Steel plate for cans and method for manufacturing the same

Abstract

The present invention provides a high-strength steel sheet for cans, particularly one having sufficiently high workability as a base material for can bodies having a neck. The steel sheet has a composition and microstructure comprising, by mass%, 0.010-0.130% C, 0.04% or less Si, 0.10-1.00% Mn, 0.007-0.100% P, 0.0005-0.0090% S, 0.001-0.100% Al, 0.0050-0.1000% N, 0.0050-0.1000% Ti, 0.0005% or more and less than 0.0020% B, and 0.08% or less Cr, satisfying the following conditions: 0.005 ≤ (Ti* / 48) / (C / 12) ≤ 0.700; and a microstructure with an upper yield strength of 550-620 MPa. The composition also includes 3% or less of unrecrystallized ferrite.

Description

[0001] This application is a divisional application filed for an application with an application date of June 8, 2020, application number 202080043123.7, and invention name “Steel plate for tanks and its manufacturing method”. Technical Field

[0002] The present invention relates to a steel plate for a tank and a manufacturing method thereof. Background Art

[0003] There is a pressing need to reduce the cost of manufacturing food and beverage can bodies and lids made of steel sheet. As a countermeasure, efforts are underway to reduce the cost of raw materials by thinning the steel sheet used. The steel sheets targeted for thinning are the bodies of two-piece cans formed by drawing, the bodies of three-piece cans formed by cylindrical forming, and the steel sheets used for lids. Simply thinning the steel sheet reduces the strength of the body and lid. Therefore, high-strength, ultra-thin steel sheets are desired for applications such as redrawn (DRD) cans and welded can bodies.

[0004] High-strength, ultra-thin can steel sheets are manufactured using the Double Reduce process (hereinafter referred to as the "DR process"), which involves secondary cold rolling at a reduction ratio of 20% or more after annealing. While steel sheets manufactured using the DR process (hereinafter referred to as "DR material") exhibit high strength, they suffer from low total elongation (lack of ductility) and poor formability.

[0005] To reduce the material cost of the lid, the can mouth is sometimes designed with a smaller diameter than the rest of the can body. This process of reducing the diameter of the can mouth is called necking. This process involves necking the can mouth using a die head or a rotating roller, reducing the diameter of the mouth and forming the neck. High-strength stock, such as DR material, can cause dents in the neck due to buckling caused by localized deformation. These dents impair the can's appearance and diminish its product value, so they should be avoided. Furthermore, thinning the material wall can lead to dents in the neck.

[0006] DR materials, commonly used as high-strength, ultra-thin steel sheets for cans, often lack ductility, making them difficult to machine onto can necks. Consequently, using DR materials requires multiple mold adjustments and multi-stage processing to produce finished products. Furthermore, DR materials achieve high strength through work hardening during secondary cold rolling. Depending on the accuracy of the secondary cold rolling, work hardening can be unevenly applied to the steel sheet, sometimes leading to localized deformation during DR material processing. This localized deformation can cause dents on can necks and should be avoided.

[0007] To avoid these drawbacks of DR materials, methods for producing high-strength steel sheets using various strengthening methods have been proposed. Patent Document 1 proposes a steel sheet that achieves high strength through microstructure refinement and optimization, resulting in excellent deep drawability and flange formability during canning, as well as excellent surface shape after canning. Patent Document 2 proposes a steel sheet for thin-walled deep-drawn cans that is soft during working but hardens after heat treatment by adjusting the amounts of Mn, P, and N in low-carbon steel. Patent Document 3 proposes a steel sheet for three-piece cans that controls the particle size of oxide inclusions, resulting in excellent weld formability, such as reduced neck wrinkling, and improved flange cracking. Patent Document 4 proposes a high-strength steel sheet for containers that achieves high strength through solid solution nitrogen by increasing the nitrogen content. By controlling the dislocation density in the through-thickness direction of the steel sheet, the steel sheet achieves a tensile strength of 400 MPa or higher and an elongation at break of 10% or higher.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 8-325670

[0011] Patent Document 2: Japanese Patent Application Laid-Open No. 2004-183074

[0012] Patent Document 3: Japanese Patent Application Laid-Open No. 2001-89828

[0013] Patent Document 4: International Publication No. 2015 / 166653 Summary of the Invention

[0014] As mentioned above, achieving thinner steel sheets for cans requires ensuring sufficient strength. On the other hand, when using steel sheets as base material for can bodies with necks, the steel sheets must exhibit high ductility. Furthermore, to prevent the formation of dents in the can necks, localized deformation of the steel sheets must be suppressed. However, the aforementioned prior art suffers from poor strength, ductility (total elongation), uniform deformability, and neck workability.

