilizing these effects, it is necessary to contain 0.05% or more of C. For this reason, the C content is set to 0.05% or more, preferably 0.06% or more, and more preferably 0.07% or more.

[0023] Furthermore, excessive carbon content leads to an increase in yield strength, a decrease in ductility, and an increase in the proportion of unrecrystallized ferrite beyond 3%, causing wrinkles when the steel sheet is processed into the bead portion of a can shell. For this reason, the carbon content should be 0.13% or less, preferably 0.12% or less, and more preferably 0.11% or less. Si: 0.01% to 0.04% Si contributes to increasing the strength of steel through solid solution strengthening. In order to obtain the desired yield strength by utilizing these effects, it is necessary to include 0.01% or more of Si. For this reason, the Si content should be 0.01% or more.

[0024] Furthermore, if the Si content exceeds 0.04%, there is a risk of serious problems occurring in corrosion resistance and surface properties. To prevent this, the Si content should be 0.04% or less, preferably 0.03% or less. ・Mn: 0.1% to 0.6% Mn is an element that contributes to strength and contributes to high strength by solid-solubilating in steel. As the objective of this invention is to obtain YP of 450 MPa or more, it is necessary to contain 0.1% or more of Mn. For this reason, the Mn content should be 0.1% or more, preferably 0.4% or more.

[0025] Furthermore, if the Mn content exceeds 0.6%, it leads to a decrease in ductility, and the proportion of unrecrystallized ferrite exceeds 3%, causing wrinkles when the steel sheet is processed into the bead portion of a can body. To prevent this, the Mn content should be 0.6% or less. ・P: 0.02% or less. P is an element with high solid solution strengthening ability. To obtain this effect, it is preferable to contain 0.001% or more P.

[0026] Furthermore, if the P content exceeds 0.02%, the corrosion resistance deteriorates. To prevent this, the P content should be 0.02% or less, preferably 0.007% or more, and even more preferably 0.018% or less. ・S: 0.02% or less S is inevitably mixed into the steel and forms inclusions such as coarse MnS, which significantly reduces local ductility. To prevent this, the S content should be 0.02% or less, preferably 0.018% or less.

[0027] Furthermore, reducing the sulfur content to less than 0.001% would incur excessive costs in steel refining. For this reason, it is preferable that the sulfur content be 0.001% or more, and more preferably 0.007% or more. Al: 0.01% or more and 0.10% or less. Al acts as a deoxidizing agent, and in order to obtain this effect, it is necessary to contain 0.01% or more of Al. For this reason, the Al content should be 0.01% or more, and preferably 0.03% or more.

[0028] Furthermore, if the Al content exceeds 0.10%, surface defects may occur in the steel sheet, and manufacturing costs will increase. To prevent this, the Al content should be 0.10% or less, preferably 0.08% or less. ・N: 0.0005% or more and 0.0040% or less. N combines with carbonitride-forming elements such as Al, Nb, and Ti to form precipitates, contributing to improved strength. To obtain this effect, it is necessary to contain 0.0005% or more of N. Therefore, the N content should be 0.0005% or more, preferably 0.0006% or more.

[0029] Furthermore, if the N content exceeds 0.0040%, the amount of dissolved N increases, causing wrinkles to occur when the steel sheet is processed into the bead portion of the can shell. To prevent this, the N content should be 0.0040% or less, preferably 0.0038% or less. ・Ti: 0.005% or more and 0.030% or less Ti combines with C and N to form carbonitrides, contributing to improved strength, and is therefore one of the important additive elements in this invention. In addition, the addition of Ti suppresses the formation of BN and enhances the strength-improving effect due to grain boundary segregation of B. Furthermore, the addition of Ti suppresses the increase in dissolved N, preventing the occurrence of wrinkles when the steel sheet is processed into the bead portion of the can shell. To obtain this effect, it is necessary to contain 0.005% or more of Ti. For this reason, the Ti content should be 0.005% or more, preferably 0.010% or more.

