Steel sheet for can and method for producing same

Abstract

Provided are a steel sheet for cans that has pressure resistance strength for can lids, processability for curled sections, and corrosion resistance necessary for the steel sheet for cans, and a method for producing the same. The steel sheet for cans has: a prescribed component composition; an Mo content as precipitated Mo among Mo being 0.013 mass% or less; a metal structure containing a ferrite phase of 70% or more in terms of an area ratio, a martensite phase of 5% to 22% in terms of the area ratio, and a proportion of unrecrystallized ferrite of 3% or less in terms of the area ratio with respect to the total structure; yield strength YS (MPa) and tensile strength TS (MPa) satisfying the following relationship of equation (1): (YS + TS) / 2 ≥ 470 MPa; and total elongation of 10% or more.

Description

Steel sheet for cans and method for producing the same

[0001] The present invention relates to a steel sheet for cans and a method for producing the same, which are excellent in the workability of can lids and the pressure resistance of can lids.

[0002] In recent years, in order to reduce costs in steel sheets for cans, thinning of the steel sheet by increasing the strength has been promoted. Specifically, application of high-strength thin steel sheets with a yield strength (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. Therefore, for example, when processed into a can lid, wrinkles are likely to occur in the curled portion around the can lid. In addition, the can lid is required to have a pressure resistance such that the can lid does not deform during the retort sterilization treatment after filling the contents. However, as the steel sheet is thinned due to the increase in strength, the lid is easily deformed. Furthermore, since the DR material undergoes rolling twice, it is more costly than the SR material.

[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 0.020% or less, 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 yield stress 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, a steel sheet for cans having high strength and excellent workability 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 describes a material containing, by mass ratio, C: 0.020-0.150%, 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: 0.005-0.025%, with the remainder being iron and unavoidable impurities, having a substantially single-phase ferrite structure, and a yield strength of 40 kgf / mm². 2 The above discloses steel sheets for cans with 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 at such a ductility level, the processability and compressive strength of the can lid are unknown.

[0009] While Patent Document 2 mentions strength and elongation, it does not evaluate the processability or pressure resistance of the can lid, and therefore the superiority or inferiority of the can lid in terms of processability and pressure resistance 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 completely fails to consider the curling properties of can lids or the pressure resistance strength. Therefore, it is difficult to obtain a steel sheet that satisfies the required workability and desired pressure resistance strength for the curled portion of a can lid using the steel sheet for cans and its manufacturing method described in Patent Document 3.

[0011] Patent Document 4 describes evaluating the pressure resistance by forming a can lid using a high-strength steel sheet for containers having a tensile strength of 400 MPa or more and an elongation at break of 10% or more. However, in Patent Document 4, the formability of the can lid is evaluated using the Erichsen value, and the processability of the curled portion is not considered at all. Therefore, it is difficult to obtain a good curled portion of a can lid using the technology described in Patent Document 4.

[0012] The present invention has been 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 the corrosion resistance necessary for a steel sheet for cans, as well as processability of the curled portion and pressure resistance strength for can lids.

[0013] The inventors diligently conducted research to solve the above problems. As a result, they found that by adjusting the steel composition and amount of precipitated Mo, the ratio of ferrite phase and martensite phase in the metal structure, the amount of unrecrystallized ferrite, and the relationship between yield strength (YS) and tensile strength (TS), a high-strength steel sheet with excellent curl-forming properties and compressive strength when processed into a can lid could be obtained. In particular, they found that controlling the amount of precipitated Mo could suppress the occurrence of wrinkles in the curled portion.

[0014] Furthermore, we found that strictly 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 precipitated Mo, the proportion of the ferrite phase, the proportion of the martensite phase, and the amount of unrecrystallized material.

[0015] 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.