[0015] Patent Document 1 proposes a steel that achieves a balance between high strength and ductility through microstructure refinement and optimization. However, Patent Document 1 completely ignores local deformation of the steel sheet, and the manufacturing method described in Patent Document 1 makes it difficult to obtain a steel sheet that meets the workability requirements for can necks.

[0016] Patent Document 2 proposes improving can strength characteristics by reducing the steel structure with P and aging with N. However, the high strength of the steel sheet due to the addition of P in Patent Document 2 easily leads to local deformation of the steel sheet, making it difficult to obtain a steel sheet with the workability required for the neck of a can body using the technology described in Patent Document 2.

[0017] Patent Document 3 achieves the desired strength through grain refinement brought about by Nb and B. However, the tensile strength of the steel sheet in Patent Document 3 is less than 540 MPa, making it insufficient for high-strength, ultra-thin can steel. Furthermore, the addition of Ca and REM is necessary to improve weld formability and surface properties, leading to a reduction in corrosion resistance in the technology of Patent Document 3. Furthermore, Patent Document 3 makes no consideration of localized deformation of the steel sheet, making it difficult to obtain steel sheet with the workability required for can necks using the manufacturing method described in Patent Document 3.

[0018] Patent Document 4 describes a method for evaluating the compressive strength of can lids formed from high-strength container steel sheets with a tensile strength of 400 MPa or greater and an elongation at break of 10% or greater. However, Patent Document 4 makes no consideration of the shape of the can neck, making it difficult to achieve a good can neck using the technique described in Patent Document 4.

[0019] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel sheet for cans having high strength and sufficiently high workability as a material for a can body having a neck, and a method for producing the same.

[0020] The main configuration of the present invention for solving the above-mentioned problems is as follows.

[0021] [1] A steel plate for cans having the following composition and structure, wherein the upper yield strength is 550 MPa to 620 MPa, wherein the composition comprises, in mass%, C: 0.010% to 0.130%, Si: 0.04% or less, Mn: 0.10% to 1.00%, P: 0.007% to 0.100%, S: 0.0005% to 0.0090%, Al: 0.001% to 0.100%, N: less than 0.0050%, Ti: 0.0050% to 0.1000%, B: more than 0.0005% and less than 0.0020%, and Cr: less than 0.08%, and when Ti*=Ti-1.5S, the relationship of 0.005 to (Ti* / 48) / (C / 12) to 0.700 is satisfied, and the remainder is Fe and unavoidable impurities, and the proportion of unrecrystallized ferrite in the said structure is less than 3%.

[0022] [2] The can steel sheet according to [1] above, wherein the above-mentioned component composition further contains, in mass %, one or more selected from the group consisting of Nb: 0.0050% to 0.0500%, Mo: 0.0050% to 0.0500%, and V: 0.0050% to 0.0500%.

[0023] [3] A method for manufacturing a steel plate for a can, comprising the following steps:

[0024] In the hot rolling process, the steel slab is heated to above 1200°C and rolled at a finishing temperature of above 850°C to produce a steel plate. The steel plate is then coiled at a temperature of 640°C to 780°C and then cooled from 500°C to 300°C at an average cooling rate of 25°C / h to 55°C / h.

[0025] a cold rolling step of cold rolling the steel sheet after the hot rolling step at a reduction ratio of 86% or more;

[0026] an annealing step of holding the cold-rolled steel sheet at a temperature range of 640°C to 780°C for 10 to 90 seconds, then cooling the steel sheet to a temperature range of 500°C to 600°C at an average cooling rate of 7°C / s to 180°C / s, and then cooling the steel sheet to a temperature below 300°C at an average cooling rate of 0.1°C / s to 10°C / s;

[0027] The steel sheet after the annealing step is subjected to temper rolling at a reduction ratio of 0.1% to 3.0%.