[0030] Furthermore, if the Ti content exceeds 0.030%, the strength increases, leading to a decrease in ductility, and the proportion of unrecrystallized ferrite exceeds 3%, causing wrinkles to occur when the steel sheet is processed into the bead portion of the can body. To prevent this, the Ti content is set to 0.030% or less, preferably 0.025% or less. ・Nb: 0.005% or more and 0.030% or less Nb combines with C and N to form carbonitrides, contributing to improved strength, and is therefore one of the important additive elements in this invention. In addition, the addition of Nb suppresses the increase in solid-solution N, preventing the occurrence of wrinkles when the steel sheet is processed into the bead portion of the can body. To obtain this effect, it is necessary to contain 0.005% or more of Nb. For this reason, the Nb content is set to 0.005% or more, preferably 0.010% or more.

[0031] Furthermore, if the Nb content exceeds 0.030%, the strength increases, leading to a decrease in ductility, and the proportion of unrecrystallized ferrite exceeds 3%, causing wrinkles when the steel sheet is processed into the bead portion of a can body. To prevent this, the Nb content should be 0.030% or less, preferably 0.025% or less. Mo: 0.01% or more and 0.05% or less Mo contributes to strength improvement by being dissolved in the steel or precipitated as carbides. To obtain this effect, it is necessary to contain 0.01% or more of Mo. For this reason, the Mo content should be 0.01% or more, preferably 0.015% or more.

[0032] Furthermore, if Mo is included in excess, the yield strength increases and ductility decreases, and the proportion of unrecrystallized ferrite exceeds 3%, causing wrinkles to occur when the steel sheet is processed into the bead portion of a can body. To prevent this, the Mo content should be 0.05% or less, preferably 0.045% or less. ・B: 0.0005% or more and 0.0050% or less. B contributes to strength improvement by segregating at grain boundaries. To obtain this effect, it is necessary to include 0.0005% or more of B. For this reason, the B content should be 0.0005% or more, preferably 0.0008% or more.

[0033] Furthermore, if B is included in a large amount exceeding 0.0050%, its effect will saturate. For this reason, the B content should be 0.0050% or less, preferably 0.0040% or less. Cu: 0.080% or less Cu contributes to strength improvement by forming solid solutions or precipitates in the steel. To obtain this effect, it is preferable to include 0.003% or more Cu, and more preferably 0.004% or more.

[0034] Furthermore, excessive Cu content increases yield strength and decreases ductility, and the proportion of unrecrystallized ferrite exceeds 3%, causing wrinkles when the steel sheet is processed into the bead portion of a can body. To prevent this, the Cu content should be 0.080% or less, preferably 0.060% or less. Cr: 0.080% or less Cr has the effect of improving hardenability and contributes to improving the strength of steel as a strengthening element. To obtain this effect, it is preferable to contain 0.003% or more Cr, and more preferably 0.004% or more.

[0035] Furthermore, excessive Cr content increases yield strength and decreases ductility, causing wrinkles to occur when the steel sheet is processed into a bead portion of a can body. To prevent this, the Cr content should be 0.080% or less, preferably 0.060% or less. • N content as solid-solution N is 0.0025% by mass or less. N, by being present in the steel in a solid-solution state, contributes to improving the strength of the steel sheet. To obtain this effect, it is preferable that the N content as solid-solution N be 0.0005% or more of the total mass of all components, and more preferably 0.0006% or more.

[0036] Furthermore, if the material contains an excessive amount of nitrogen as solid solution, wrinkles will occur when the steel sheet is processed into the bead portion of the can body. To prevent this, the nitrogen content as solid solution should be 0.0025% or less of the total mass of all components, preferably 0.0020% or less.

[0037] The steel sheet for cans in this embodiment has a composition in which the above-mentioned elemental components are essential, with the remainder being iron and unavoidable impurities.

[0038] By containing the above-mentioned essential elements, the steel sheet for cans of this embodiment can possess the desired properties. In addition to the above-mentioned essential elements, the following elements may be included as needed: ・V: 0.005% to 0.100%, Ni: 0.005% to 0.150%, one or more of these. V and Ni have the effect of improving hardenability and are therefore useful as strengthening elements for steel. In order to effectively exert this effect, it is preferable that Cr and Ni each be contained at 0.005% or more. For this reason, when V and Ni are included, the V content should be 0.005% or more, and the Ni content should be 0.005% or more. Preferably, the V content should be 0.01% or more, and the Ni content should be 0.01% or more.