[0016] [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 0.0080%, Ti: 0.005% to 0.030%, Nb: 0.005% to 0.030%, Mo: 0.010% to 0.050%, B: 0.0005% to 0.0050%, Cu: 0.003% to 0.500%, Cr: 0.003% to 0.300%, N A steel sheet for cans having a composition containing i: 0.005% to 0.500%, V: 0.005% to 0.100%, Sn: 0.003% to 0.050%, with the remainder being iron and unavoidable impurities, wherein the Mo content as precipitated Mo is 0.013% by mass or less, and the metal structure contains a ferrite phase of 70% or more by area ratio and a martensite phase of 5% to 22% by area ratio, with the proportion of unrecrystallized ferrite being 3% or less by area ratio of the total structure, and the yield strength YS (MPa) and tensile strength TS (MPa) satisfy the following relationship (1) equation (YS + TS) / 2 ≥ 470 MPa ... (1), and the total elongation is 10% or more.

[0017] [2] A method for manufacturing steel sheets for cans as described in [1], comprising: a hot rolling step in which a steel slab having the above-mentioned component composition is heated to 1150°C or higher and hot-rolled under conditions 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 annealing step in which the cold-rolled sheet having undergone the cold-rolling step is annealed to an annealing temperature of 640°C or higher and 860°C or lower at an average heating rate of 5 to 40°C / second. A method for manufacturing steel sheets for cans, comprising: an annealing step of raising the temperature, holding it at the annealing temperature for 10 seconds or more and 90 seconds or less, primary cooling to a cooling stop temperature of 250°C or more and 350°C or less at an average cooling rate of 10°C / second or more and 230°C / second or less, and secondary cooling to a cooling stop temperature of 150°C or more and 200°C or less at an average cooling rate of 0.2°C / second or more and 10.0°C / second or less; and a temper rolling step of subjecting the annealed sheet that has undergone the annealing step to temper rolling with a temper rolling ratio of 0.5% or more and 5.0% or less.

[0018] According to the present invention, it is possible to provide a steel sheet that possesses the corrosion resistance necessary for can steel sheets while also having excellent curling properties and compressive strength for can lids. Because the steel sheet for cans 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 can lids that undergo curling. Furthermore, by applying steel sheets manufactured by the manufacturing method 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.

[0019] <Steel Sheet for Cans> The 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 the product has excellent curl workability, 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 carbides 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.

[0020] Furthermore, if carbon is present in excess, the proportion of the martensite phase will exceed 22% and the proportion of the ferrite phase will be less than 70%, reducing ductility, and the proportion of unrecrystallized ferrite will exceed 3%, causing wrinkles to occur when the steel sheet is processed into the curled part of a can lid. To prevent this, 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.

[0021] 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-solubilizing in steel. As the objective of this invention is to obtain the desired compressive strength, 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.

[0022] Furthermore, if the Mn content exceeds 0.6%, the proportion of the martensite phase will exceed 22% and the proportion of the ferrite phase will be less than 70%, resulting in decreased ductility. Additionally, the proportion of unrecrystallized ferrite will exceed 3%, causing wrinkles to occur when the steel sheet is processed into the curled part of a can lid. 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 include P at 0.001% or more.

[0023] 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.

[0024] 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.

[0025] 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 0.0080% 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, the N content should be 0.0005% or more, preferably 0.0006% or more.

[0026] Furthermore, if the N content exceeds 0.0080%, the ductility decreases. To prevent this, the N content is set to 0.0080% or less, preferably 0.0075% or less. Ti: 0.005% or more and 0.030% or less Ti is one of the important additive elements in this invention because it combines with C and N to form carbonitrides and contributes to strength improvement. In addition, the addition of Ti suppresses the formation of BN and enhances the strength improvement effect due to grain boundary segregation of B. To obtain this effect, it is necessary to contain 0.005% or more of Ti. For this reason, the Ti content is set to 0.005% or more, preferably 0.007% or more, and more preferably 0.010% or more.

[0027] 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 curled part of a can lid. To prevent this, the Ti content should be 0.030% or less, preferably 0.028% or less, and more preferably 0.025% or less. ・Nb: 0.005% or more and 0.030% or less Nb combines with C and N to form carbonitrides and contributes to strength improvement, making it one of the important additive elements in this invention. To obtain this effect, it is necessary to contain 0.005% or more of Nb. For this reason, the Nb content should be 0.005% or more, preferably 0.007% or more, and more preferably 0.010% or more.