[0028] The steel slab has a component composition comprising, by mass%, C: 0.010% to 0.130%, Si: 0.04% or less, Mn: 0.10% to 1.00%, P: 0.007% to 0.100%, S: 0.0005% to 0.0090%, Al: 0.001% to 0.100%, N: 0.0050% or less, Ti: 0.0050% to 0.1000%, B: 0.0005% or more and less than 0.0020%, and Cr: 0.08% or less, and when Ti*=Ti-1.5S, the relationship of 0.005≤(Ti* / 48) / (C / 12)≤0.700 is satisfied, and the remainder is Fe and unavoidable impurities.

[0029] [4] The method for manufacturing a can steel sheet according to [3] above, wherein the above-mentioned component composition further contains, in terms of mass%, one or more selected from the group consisting of Nb: 0.0050% to 0.0500%, Mo: 0.0050% to 0.0500%, and V: 0.0050% to 0.0500%.

[0030] According to the present invention, it is possible to obtain a steel sheet for cans which has high strength and which has sufficiently high processing accuracy as a material for a can body having a neck. DETAILED DESCRIPTION

[0031] The present invention will be described based on the following embodiments. First, the composition of a can steel sheet according to one embodiment of the present invention will be described. It should be noted that all composition units are "mass %" and, unless otherwise specified, are expressed simply as "%."

[0032] C: 0.010%~0.130%

[0033] It is important for the can steel plate of this embodiment to have an upper yield strength of 550 MPa or higher. Therefore, it is extremely important to utilize the precipitation strengthening brought about by the Ti-based carbides generated by the inclusion of Ti. In order to utilize the precipitation strengthening brought about by the Ti-based carbides, the C content of the can steel plate becomes important. If the C content is less than 0.010%, the strength-enhancing effect caused by the above-mentioned precipitation strengthening is reduced, and the upper yield strength is less than 550 MPa. Therefore, the lower limit of the C content is set to 0.010%, preferably 0.015% or higher. On the other hand, if the C content exceeds 0.130%, hypoperitectic cracks will occur during the cooling process of the steel during melting, and the steel plate will be over-hardened, thereby reducing ductility. In addition, if the proportion of unrecrystallized ferrite exceeds 3%, dents will be generated when the steel plate is processed into the neck of the can body. Therefore, the upper limit of the C content is set to 0.130%. It should be noted that if the C content is 0.060% or less, the strength of the hot-rolled sheet is suppressed, the deformation resistance during cold rolling becomes even smaller, and surface defects are less likely to occur even when the rolling speed is increased. Therefore, from the perspective of ease of production, the C content is preferably set to 0.060% or less. The C content is more preferably 0.015% to 0.060%.

[0034] Si: 0.04% or less

[0035] Si is an element that increases the strength of steel through solid solution strengthening. To achieve this effect, the Si content is preferably set to 0.01% or higher. However, if the Si content exceeds 0.04%, corrosion resistance is severely impaired. Therefore, the Si content is set to 0.04% or lower. The Si content is preferably 0.03% or lower, and more preferably 0.01% to 0.03%.

[0036] Mn: 0.10%~1.00%

[0037] Mn increases the strength of steel by solid solution strengthening. If the Mn content is less than 0.10%, it is impossible to ensure an upper yield strength of 550 MPa or more. Therefore, the lower limit of the Mn content is set to 0.10%. On the other hand, if the Mn content exceeds 1.00%, not only the corrosion resistance and surface properties deteriorate, but also the proportion of unrecrystallized ferrite exceeds 3%, which causes local deformation and deteriorates the uniform deformation ability. Therefore, the upper limit of the Mn content is set to 1.00%. The Mn content is preferably 0.20% or more, preferably 0.60% or less, and more preferably 0.20% to 0.60%.

[0038] P: 0.007%~0.100%

[0039] P is an element with a strong solid solution strengthening ability. To achieve this effect, it is necessary to contain 0.007% or more of P. Therefore, the lower limit of the P content is set to 0.007%. On the other hand, if the P content exceeds 0.100%, the steel plate will be over-hardened, resulting in reduced ductility and further deterioration of corrosion resistance. Therefore, the upper limit of the P content is set to 0.100%. The P content is preferably 0.008% or more, preferably 0.015% or less, and more preferably 0.008% to 0.015%.