[0039] Furthermore, since V and Ni are expensive elements, and no further improvement in effect can be expected beyond their respective upper limits, when V and Ni are included, the V content should be 0.100% or less, and the Ni content should be 0.150% or less. [Metal structure] Next, the appropriate range and reasons for limiting the metal structure, which is an important requirement for the steel sheet for cans in this embodiment, will be explained. In the following, area ratio means the area ratio to the entire steel sheet structure. ・Ferrite phase ratio: 90% or more in area ratio Ferrite is formed during cooling after annealing and contributes to improving the ductility of the steel. If the ferrite phase ratio is less than 90% in area ratio, it becomes difficult to secure the desired ductility. Therefore, the ferrite phase ratio should be 90% or more in area ratio, preferably 92% or more. On the other hand, if the ferrite phase ratio is 99.5% or less in area ratio, it becomes easier to maintain a uniform steel sheet shape. Therefore, it is preferable that the ferrite phase ratio be 99.5% or less in area ratio.

[0040] Note that the proportion of the ferrite phase referred to here does not include the proportion of unrecrystallized ferrite, which will be discussed later. ・Proportion of unrecrystallized ferrite: 3% or less in area relative to the total structure If the proportion of unrecrystallized ferrite exceeds 3% in area relative to the total structure, wrinkles will occur when the steel sheet is processed into the bead portion of the can body. Therefore, the proportion of unrecrystallized ferrite should be 3% or less in area relative to the total structure. The mechanism by which wrinkles occur is not clear, but it is thought that if a large amount of unrecrystallized ferrite is present, the processing stress concentrates at the locations where the unrecrystallized ferrite is present during bead processing, leading to the occurrence of wrinkles. The proportion of unrecrystallized ferrite is preferably 2.7% or less in area relative to the total structure, and more preferably 2.5% or less.

[0041] Furthermore, if the proportion of unrecrystallized ferrite is to be less than 0.5% in terms of area relative to the total structure, the annealing temperature must be increased, making it impossible to maintain a uniform shape of the steel sheet during annealing. For this reason, it is preferable that the proportion of unrecrystallized ferrite be 0.5% or more, and more preferably 0.8% or more, in terms of area relative to the total structure.

[0042] In the above-mentioned metal structure, the remainder other than the ferrite phase does not need to be particularly limited. For example, the remainder may include retained austenite, cementite, pearlite, bainite, martensite, etc. The remainder is preferably 5% or less. - Yield strength: 450 MPa or more To ensure the pressure resistance of the can body, etc., the yield strength of the steel sheet for cans of this embodiment is 450 MPa or more. Also, even if the YP exceeds 600 MPa, since the pressure resistance saturates, the yield strength of the steel sheet for cans of this embodiment is preferably 600 MPa or less. - Elongation: 15% or more To ensure the bead workability of the can body, the elongation of the steel sheet for cans of this embodiment is 15% or more. - Plate thickness: 0.10 to 0.60 mm (preferred condition) Currently, for the purpose of reducing the can manufacturing cost, the thinning of the steel sheet is being promoted. However, as the steel sheet is thinned, that is, as the plate thickness of the steel sheet is reduced, there are concerns about a decrease in the strength of the can body and forming defects during processing. The present invention aims to prevent the strength of the can body, for example, the pressure resistance of the can body, from decreasing and prevent forming defects such as wrinkles from occurring during the bead processing of the can body even when the plate thickness of the steel sheet is thin. That is, the effect of the present invention of high strength and high processing accuracy is显著地 exhibited when the plate thickness of the steel sheet is thin.

[0043] From this perspective, the plate thickness of the steel sheet for cans of this embodiment is preferably 0.60 mm or less, more preferably 0.40 mm or less, and even more preferably 0.30 mm or less. Also, if the plate thickness of the steel sheet is <0.10> mm, can manufacturing becomes easier, so the plate thickness of the steel sheet for cans of this embodiment is preferably <0.10> mm or more. <Manufacturing method of steel sheet for cans> Next, the manufacturing method of the steel sheet for cans according to an embodiment of the present invention will be described below.