[0028] 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 to occur when the steel sheet is processed into the curled part of a can lid. To prevent this, the Nb content should be 0.030% or less, preferably 0.028% or less, and more preferably 0.025% or less. Mo: 0.010% or more and 0.050% 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 0.010% or more of Mo. For this reason, the Mo content should be 0.010% or more, preferably 0.012% or more, and more preferably 0.015% or more.

[0029] Furthermore, if Mo is included in excess, the yield strength increases and ductility decreases, and the amount of precipitated Mo, as described later, exceeds 0.010%, causing wrinkles to occur when the steel sheet is processed into the curled part of a can lid. To prevent this, the Mo content should be 0.050% or less, preferably 0.047% or less, and more 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.

[0030] 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: 0.003% or more and 0.500% or less Cu has the effect of solid solution in steel or improving hardenability, and contributes to improving the pressure resistance strength of the lid, making it one of the important additive elements in this invention. In order to obtain this effect, it is necessary to include 0.003% or more of Cu. For this reason, the Cu content should be 0.003% or more, preferably 0.007% or more, and more preferably 0.010% or more.

[0031] Furthermore, if Cu is included in excess, the proportion of the martensite phase will exceed 22%, reducing ductility and causing wrinkles when the steel sheet is processed into the curled portion of the can lid. To prevent this, the Cu content should be 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less. Cr: 0.003% or more and 0.300% or less Cr has the effect of improving hardenability and contributes to improving the pressure resistance strength of the lid, and is therefore one of the important additive elements in this invention. In order to obtain this effect, it is necessary to include 0.003% or more of Cr. For this reason, the Cr content should be 0.003% or more, preferably 0.007% or more, and more preferably 0.010% or more.

[0032] Furthermore, if Cr is included in excess, the proportion of the martensite phase will exceed 22%, reducing ductility and causing wrinkles when the steel sheet is processed into the curled portion of the can lid. To prevent this, the Cr content should be 0.300% or less, preferably 0.270% or less, and more preferably 0.250% or less. Ni: 0.005% or more and 0.500% or less. Ni has the effect of improving hardenability and contributes to improving the pressure resistance strength of the lid, and is therefore one of the important additive elements in this invention. In order to obtain this effect, it is necessary to include 0.005% or more of Ni. For this reason, the Ni content should be 0.005% or more, preferably 0.007% or more, and more preferably 0.010% or more.

[0033] Furthermore, if Ni is included in excess, the proportion of the martensite phase will exceed 22%, reducing ductility and causing wrinkles when the steel sheet is processed into the curled part of a can lid. To prevent this, the Ni content should be 0.500% or less, preferably 0.300% or less, and more preferably 0.150% or less. ・V: 0.005% or more and 0.100% or less V combines with C and N to form carbonitrides and contributes to improving strength, making it one of the important additive elements in this invention. To obtain this effect, it is necessary to include 0.005% or more of V. For this reason, the V content should be 0.005% or more, preferably 0.007% or more, and more preferably 0.010% or more.

[0034] Furthermore, if the amount of V exceeds 0.100%, 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 curled part of the can lid. To prevent this, the V content should be 0.100% or less, preferably 0.060% or less, and more preferably 0.040% or less. ・Sn: 0.003% or more and 0.050% or less Sn has the effect of improving hardenability and contributes to improving the pressure resistance strength of the lid, and is therefore one of the important additive elements in this invention. In order to obtain this effect, it is necessary to contain 0.003% or more of Sn. For this reason, the Sn content should be 0.003% or more, preferably 0.004% or more, and more preferably 0.005% or more.