[0040] S: 0.0005%~0.0090%

[0041] The can steel plate of this embodiment achieves high strength through precipitation strengthening by Ti-based carbides. S easily forms Ti and TiS. If TiS forms, the amount of Ti-based carbides useful for precipitation strengthening decreases, preventing high strength. Specifically, if the S content exceeds 0.0090%, a large amount of TiS forms, reducing strength. Therefore, the upper limit of the S content is set to 0.0090%. The S content is preferably 0.0080% or less. On the other hand, if the S content is less than 0.0005%, the cost of desulfurization becomes excessive. Therefore, the lower limit of the S content is set to 0.0005%.

[0042] Al: 0.001%~0.100%

[0043] Al is an element contained as a deoxidizer and is also useful for steel refinement. If the Al content is less than 0.001%, the deoxidizer effect is insufficient, resulting in solidification defects and increased steelmaking costs. Therefore, the lower limit of the Al content is set to 0.001%. On the other hand, if the Al content exceeds 0.100%, surface defects may occur. Therefore, the upper limit of the Al content is set to 0.100% or less. It should be noted that if the Al content is set to 0.010% to 0.060%, Al can function more effectively as a deoxidizer, which is preferred.

[0044] N: 0.0050% or less

[0045] The can steel plate of this embodiment achieves high strength through precipitation strengthening by Ti-based carbides. Nitrogen easily forms TiN with Ti. If TiN forms, the amount of Ti-based carbides useful for precipitation strengthening decreases, preventing high strength. Furthermore, excessive N content can easily cause slab cracking in the lower correction zone, where the temperature drops during continuous casting. Therefore, the upper limit of the N content is set to 0.0050%. While the lower limit of the N content does not need to be specifically set, it is preferable to have an N content exceeding 0.0005% from the perspective of steelmaking costs.

[0046] Ti: 0.0050% to 0.1000% or less

[0047] Ti is an element with a high carbide-forming ability and is effective in precipitating fine carbides. As a result, the upper yield strength is improved. In this embodiment, the upper yield strength can be adjusted by adjusting the Ti content. This effect is achieved by setting the Ti content to 0.0050% or more, so the lower limit of the Ti content is set to 0.0050%. On the other hand, Ti causes a rise in recrystallization temperature, so if the Ti content exceeds 0.1000%, the proportion of unrecrystallized ferrite exceeds 3% during annealing at 640-780°C, resulting in dents when the steel sheet is processed into the neck of the can body. Therefore, the upper limit of the Ti content is set to 0.1000%. The Ti content is preferably 0.0100% or more, preferably 0.0800% or less, and more preferably 0.0100% to 0.0800%.

[0048] B: 0.0005% or more and less than 0.0020%

[0049] B is effective in refining the ferrite grain size and improving the upper yield strength. In this embodiment, the upper yield strength can be adjusted by adjusting the B content. This effect is achieved by setting the B content to 0.0005% or more, so the lower limit of the B content is set to 0.0005%. On the other hand, B causes a rise in the recrystallization temperature, so if the B content is 0.0020% or more, the proportion of unrecrystallized ferrite exceeds 3% during annealing at 640°C to 780°C, resulting in dents when the steel sheet is processed into the neck of the can body. Therefore, the B content is less than 0.0020%. The B content is preferably 0.0006% or more, preferably 0.0018% or less, and more preferably 0.0006% to 0.0018%.

[0050] Cr: 0.08% or less

[0051] Cr is an element that forms carbonitrides. While Cr carbonitrides have less strengthening power than Ti-based carbides, they contribute to higher strength in steel. To fully achieve this effect, the Cr content is preferably set at 0.001% or higher. However, if the Cr content exceeds 0.08%, excessive Cr carbonitride formation inhibits the formation of Ti-based carbides, which contribute most to the steel's strengthening ability, and the desired strength cannot be achieved. Therefore, the Cr content is set at 0.08% or lower.