[0044] In the following description, the temperature is based on the surface temperature of the steel sheet. Also, the average cooling rate is the average value obtained by calculating using the cooling start temperature, the cooling end temperature, and the cooling time from the cooling start temperature to the cooling end temperature based on the surface temperature of the steel sheet. For example, the average cooling rate from <500> °C to <300> °C is expressed as {(<500> °C) - (<300> °C)} / (cooling time from <500> °C to <300> °C).

[0045] When manufacturing the steel sheet for cans according to this embodiment, molten steel is adjusted to the above-described component composition by a known method using a converter or the like, and then made into a slab by, for example, a continuous casting method.

[0046] The manufacturing method of the steel sheet for cans of this embodiment includes a hot rolling process, a cold rolling process, an annealing process, and a temper rolling process. In the hot rolling process, a slab having the above-described component composition is heated at 1200°C or higher, and hot rolling is performed under conditions where the finishing temperature is 800°C or higher and 950°C or lower, and the coiling temperature is 550°C or higher and 750°C or lower. In the cold rolling process, cold rolling with a reduction ratio of 85% or higher is performed on the hot-rolled sheet that has undergone the hot rolling process. In the annealing process, the cold-rolled sheet that has undergone the cold rolling process is heated to an annealing temperature of 640°C or higher and 760°C or lower at an average heating rate of 5 to 40°C / second. Then, after holding at this annealing temperature for 10 seconds or longer and 90 seconds or shorter, it is cooled to a cooling stop temperature of 250°C or higher and 450°C or lower at an average cooling rate of 15°C / second or higher and 150°C / second or lower. In the temper rolling process, temper rolling is performed on the annealed sheet that has undergone the annealing process at a temper rolling rate of 0.5% or higher and 5.0% or lower. [Hot rolling process] - Slab heating temperature: 1200°C or higher When the slab heating temperature in the hot rolling process is less than 1200°C, the amount of dissolved N increases, and wrinkles occur when the steel sheet is processed into the bead portion of the can body. Therefore, the slab heating temperature is 1200°C or higher, preferably 1210°C or higher. Also, since the effect saturates even if the slab heating temperature exceeds 1350°C, the slab heating temperature is preferably 1350°C or lower. - Finishing temperature: 800°C or higher and 950°C or lower When the finishing temperature in the hot rolling process exceeds 950°C, the ferrite structure after hot rolling becomes coarse, so the ferrite structure after the annealing process also becomes coarse and the yield strength decreases. Therefore, the finishing temperature is 950°C or lower, preferably 930°C or lower.

[0047] Furthermore, if the finishing temperature is less than 800°C, the proportion of unrecrystallized ferrite will exceed 3% in terms of area relative to the total structure, causing wrinkles to occur when the steel sheet is processed into the bead portion of the can body. To prevent this, the finishing temperature should be 800°C or higher, preferably 830°C or higher. ・Winding temperature: 550°C or higher and 750°C or lower In the manufacturing method of steel sheets for cans of this embodiment, the desired yield strength is achieved by controlling the winding temperature and the heating rate up to the annealing temperature, thereby precipitating carbonitrides containing Nb and Ti in the ferrite. If the winding temperature exceeds 750°C, sufficient carbonitrides contributing to the yield strength will not precipitate during winding, and furthermore, some of the ferrite in the steel sheet after continuous annealing will coarseen, causing the steel sheet to soften and the yield strength to fall below 450 MPa. In addition, scale will form on the surface of the steel sheet, making it more susceptible to surface defects. For this reason, the winding temperature should be 750°C or lower, preferably 730°C or lower.

[0048] Furthermore, if the winding temperature is less than 550°C, the proportion of unrecrystallized ferrite in the steel sheet after the annealing process will exceed 3% in terms of area relative to the total structure, causing wrinkles to occur when the steel sheet is processed into the bead portion of the can body. To prevent this, the winding temperature should be 550°C or higher, preferably 560°C or higher.