[0035] Furthermore, if Sn is included in excess, the proportion of unrecrystallized ferrite will exceed 3%, causing wrinkles to occur when the steel sheet is processed into the curled part of a can lid. To prevent this, the Sn content should be 0.050% or less, preferably 0.015% or less, and more preferably 0.010% or less. ・Mo content as precipitated Mo should be 0.013% by mass or less Mo, by being present in the steel in a precipitated state, contributes to improving the strength of the steel sheet. To obtain this effect, it is preferable that the Mo content as precipitated Mo be 0.001% by mass or more, and more preferably 0.0002% by mass or more, relative to the total mass of all components.

[0036] Furthermore, if the Mo content as precipitated Mo is excessive, wrinkles will occur when the steel sheet is processed into the curled portion of the can lid. To prevent this, the Mo content as precipitated Mo should be 0.013% by mass or less relative to the total mass of all components, preferably 0.010% by mass or less, and more preferably 0.008% by mass 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] Inevitable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap. Examples of impurities include O, H, Zn, Pb, As, Sb, Bi, or Co. [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: 70% or more in area ratio Ferrite is formed during cooling after annealing and contributes to improving the ductility of steel. If the ferrite phase ratio is less than 70% in area ratio, it becomes difficult to secure the desired ductility. Therefore, the ferrite phase ratio should be 70% or more in area ratio, preferably 71% or more.

[0039] Furthermore, if the proportion of the ferrite phase is 99.5% or less by area, it becomes easier to maintain a uniform shape in the steel sheet. Therefore, it is preferable that the proportion of the ferrite phase be 99.5% or less by area.

[0040] Note that the ratio of the ferrite phase as mentioned here does not include the ratio of the unrecrystallized ferrite described later. - Ratio of martensite phase: 5% or more and 22% or less in terms of area ratio Martensite has the effect of increasing the yield strength YS (MPa) and tensile strength TS (MPa) of the steel sheet and improving the pressure resistance of the can lid. When the ratio of the martensite phase is less than 5% in terms of area ratio, the increase in the strength of the steel sheet becomes small, and the pressure resistance of the can lid becomes low. Therefore, the ratio of the martensite phase shall be 5% or more in terms of area ratio, preferably 6% or more, and more preferably 7% or more.

[0041] Also, if the ratio of the martensite phase exceeds 22% in terms of area ratio, the steel sheet is excessively strengthened and the total elongation decreases. To prevent this, the ratio of the martensite phase shall be 22% or less in terms of area ratio, preferably 21% or less, and more preferably 20% or less. - Ratio of unrecrystallized ferrite: 3% or less in terms of area ratio to the entire structure When the ratio of the unrecrystallized ferrite exceeds 3% in terms of area ratio to the entire structure, wrinkles occur when the steel sheet is processed into the curled part of the can lid. Therefore, the ratio of the unrecrystallized ferrite shall be 3% or less in terms of area ratio to the entire structure. Although the mechanism of wrinkle generation is not clear, it is considered that when a large amount of unrecrystallized ferrite exists, the processing stress concentrates at the position where the unrecrystallized ferrite exists during curling, leading to wrinkle generation. The ratio of the unrecrystallized ferrite is preferably 2.8% or less, and more preferably 2.5% or less in terms of area ratio to the entire structure.

[0042] Also, if the ratio of the unrecrystallized ferrite is to be less than 0.5% in terms of area ratio to the entire structure, it is necessary to increase the annealing temperature, and the shape of the steel sheet cannot be kept uniform during annealing. For this reason, the ratio of the unrecrystallized ferrite is preferably 0.5% or more, and more preferably 0.8% or more in terms of area ratio to the entire structure.

[0043] In addition, the remainder of the metal structure described above, other than the ferrite phase and martensite phase, does not need to be particularly limited. For example, the remainder may include retained austenite, cementite, pearlite, bainite, etc. Preferably, the remainder is 5% or less. ・(YS + TS) / 2 ≥ 470 MPa In order to ensure the pressure resistance strength of the can lid, the yield strength YS (MPa) and tensile strength TS (MPa) of the steel sheet for the can in this embodiment must satisfy the following relationship (1).