[0052] 0.005≤(Ti* / 48) / (C / 12)≤0.700

[0053] In order to obtain high strength and suppress local deformation during processing, the value of (Ti* / 48) / (C / 12) is important. Here, Ti* is defined by Ti*=Ti-1.5S. Ti forms fine precipitates (Ti-based carbides) with C, which contribute to the high strength of steel. C that does not form Ti-based carbides exists in the steel in the form of cementite or solid solution C. This solid solution C becomes the cause of local deformation when the steel plate is processed, and produces dents when the steel plate is processed into the neck of the can body. In addition, Ti easily combines with S to form TiS. If TiS is formed, the amount of Ti-based carbides useful for precipitation strengthening is reduced, and high strength cannot be obtained. The present inventors found that by controlling the value of (Ti* / 48) / (C / 12), the high strength brought by Ti-based carbides can be achieved, and the dents caused by local deformation during the processing of the steel plate can be suppressed, thereby completing the present invention. Specifically, if (Ti* / 48) / (C / 12) is less than 0.005, the amount of Ti-based carbides that contribute to high strength of the steel decreases, resulting in an upper yield strength of less than 550 MPa. Furthermore, if the proportion of unrecrystallized ferrite exceeds 3%, sink marks may occur when the steel sheet is processed into the neck of a can. Therefore, (Ti* / 48) / (C / 12) is set to 0.005 or higher. On the other hand, if (Ti* / 48) / (C / 12) exceeds 0.700, the proportion of unrecrystallized ferrite exceeds 3% during annealing at 640°C to 780°C, resulting in sink marks when the steel sheet is processed into the neck of a can. Therefore, (Ti* / 48) / (C / 12) is set to 0.700 or lower. (Ti* / 48) / (C / 12) is preferably 0.090 or higher, preferably 0.400 or lower, and more preferably 0.090 to 0.400.

[0054] The remainder other than the above-mentioned components is Fe and inevitable impurities.

[0055] While the basic components of the present invention have been described above, the following elements may be appropriately contained as needed.

[0056] Nb: 0.0050%~0.0500%

[0057] Like Ti, Nb is an element with a high carbide-forming ability and is effective in precipitating fine carbides. Therefore, the upper yield strength is improved. In this embodiment, the upper yield strength is adjusted by adjusting the Nb content. Since this effect is achieved by setting the Nb content to 0.0050% or more, when adding Nb, the lower limit of the Nb content is preferably set to 0.0050%. On the other hand, Nb causes an increase in the recrystallization temperature. Therefore, if the Nb content exceeds 0.0500%, the proportion of unrecrystallized ferrite exceeds 3% during annealing at 640°C to 780°C, resulting in dents when the steel sheet is processed into the neck of a can body. Therefore, when adding Nb, the upper limit of the Nb content is preferably set to 0.0500%. The Nb content is more preferably 0.0080% or more, more preferably 0.0300% or less, and more preferably 0.0080% to 0.0300%.

[0058] Mo: 0.0050%~0.0500%

[0059] Like Ti and Nb, Mo is an element with a high carbide-forming ability and is effective in precipitating fine carbides. As a result, the upper yield strength is improved. In this embodiment, the upper yield strength can be adjusted by adjusting the Mo content. This effect is achieved by setting the Mo content to 0.0050% or more, so when adding Mo, it is preferable to set the lower limit of the Mo content to 0.0050%. On the other hand, Mo causes a rise in the recrystallization temperature, so if the Mo content exceeds 0.0500%, the proportion of unrecrystallized ferrite exceeds 3% during annealing at 640°C to 780°C, resulting in dents when the steel sheet is processed into the neck of a can body. Therefore, when adding Mo, it is preferable to set the upper limit of the Mo content to 0.0500%. The Mo content is more preferably 0.0080% or more, more preferably 0.0300% or less, and even more preferably 0.0080% to 0.0300%.

[0060] V: 0.0050%~0.0500%

[0061] V refines the ferrite grain size and is effective in improving the upper yield strength. In this embodiment, the upper yield strength can be adjusted by adjusting the V content. This effect is achieved by setting the V content to 0.0050% or more, so when adding V, it is preferred to set the lower limit of the V content to 0.0050%. On the other hand, V causes a rise in the recrystallization temperature, so if the V content exceeds 0.0500%, the proportion of unrecrystallized ferrite exceeds 3% during annealing at 640°C to 780°C, resulting in dents when the steel sheet is processed into the neck of a can body. Therefore, when adding V, it is preferred to set the upper limit of the V content to 0.0500%. The V content is more preferably 0.0080% or more, more preferably 0.0300% or less, and even more preferably 0.0080% to 0.0300%.