[0049] A hot-rolled sheet is obtained through the hot-rolling process described above. [Pickling] After the hot-rolling process, it is preferable to perform pickling as needed. The pickling can be any method that can remove scale from the surface of the steel sheet, and there is no particular need to limit the conditions of the pickling. Alternatively, scale from the surface of the steel sheet may be removed by methods other than pickling. [Cold-rolling process] ・Reduction ratio in cold rolling: 85% or more By setting the reduction ratio in cold rolling to 85% or more, the strain applied to the steel sheet in the cold-rolling process is increased, and the yield strength of the steel sheet for cans after the annealing process can be increased to 450 MPa or more. In order to obtain this effect, the reduction ratio is set to 85% or more, preferably to 86% or more.

[0050] Furthermore, if the reduction ratio exceeds 95%, the rolling load increases significantly, and the load on the rolling mill increases. To avoid this, it is preferable to keep the reduction ratio at 95% or less, and more preferably between 87% and 94%.

[0051] A cold-rolled sheet is obtained through the cold rolling process described above. Note that other processes, such as an annealing process to soften the hot-rolled sheet, may be appropriately included between the hot-rolling and cold-rolling processes. Alternatively, the cold-rolling process may be performed immediately after the hot-rolling process without pickling. [Annealing Process] ・Average heating rate to annealing temperature: 5 to 40°C / second In this embodiment of the method for manufacturing steel sheets for cans, the desired yield strength is achieved by controlling the winding temperature and the heating rate to the annealing temperature, thereby precipitating carbonitrides containing Nb and Ti in the ferrite. To obtain this effect, it is important to control the average heating rate to the annealing temperature within the range of 5 to 40°C / second.

[0052] If the average heating rate is less than 5°C / second, the carbonitride obtained in the hot rolling process will melt during heating, and a sufficient amount of carbonitride cannot be secured after the annealing process. As a result, the ferrite will not be strengthened by precipitates, making it difficult to secure the desired yield strength. To prevent this, the average heating rate to the annealing temperature should be 5°C / second or higher, preferably 8°C / second or higher.

[0053] Furthermore, if the average heating rate exceeds 40°C / second, the area ratio of the ferrite phase will be less than 90%, making it difficult to secure the desired ductility. In addition, the proportion of unrecrystallized ferrite in the steel sheet after the annealing process will exceed 3% in terms of area ratio to the total structure, causing wrinkles to occur when the steel sheet is processed into the bead portion of the can body. To prevent this, the average heating rate to the annealing temperature should be 40°C / second or less, preferably 35°C / second or less. Annealing temperature: 640°C or higher and 760°C or lower If the annealing temperature is less than 640°C, the area ratio of the ferrite phase will be less than 90%, making it difficult to secure the desired ductility. Furthermore, the proportion of unrecrystallized ferrite in the steel sheet after the annealing process will exceed 3% in terms of area ratio to the total structure, causing wrinkles to occur when the steel sheet is processed into the bead portion of the can body. To prevent this, the annealing temperature should be 640°C or higher, preferably 660°C or higher.

[0054] Furthermore, if the annealing temperature exceeds 760°C, precipitates containing Nb and Ti generated in the hot rolling process will dissolve, making it impossible to obtain the desired yield strength after the annealing process. To prevent this, the annealing temperature should be 760°C or lower, preferably 740°C or lower. Holding time in the temperature range of 640°C to 760°C: 10 seconds to 90 seconds If the holding time in the annealing temperature range of 640°C to 760°C is less than 10 seconds, the proportion of unrecrystallized ferrite in the steel sheet after the annealing process will exceed 3% in terms of area relative to the total structure, causing wrinkles to occur when the steel sheet is processed into the bead portion of the can body. To prevent this, the holding time in the annealing temperature range of 640°C to 760°C should be 10 seconds or more, preferably 15 seconds or more.

[0055] Furthermore, if the holding time at an annealing temperature of 640°C to 760°C exceeds 90 seconds, precipitates containing Nb and Ti, which mainly precipitate during the hot rolling winding process, become coarser during holding at the annealing temperature, reducing the yield strength. To prevent this, the holding time at an annealing temperature of 640°C to 760°C should be 90 seconds or less, preferably 50 seconds or less.