[0044] (YS + TS) / 2 ≥ 470 MPa ……(1) Also, even if the value of (YS + TS) / 2 exceeds 620 MPa, the pressure resistance strength will saturate, so it is preferable that the value of (YS + TS) / 2 for the steel sheet for cans in this embodiment be 620 MPa or less. ・Total elongation: 10% or more In order to ensure the curling properties of the can lid, the total elongation of the steel sheet for cans in this embodiment is 10% or more, preferably 12% or more. Also, even if the total elongation exceeds 35%, the curling properties will saturate, so it is preferable that the total elongation of the steel sheet for cans in this embodiment be 35% or less. ・Sheet thickness: 0.10 to 0.60 mm (preferred conditions) The sheet thickness of the steel sheet for cans in this embodiment is not particularly limited, but it is preferable that the sheet thickness be in the range of 0.10 to 0.60 mm.

[0045] Currently, thinning of steel sheets is being pursued to reduce can manufacturing costs. However, as steel sheets are thinned, i.e., the thickness of the steel sheet is reduced, there are concerns about a decrease in can body strength and forming defects during processing. The present invention aims to prevent a decrease in can body strength, such as the pressure resistance of the can lid, even when the steel sheet thickness is thin, and to prevent forming defects such as wrinkles occurring during the curling process of the can lid. In other words, the effects of the present invention, which are high strength and high processing accuracy, are particularly evident when the steel sheet thickness is thin.

[0046] From this perspective, the thickness of the steel sheet for cans in 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 thickness of the steel sheet is 0.10 mm, can manufacturing becomes easier. Therefore, the thickness of the steel sheet for cans in this embodiment is preferably 0.10 mm or more. <Method for manufacturing steel sheet for cans> Next, the method for manufacturing a steel sheet for cans according to an embodiment of the present invention will be described below.

[0047] 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, 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).

[0048] 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, an electric furnace, etc., and then made into a slab by, for example, the continuous casting method.

[0049] Note that the melting method of the slab having the above-described component composition is not particularly limited. Any known melting method such as an electric furnace or a converter can be adopted, and secondary refining may be performed in a vacuum degassing furnace. Thereafter, from the viewpoints of productivity and quality stability, it is preferable to manufacture a slab (steel material) by the continuous casting method, but the slab may be manufactured by a known casting method such as the ingot - block rolling method. Since the steel sheet for cans according to this embodiment has Cu, Ni, and Sn as essential elements, if iron scrap containing these three elements is melted in an electric furnace as a raw material, it is also possible to manufacture without going through the iron ore reduction process, which is also beneficial from the viewpoints of carbon neutrality and resource circulation.

[0050] The manufacturing method for steel sheets for cans according to this embodiment includes a hot rolling step, a cold rolling step, an annealing step, and a temper rolling step. In the hot rolling step, a slab having the above-described component composition is heated to 1150°C or higher and hot-rolled under conditions that the finishing temperature is 800°C to 950°C and the coiling temperature is 550°C to 750°C. In the cold rolling step, the hot-rolled sheet that has gone through the hot rolling step is cold-rolled to a reduction ratio of 85% or higher. In the annealing step, the cold-rolled sheet that has gone through the cold-rolling step is heated to an annealing temperature of 640°C to 860°C at an average heating rate of 5 to 40°C / second. Then, after holding at this annealing temperature for 10 seconds to 90 seconds, primary cooling is performed at an average cooling rate of 10°C / sec to 230°C / sec to a cooling stop temperature of 250°C to 350°C, and secondary cooling is performed at an average cooling rate of 0.2°C / sec to 10.0°C / sec to a cooling stop temperature of 150°C to 250°C. In the temper rolling process, the annealed sheet that has undergone the annealing process is subjected to temper rolling with a temper rolling ratio of 0.5% to 5.0%. [Hot rolling process] ・Slab heating temperature: 1150°C or higher If the slab heating temperature in the hot rolling process is less than 1150°C, the amount of carbonitrides that contribute to the strength of the steel sheet cannot be sufficiently secured after the annealing process, resulting in a decrease in strength. Therefore, the slab heating temperature should be 1150°C or higher, preferably 1160°C or higher. Furthermore, since the effect saturates even if the slab heating temperature exceeds 1350°C, it is preferable to keep the slab heating temperature below 1350°C. Finishing temperature: 800°C to 950°C If the finishing temperature in the hot rolling process exceeds 950°C, the ferrite structure after hot rolling becomes coarser, and the ferrite structure after the annealing process also becomes coarser, reducing the yield strength. For this reason, the finishing temperature should be below 950°C, preferably below 930°C.