[0062] Next, the mechanical properties of the steel sheet for cans according to this embodiment will be described.

[0063] Upper yield strength: 550MPa~620MPa

[0064] To ensure the welded can's resistance to dents, i.e., dent strength, and the can lid's compressive strength, the steel plate's upper yield strength is set at 550 MPa or higher. On the other hand, if the steel plate's upper yield strength exceeds 620 MPa, dents will form when the steel plate is processed into the can's neck. Therefore, the steel plate's upper yield strength is between 550 MPa and 620 MPa.

[0065] The yield strength can be measured according to the tensile testing method for metallic materials specified in JIS Z 2241:2011. This yield strength can be achieved by adjusting the composition, the coiling temperature in the hot rolling process, the cooling rate in the cooling process after coiling in the hot rolling process, the reduction ratio in the cold rolling process, the soaking temperature and holding time in the annealing process, the cooling rate in the annealing process, and the reduction ratio in the temper rolling process. Specifically, a yield strength of 550 MPa to 620 MPa can be obtained by setting the above-mentioned composition, setting the coiling temperature in the hot rolling process to 640°C to 780°C, setting the average cooling rate from 500°C to 300°C after coiling to 25°C / h to 55°C / h, setting the reduction ratio in the cold rolling process to 86% or more, setting the holding time in the temperature range of 640°C to 780°C to 10s to 90s in the annealing process, primary cooling to the temperature range of 500°C to 600°C at an average cooling rate of 7°C / s to 180°C / s, and secondary cooling to 300°C at an average cooling rate of 0.1°C / s to 10°C / s, and setting the reduction ratio in the tempering rolling process to 0.1% to 3.0%.

[0066] Next, the metal structure of the steel sheet for cans according to the present invention will be described.

[0067] Ratio of unrecrystallized ferrite: 3% or less

[0068] If the proportion of unrecrystallized ferrite in the metal structure exceeds 3%, dents caused by local deformation will occur during processing, for example, when the steel plate is processed into the neck of a can body. Therefore, the proportion of unrecrystallized ferrite in the metal structure is set to 3% or less. The mechanism of local deformation during processing is not clear, but it is speculated that if a large amount of unrecrystallized ferrite is present, the balance of interaction between unrecrystallized ferrite and dislocations will collapse during processing until dents are generated. The proportion of unrecrystallized ferrite in the metal structure is preferably 2.7% or less. If the proportion of unrecrystallized ferrite in the metal structure is set to 0.5% or more, the annealing temperature can be made relatively low, so it is preferred, and more preferably 0.8% or more.

[0069] The proportion of unrecrystallized ferrite in the metal structure can be measured according to the following method. After grinding the cross section in the thickness direction parallel to the rolling direction of the steel plate, it is corroded using a corrosive solution (3% by volume nitric alcohol). Next, an optical microscope is used to observe the area from a depth position of 1 / 4 of the plate thickness (a position of 1 / 4 of the plate thickness in the thickness direction from the surface of the above-mentioned cross section) to a position of 1 / 2 of the plate thickness in 10 fields of view at a magnification of 400 times. Next, using a tissue photograph taken through an optical microscope, the unrecrystallized ferrite is determined by visual judgment, and the area ratio of the unrecrystallized ferrite is calculated by image analysis. Here, the unrecrystallized ferrite is a metal structure that is elongated in the rolling direction under an optical microscope at a magnification of 400 times. In each field of view, the area ratio of the unrecrystallized ferrite is calculated, and the value obtained by averaging the area ratios of the 10 fields of view is taken as the proportion of the unrecrystallized ferrite in the metal structure.

[0070] Plate thickness: 0.4mm or less

[0071] Currently, thinning of steel plates is being pursued to reduce can manufacturing costs. However, thinning of steel plates, i.e., reduction in plate thickness, can lead to reduced can body strength and poor forming during processing. In contrast, the can steel plate of this embodiment does not reduce can body strength, such as the compressive strength of the can lid, even when the plate thickness is thin, nor does it cause poor forming due to dents during processing. In other words, the effects of the present invention, namely, high strength and high processing accuracy, can be significantly exerted even with a thin plate thickness. Therefore, from this perspective, the plate thickness of the can steel plate is preferably set to 0.4 mm or less. It should be noted that the plate thickness can be 0.3 mm or less, or 0.2 mm or less.