[0056] A continuous annealing apparatus can be used for the annealing process. Alternatively, other processes, such as other annealing processes to soften the hot-rolled sheet, may be included between the cold-rolling process and the annealing process, or the annealing process may be performed immediately after the cold-rolling process. • Average cooling rate: 15°C / sec to 150°C / sec. If the average cooling rate from the annealing temperature to the cooling stop temperature is less than 15°C / sec, precipitates containing Nb and Ti will become coarser during cooling, reducing the yield strength. To prevent this, the average cooling rate from the annealing temperature to the cooling stop temperature should be 15°C / sec or higher, preferably 20°C / sec or higher. Cooling from the annealing temperature to the cooling stop temperature can be carried out by gas cooling, or a combination of one or more methods such as mist cooling, roll cooling, and water cooling.

[0057] Furthermore, if the average cooling rate from the annealing temperature to the cooling stop temperature exceeds 150°C / second, a good bead cannot be obtained. To prevent this, the average cooling rate from the annealing temperature to the cooling stop temperature should be 150°C / second or less, preferably 130°C / second or less. Cooling stop temperature: 250°C or more and less than 450°C If the cooling stop rate is less than 250°C, the steel plate will harden excessively and its ductility will decrease, causing wrinkles to occur when the steel plate is processed into the bead portion of the can body. To prevent this, the cooling stop temperature should be 250°C or higher, preferably 270°C or higher.

[0058] Furthermore, if the cooling stop temperature is set to 450°C or higher, the amount of dissolved nitrogen increases, causing wrinkles to form when the steel plate is processed into the bead portion of the can body. To prevent this, the cooling stop temperature should be set to less than 450°C, preferably less than 430°C.

[0059] After the annealing process described above, an annealed sheet is obtained. [Tempered Rolling Process] ・Tempered rolling ratio: 0.5% or more and 5.0% or less By performing tempered rolling, the surface roughness of the steel sheet can be adjusted and the shape corrected, and the yield strength can be improved by introducing strain into the annealed sheet. In order to obtain this effect, the tempered rolling ratio must be 0.5% or more. For this reason, the tempered rolling ratio is set to 0.5% or more, preferably 0.6% or more.

[0060] Furthermore, if the temper rolling ratio exceeds 5.0%, excessive strain is introduced into the steel sheet, reducing its ductility and causing wrinkles to form when the steel sheet is processed into the bead portion of the can shell. To prevent this, the temper rolling ratio should be 5.0% or less, preferably 2.0% or less.

[0061] The steel sheet for cans of the present invention is obtained through the temper rolling process described above.

[0062] Furthermore, in the manufacturing method of steel sheets for cans according to the present invention, various processes may be carried out after the temper rolling process. For example, the steel sheets for cans according to the present invention may have a plating layer on the surface of the steel sheet. Examples of plating layers include a Sn plating layer, a tin-free Cr plating layer, a Ni plating layer, a Sn-Ni plating layer, etc. Therefore, a plating process may be carried out after the temper rolling process to form a plating layer on the surface of the steel sheet. Alternatively, a paint baking process, a film lamination process, etc. may be carried out after the temper rolling process. Note that since the film thickness of the plating layer, laminate film, etc. is sufficiently small compared to the thickness of the steel sheet, its influence on the mechanical properties of the steel sheets for cans can be ignored.

[0063] Examples of the steel sheet for cans and its manufacturing method according to the present invention will be described below. However, the steel sheet for cans and its manufacturing method according to the present invention are not limited to the examples shown below.

[0064] Steel containing the component composition shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted in a converter and continuously cast to obtain steel slabs, which were the raw material for the steel. Next, these steel slabs were hot-rolled under the hot-rolling conditions shown in Tables 2 and 3, and then pickled after hot-rolling. Furthermore, cold-rolling was performed at the reduction ratios shown in Tables 2 and 3, continuous annealing was performed under the annealing conditions shown in Tables 2 and 3, and temper-rolling was performed at the reduction ratios shown in Tables 2 and 3 to obtain steel plates.