[0051] 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 curled part of the can lid. 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 value of equation (1) above, which defines the relationship between yield strength and tensile strength, will fall below 470 MPa. In addition, scale will form on the surface of the steel sheet, making it more susceptible to surface defects. Therefore, the winding temperature should be 750°C or lower, preferably 730°C or lower.

[0052] Furthermore, if the winding temperature is less than 550°C, the proportion of unrecrystallized ferrite in the microstructure of the steel sheet after continuous annealing will exceed 3% in terms of area relative to the total microstructure, causing wrinkles to occur when the steel sheet is processed into the curled portion of the can lid. To prevent this, the winding temperature should be 550°C or higher, preferably 560°C or higher.

[0053] 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 the scale from the surface of the steel sheet, and there is no particular need to limit the conditions of the pickling. Alternatively, the scale from the surface of the steel sheet may be removed by a method 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 increases, so that the value of equation (1) above, which defines the relationship between the yield strength and tensile strength of the steel sheet after annealing, can be set to 470 MPa or more. In order to obtain this effect, the reduction ratio is set to 85% or more, preferably to 86% or more.

[0054] 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%.

[0055] 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, Ti, and V in the ferrite. To obtain this effect, it is important to control the average heating rate to the annealing temperature to 5 to 40°C / second.

[0056] 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.

[0057] Furthermore, if the average heating rate exceeds 40°C / second, the area ratio of the ferrite phase will be less than 70%, 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 curled part of a can lid. 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 860°C or lower If the annealing temperature is less than 640°C, the area ratio of the ferrite phase will be less than 70%, 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 curled part of a can lid. To prevent this, the annealing temperature should be 640°C or higher, preferably 680°C or higher.

[0058] Furthermore, if the annealing temperature exceeds 860°C, precipitates containing Nb, Ti, and V 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 860°C or lower, preferably 840°C or lower. Holding time in the temperature range of 640°C to 860°C: 10 seconds to 90 seconds If the holding time in the annealing temperature range of 640°C to 860°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 curled part of a can lid. To prevent this, the holding time in the annealing temperature range of 640°C to 860°C should be 10 seconds or more, preferably 15 seconds or more.

[0059] Furthermore, if the holding time at an annealing temperature of 640°C to 860°C exceeds 90 seconds, precipitates containing Nb, Ti, and V, which are mainly precipitated 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 860°C should be 90 seconds or less, preferably 50 seconds or less.

[0060] A continuous annealing apparatus can be used for the annealing process. Other processes may be appropriately 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 of primary cooling: 10°C / sec to 230°C / sec. If the average cooling rate from the annealing temperature to the cooling stop temperature is less than 10°C / sec, precipitates containing Nb, Ti, and V in the steel sheet after the annealing process 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 10°C / sec or more, preferably 15°C / sec or more. 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.

[0061] Furthermore, if the average cooling rate from the annealing temperature to the cooling stop temperature exceeds 230°C / second, the proportion of the martensite phase in the steel sheet after the annealing process will exceed 22%, and the proportion of the ferrite phase will be less than 70%, resulting in reduced ductility and the formation of wrinkles when the steel sheet is processed into the curled portion of a can lid. To prevent this, the average cooling rate from the annealing temperature to the cooling stop temperature should be 230°C / second or less, preferably 200°C / second or less. Primary cooling stop temperature: 250°C or higher and 350°C or lower If the primary cooling stop temperature is less than 250°C, the proportion of the martensite phase in the steel sheet after the annealing process will exceed 22%, and the proportion of the ferrite phase will be less than 70%, resulting in reduced ductility and the formation of wrinkles when the steel sheet is processed into the curled portion of a can lid. To prevent this, the cooling stop temperature should be 250°C or higher, preferably 270°C or higher.