[0072] Next, a method for manufacturing a can steel plate according to one embodiment of the present invention will be described. The following temperatures are based on the surface temperature of the steel plate. The average cooling rate is calculated based on the surface temperature of the steel plate as follows. 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).

[0073] When manufacturing the steel plate for cans of the present embodiment, molten steel is adjusted to the above-mentioned component composition by a known method using a converter or the like, and then slabs are formed by, for example, continuous casting.

[0074] Slab heating temperature: above 1200℃

[0075] If the slab heating temperature during the hot rolling process is less than 1200°C, unrecrystallized structure will remain in the steel sheet after annealing, causing sink marks when the steel sheet is processed into the neck of a can body. Therefore, the lower limit of the slab heating temperature is set to 1200°C. The slab heating temperature is preferably 1220°C or higher. The effect is saturated even if the slab heating temperature exceeds 1350°C, so the upper limit is preferably set to 1350°C.

[0076] Finish rolling temperature: above 850℃

[0077] If the final temperature of the hot rolling process is less than 850°C, unrecrystallized structure caused by the hot-rolled steel sheet remains in the annealed steel sheet, causing sink marks due to local deformation during processing of the steel sheet. Therefore, the lower limit of the finish rolling temperature is set to 850°C. On the other hand, a finish rolling temperature of 950°C or lower is preferred because it suppresses the formation of oxide scale on the steel sheet surface and achieves better surface properties.

[0078] Winding temperature: 640℃~780℃

[0079] If the coiling temperature in the hot rolling process is less than 640°C, cementite will precipitate in large quantities in the hot-rolled steel sheet. Moreover, the proportion of unrecrystallized ferrite in the metal structure after annealing exceeds 3%, and dents caused by local deformation will occur when the steel sheet is processed into the neck of the can body. Therefore, the lower limit of the coiling temperature is set to 640°C. On the other hand, if the coiling temperature exceeds 780°C, part of the ferrite of the steel sheet after continuous annealing will coarsen, the steel sheet will soften, and the upper yield strength will be less than 550MPa. Therefore, the upper limit of the coiling temperature is set to 780°C. The coiling temperature is preferably above 660°C, more preferably below 760°C, and more preferably between 660°C and 760°C.

[0080] Average cooling rate from 500℃ to 300℃: 25℃ / h~55℃ / h

[0081] If the average cooling rate from 500°C to 300°C after coiling is less than 25°C / h, cementite will precipitate in large quantities in the hot-rolled steel sheet. As a result, the proportion of unrecrystallized ferrite in the metal structure after annealing exceeds 3%, and dents caused by local deformation will occur when the steel sheet is processed into the neck of the can body. In addition, the amount of fine Ti-based carbides that contribute to strength will decrease, and the strength of the steel sheet will decrease. Therefore, the lower limit of the average cooling rate from 500°C to 300°C after coiling is set to 25°C / h. On the other hand, if the average cooling rate from 500°C to 300°C after coiling exceeds 55°C / h, the solid solution C present in the steel will increase, and dents caused by the solid solution C will occur when the steel sheet is processed into the neck of the can body. Therefore, the upper limit of the average cooling rate from 500°C to 300°C after coiling is set to 55°C / h. The average cooling rate from 500°C to 300°C after winding is preferably 30°C / h or higher, preferably 50°C / h or lower, and more preferably 30°C / h to 50°C / h. It should be noted that the above average cooling rate can be achieved by air cooling. The "average cooling rate" is based on the average temperature at the edge and center of the coil width.

[0082] Pickling

[0083] Afterwards, it is preferred to perform pickling as needed. Pickling does not require any particular restrictions as long as it can remove the surface oxide scale. Alternatively, the oxide scale may be removed by methods other than pickling.

[0084] Cold rolling reduction rate: more than 86%

[0085] If the reduction ratio of the cold rolling process is less than 86%, the strain imparted to the steel sheet by cold rolling is reduced, making it difficult to set the upper yield strength of the steel sheet after annealing to 550 MPa or more. Therefore, the reduction ratio of the cold rolling process is set to 86% or more. The reduction ratio of the cold rolling process is preferably 87% or more, preferably 94% or less, and more preferably 87% to 94%. It should be noted that other processes may be appropriately included after the hot rolling process and before the cold rolling process, such as an annealing process for softening the hot-rolled sheet. In addition, the cold rolling process may be performed without immediately performing pickling after the hot rolling process.