[0065] The steel sheet obtained in this manner was then continuously plated with conventional Sn, resulting in a plating layer adhesion amount of 11.2 g / m² on one side. 2Sn-plated steel sheets (tinplate) were obtained. Subsequently, the Sn-plated steel sheets, which had undergone heat treatment equivalent to a paint baking process at 210°C for 10 minutes, were used as the test subjects, and the following test items were evaluated. <Tensile Test> Tensile tests were performed on the Sn-plated steel sheets of the test subject in accordance with the test method specified in Japanese Industrial Standard JIS Z 2241:2011 "Method for Tensile Testing of Metallic Materials", and the yield strength and total elongation were measured. That is, JIS No. 5 tensile test specimens were taken in accordance with the method specified in Japanese Industrial Standard JIS Z 2201 "Tensile Test Specimens for Metallic Materials", so that the direction parallel to the rolling direction was the tensile direction. Then, a gauge mark of 50 mm (L) was given to the parallel part of the tensile test specimen, and a tensile test was performed at a tensile speed of 10 mm / min in accordance with the provisions of JIS Z 2241 until the tensile test specimen broke. In this invention, yield strength (YP) refers to the upper yield point if one exists, or the 0.2% proof stress if there is no upper yield point. The results of this yield strength measurement are shown in Tables 2 and 3. <Investigation of Metallographic Structure> The metallographic structure was investigated by polishing a cross section of the Sn-plated steel sheet in the thickness direction parallel to the rolling direction, then etching this cross section with an etching solution (3 vol% nital), and observing it with a scanning electron microscope (SEM). Specifically, using an SEM, at a magnification of 1000x, five randomly selected fields of view were observed, covering the region from a depth of 1 / 4 of the sheet thickness (1 / 4 of the sheet thickness in the thickness direction from the surface of the steel sheet in the above cross section) to a depth of 1 / 2 of the sheet thickness.

[0066] Then, for each field of view, the unrecrystallized ferrite in the metal structure was identified by visual inspection using images taken with a SEM, and the proportion of unrecrystallized ferrite (area ratio to the total structure) was determined by image analysis. Here, unrecrystallized ferrite is a metal structure that exhibits an elongated shape in the rolling direction when observed at a magnification of 1000x with an SEM. The average value of the area ratio of unrecrystallized ferrite to the total structure obtained in this way at five locations in the field of view was defined as the area ratio of unrecrystallized ferrite to the total structure. Image analysis software (particle analysis, manufactured by Nippon Steel & Sumitomo Metal Technology Co., Ltd.) was used for the above image analysis.

[0067] Furthermore, the area fraction of the ferrite phase was investigated using the same method as for unrecrystallized ferrite. Specifically, the cross-section of the Sn-plated steel sheet under test was polished in the thickness direction parallel to the rolling direction, and then this cross-section was etched with an etching solution (3 vol% nital). The area fraction of the ferrite phase was then investigated by observation with a scanning electron microscope (SEM). In particular, using an SEM, at a magnification of 2000x, three randomly selected fields of view were observed at a depth of 1 / 2 of the sheet thickness.

[0068] Then, for each field of view, the area ratio of the ferrite phase within a randomly set 50 μm x 50 μm square region was determined by binarization using image processing software (Photoshop, Adobe). The average of the area ratios of the ferrite phase obtained in this way at three locations in the field of view was defined as the ferrite area ratio. In addition, in SEM observations, black areas observed as clumpy shapes were identified as ferrite.

[0069] The results of the investigation of this metal structure are shown in Tables 2 and 3. <Solid-Soluble Nitrate> The amount of solid-soluble nitrogen in the Sn-plated steel sheet under test was evaluated by subtracting the amount of NasAlN (amount of Nitrate in AlN), measured by extraction analysis with 10% Br methanol, from the total amount of Nitrate in the steel sheet. The results of this investigation of solid-soluble Nitrate are shown in Tables 2 and 3. <Corrosion Resistance> For the Sn-plated steel sheet under test, an optical microscope was used to analyze an area of ​​2.7 mm². 2The measurement area was observed at 50x magnification, and the number of hole-like areas where the Sn plating was thinned was counted. The results of this corrosion resistance investigation are shown in Tables 2 and 3. In Tables 2 and 3, "○" indicates that there were fewer than 20 hole-like areas, "△" indicates that there were 20 or more but 25 or less, and "×" indicates that there were more than 25. <Presence or absence of wrinkling in the bead area> A rectangular blank was taken from the Sn-plated steel sheet to be tested, and a can body was fabricated by sequentially performing roll processing and wire seam welding. Then, a bead area was applied to the top of this can body by spin neck processing. This bead area was visually observed at eight locations around the can body to check for the presence or absence of wrinkling. The results of this investigation are shown in Tables 2 and 3. In Tables 2 and 3, "Wrinkle occurrence: Present" indicates that wrinkles occurred at at least one of the eight circumferential locations, while "Wrinkle occurrence: Absent" indicates that no wrinkles occurred at any of the eight circumferential locations. <Pressure resistance strength> Rectangular blanks were taken from the Sn-plated steel sheet to be tested, and a can body was fabricated by sequentially performing roll processing and wire seam welding. A pressure resistance strength test was then conducted on this can body with lids attached to the top and bottom as the test specimen.