[0062] Furthermore, if the cooling stop temperature for primary cooling exceeds 350°C, wrinkles will occur when the steel sheet is processed into the curled portion of the can lid. To prevent this, the cooling stop temperature should be 350°C or lower, preferably 330°C or lower. Average cooling rate for secondary cooling: 0.2°C / sec or more and 10.0°C / sec or less. If the average cooling rate from the cooling stop temperature for primary cooling to the cooling stop temperature for secondary cooling is less than 0.2°C / sec, precipitates containing Nb, Ti, and V in the steel sheet after the annealing process will become coarser during cooling, reducing the yield strength and decreasing the pressure resistance when the steel sheet is processed into a can lid. To prevent this, the average cooling rate from the annealing temperature to the cooling stop temperature should be 0.2°C / sec or higher, preferably 0.3°C / sec or higher. Note that cooling from the cooling stop temperature for primary cooling to the cooling stop temperature for secondary cooling can be carried out by gas cooling, or a combination of one or more methods such as mist cooling, roll cooling, and water cooling.

[0063] Furthermore, if the average cooling rate from the cooling stop temperature of the primary cooling to the cooling stop temperature of the secondary cooling exceeds 10.0°C / second, the proportion of the martensite phase in the steel sheet after the annealing process exceeds 22%, reducing ductility and causing wrinkles when the steel sheet is processed into the curled portion of the can lid. To prevent this, the average cooling rate from the cooling stop temperature of the primary cooling to the cooling stop temperature of the secondary cooling should be 10.0°C / second or less, preferably 5.0°C / second or less. ・Cooling stop temperature of secondary cooling: 150°C or more and 200°C or less If the cooling stop temperature of secondary cooling is less than 150°C, the proportion of the martensite phase in the steel sheet after the annealing process exceeds 22%, reducing ductility and causing wrinkles when the steel sheet is processed into the curled portion of the can lid. To prevent this, the cooling stop temperature of secondary cooling should be 150°C or higher, preferably 155°C or higher.

[0064] Furthermore, if the cooling stop temperature for secondary cooling exceeds 200°C, precipitates containing Nb, Ti, and V in the steel sheet after the annealing process become coarser during cooling, reducing the yield strength and decreasing the pressure resistance when the steel sheet is processed into a can lid. To prevent this, the cooling stop temperature for secondary cooling should be 200°C or lower, preferably 195°C or lower.

[0065] 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.

[0066] 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 curled portion of the can lid. To prevent this, the temper rolling ratio should be 5.0% or less, preferably 2.0% or less.

[0067] The steel sheet for cans of the present invention is obtained through the temper rolling process described above.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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 subject and evaluated for each of the following test items. <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 (YS), tensile strength (TS), 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 (YS) 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.

[0072] 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.

[0073] Furthermore, the area fractions of the ferrite phase and martensite were 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 fractions of the ferrite phase and martensite were 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.

[0074] Then, for each field of view, the area ratio of the ferrite phase and the area ratio of martensite within a randomly set 50 μm x 50 μm square area was determined by binarization using image processing software (Photoshop, Adobe). The average values ​​of the area ratio of the ferrite phase and the area ratio of martensite obtained in this way for the three fields of view were defined as the area ratio of ferrite and the area ratio of martensite, respectively. In SEM observation, black areas observed as clumps were identified as ferrite, and white areas were identified as martensite.