[0070] The pressure resistance test was conducted using a CMC-KUHNKE PNL-3153 panel strength tester. The test specimen was placed in the tester's sealed space, and the pressure in the sealed space was increased under test conditions of an initial pressure of 0.01 MPa and a pressure increase rate of 0.4 MPa / min. The pressure value at which the can body deformed was defined as the pressure resistance value. The results of this pressure resistance test are shown in Tables 2 and 3. In Tables 2 and 3, a pressure resistance value of 0.060 MPa or higher is indicated by "○", and a pressure resistance value of less than 0.060 MPa is indicated by "×".

[0071]

[0072]

[0073]

[0074] Steel sheets No. 1-30, 50, 54, 57, 67, 71, and 73-79, which are examples of inventions that satisfy the requirements of the steel sheet for cans and the method for manufacturing the same of the present invention, achieved a yield strength of 450 MPa or more and a total elongation of 15% or more, and were confirmed to have excellent workability and pressure resistance of the bead portion.

[0075] In contrast, steel plates No. 31-49, 51-53, 55, 56, 58-66, 68-70, 72, and 80-98 are comparative examples that do not satisfy any of the requirements for the steel plate for cans and its manufacturing method of the present invention. These plates failed to achieve the desired level in any of the following: yield strength, total elongation, workability of the bead portion, or compressive strength, and their performance was significantly inferior to that of the steel plate in the inventive example.

Claims

1. The composition has the following components by mass%, containing C: 0.05% to 0.13%, Si: 0.01% to 0.04%, Mn: 0.1% to 0.6%, P: 0.02% or less, S: 0.02% or less, Al: 0.01% to 0.10%, N: 0.0005% to 0.0040%, Ti: 0.005% to 0.030%, Nb: 0.005% to 0.030%, Mo: 0.01% to 0.05%, B: 0.0005% to 0.0050%, Cu: 0.080% or less, Cr: 0.080% or less, with the remainder being iron and unavoidable impurities. A steel sheet for cans, wherein the N content as solid-solution N is 0.0025% by mass or less, the metal structure contains 90% or more of ferrite phase by area, the proportion of unrecrystallized ferrite is 3% or less by area relative to the total structure, the yield strength is 450 MPa or more, and the total elongation is 15% or more.

2. The steel sheet for cans according to claim 1, further comprising, in terms of mass%, one or more of the following in its component composition: V: 0.005% or more and 0.100% or less, and Ni: 0.005% or more and 0.150% or less.

3. A method for manufacturing a steel sheet for cans according to claim 1 or 2, comprising: a hot rolling step of heating a steel slab having the above-mentioned component composition to 1200°C or higher, and hot rolling under conditions such that the finishing temperature is 800°C or higher and 950°C or lower, and the winding temperature is 550°C or higher and 750°C or lower; a cold rolling step of cold rolling the hot-rolled sheet that has undergone the hot-rolling step to a reduction ratio of 85% or higher; an annealing step of heating the cold-rolled sheet that has undergone the cold-rolling step to an annealing temperature of 640°C or higher and 760°C or lower at an average heating rate of 5 to 40°C / second, holding at the annealing temperature for 10 seconds or higher and 90 seconds or lower, and then cooling to a cooling stop temperature of 250°C or higher and 450°C or lower at an average cooling rate of 15°C / second or higher and 150°C / second or lower; A method for manufacturing steel sheets for cans, comprising: a temper rolling step of subjecting the annealed sheet that has undergone the aforementioned annealing step to temper rolling with a temper rolling ratio of 0.5% or more and 5.0% or less.

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