[0075] The results of the investigation of this metal structure are shown in Tables 2 and 3. <Amount of deposited Mo> After removing the Sn plating from the Sn-plated steel sheet of the test subject by pickling, the precipitate was extracted using a 10% AA electrolyte, filtered, and subjected to mixed acid decomposition, and the amount of deposited Mo was measured by ICP-AES (inductively coupled plasma atomic emission spectroscopy). The results of this investigation are shown in Tables 2 and 3. <Corrosion resistance> For the Sn-plated steel sheet of the test subject, an area of ​​2.7 mm² was measured using an optical microscope. 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 wrinkles in the curled area> A blank was taken from the Sn-plated steel sheet to be tested, and a can lid was fabricated by sequentially performing shell processing and curl processing. The curled area of ​​the fabricated can lid was then visually observed at eight locations in the circumferential direction to check for the presence or absence of wrinkles. The results of this investigation are shown in Tables 2 and 3. In Tables 2 and 3, "Wrinkle occurrence: Yes" indicates that wrinkles occurred at at least one of the eight circumferential locations, and "Wrinkle occurrence: No" indicates that no wrinkles occurred at any of the eight circumferential locations. <Pressure Resistance> A rectangular blank was taken from the Sn-plated steel sheet to be tested, and a can lid was fabricated by sequentially performing shell processing and curl processing. Then, a pressure resistance test was conducted using this can lid as the test specimen.

[0076] The pressure resistance test was conducted using a CMC-KUHNKE buckle tester BCL-3153L under test conditions of an initial pressure of 0.5 bar and a pressure increase rate of 0.3 bar / min. The pressure was increased from one side of the can lid while the curled portion of the lid was fixed. The pressure value at which the can lid 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 1.3 bar or higher is indicated by "○", and a pressure resistance value of less than 1.3 bar is indicated by "×".

[0077]

[0078]

[0079]

[0080] Steel plates No. 1 to 38, 63, 78, 81, 86, 87, and 89, which are examples of inventions that satisfy the requirements of the steel plate for cans and the method for manufacturing the same according to the present invention, were confirmed to have excellent processability and pressure resistance of the curled portion of the can lid while possessing the corrosion resistance necessary for a steel plate for cans.

[0081] In contrast, steel plates No. 39-61, 62, 64-77, 79, 80, 82-85, 88, and 90-105 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 steel plates No. 39-61, 62, 64-77, 79, 80, 82-85, 88, and 90-105 failed to achieve the desired level in terms of corrosion resistance, processability of the curled portion of the can lid, or pressure resistance, and their performance was significantly inferior to that of the steel plate in the inventive example.

Claims

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 0.0080%, Ti: 0.005% to 0.030%, Nb: 0.005% to 0.030%, Mo: 0.010% to 0.050%, B: 0.0005% to 0.0050%, Cu: 0.003% to 0.500%, Cr: 0.003% to 0.300%, Ni: 0.005% to 0.500% A steel sheet for cans having a composition containing V: 0.005% to 0.100%, Sn: 0.003% to 0.050%, with the remainder being iron and unavoidable impurities, wherein the Mo content as precipitated Mo is 0.013% by mass or less, and the metal structure contains a ferrite phase of 70% or more by area ratio and a martensite phase of 5% to 22% by area ratio, with the proportion of unrecrystallized ferrite being 3% or less by area ratio of the total structure, and the yield strength YS (MPa) and tensile strength TS (MPa) satisfy the following relationship (1) equation (YS + TS) / 2 ≥ 470 MPa ... (1), and the total elongation is 10% or more.

2. A method for manufacturing a steel sheet for cans according to claim 1, comprising: a hot rolling step of heating a steel slab having the above-mentioned component composition at 1150°C or higher, and hot rolling under conditions 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; and 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 comprising: heating the cold-rolled sheet that has undergone the cold-rolling step to an annealing temperature of 640°C to 860°C at an average heating rate of 5 to 40°C / second, holding it at the annealing temperature for 10 to 90 seconds, primary cooling to a cooling stop temperature of 250°C to 350°C at an average cooling rate of 10°C to 230°C / second, and secondary cooling to a cooling stop temperature of 150°C to 200°C at an average cooling rate of 0.2°C to 10.0°C / second; and temper rolling step of subjecting the annealed sheet that has undergone the annealing step to temper rolling with a temper rolling ratio of 0.5% to 5.0%.

Images