[0086] Maintaining temperature: 640℃~780℃

[0087] If the holding temperature of the annealing process exceeds 780°C, sheet metal defects such as thermal buckling are likely to occur during annealing. In addition, the ferrite grain size of the steel sheet partially coarsens, the steel sheet softens, and the upper yield strength is less than 550 MPa. Therefore, the holding temperature is set to 780°C or less. On the other hand, if the annealing temperature is less than 640°C, the recrystallization of the ferrite grains is incomplete, the proportion of unrecrystallized ferrite exceeds 3%, and dents are generated when the steel sheet is processed into the neck of the can body. Therefore, the holding temperature is set to 640°C or more. It should be noted that the holding temperature is preferably 660°C or more, preferably 740°C or less, and more preferably 660°C to 740°C.

[0088] Holding time in the temperature range of 640℃~780℃: 10s~90s

[0089] If the holding time exceeds 90 seconds, Ti-based carbides, which precipitate primarily during the coiling process of hot rolling, become coarse during temperature rise, reducing strength. On the other hand, if the holding time is less than 10 seconds, the recrystallization of ferrite grains becomes incomplete, leaving unrecrystallized ferrite. The proportion of unrecrystallized ferrite exceeds 3%, causing sink marks when the steel sheet is processed into the neck of a can.

[0090] A continuous annealing device can be used during annealing. In addition, other steps can be appropriately included after the cold rolling step and before the annealing step, such as an annealing step for softening the hot-rolled sheet, or the annealing step can be performed immediately after the cold rolling step.

[0091] Primary cooling: Cooling to a temperature range of 500°C to 600°C at an average cooling rate of 7°C / s to 180°C / s

[0092] After the above-mentioned holding period, the steel sheet is cooled to a temperature range of 500°C to 600°C at an average cooling rate of 7°C / s to 180°C / s. If the average cooling rate exceeds 180°C / s, the steel sheet will be excessively hardened, resulting in dents when the steel sheet is processed into the neck of a can body. On the other hand, if the average cooling rate is less than 7°C / s, the Ti-based carbides will become coarse, and the strength will be reduced. The average cooling rate is preferably 20°C / s or higher, preferably 160°C / s or lower, and more preferably 20°C / s to 160°C / s. In addition, if the cooling stop temperature of the primary cooling after holding is less than 500°C, the steel sheet will be excessively hardened, resulting in dents when the steel sheet is processed into the neck of a can body. Therefore, the cooling stop temperature is set to 500°C or higher. The cooling stop temperature of the primary cooling after holding is preferably set to 520°C or higher. If the cooling stop temperature of the primary cooling after holding exceeds 600°C, the Ti-based carbides will become coarse, resulting in a reduction in strength, so the cooling stop temperature is set to 600°C or lower.

[0093] Secondary cooling: Cool to below 300℃ at an average cooling rate of 0.1℃ / s to 10℃ / s

[0094] In the secondary cooling after the primary cooling, the steel plate is cooled to a temperature range below 300°C at an average cooling rate of 0.1°C / s to 10°C / s. If the average cooling rate exceeds 10°C / s, the steel plate is excessively hardened, and dents are generated when the steel plate is processed into the neck of the can body. On the other hand, if the average cooling rate is less than 0.1°C / s, the Ti-based carbides become coarse and the strength is reduced. The average cooling rate is preferably greater than 1.0°C / s, preferably less than 8.0°C / s, and more preferably 1.0°C / s to 8.0°C / s. Cool to below 300°C in the secondary cooling. When the secondary cooling is stopped when the temperature exceeds 300°C, the steel plate is excessively hardened, and dents are generated when the steel plate is processed into the neck of the can body. It is preferred to perform secondary cooling to below 290°C.

[0095] Tempering rolling reduction rate: 0.1% to 3.0%

[0096] If the reduction ratio of the temper rolling after annealing exceeds 3.0%, excessive work hardening will be introduced into the steel plate, resulting in an excessive increase in the strength of the steel plate. When the steel plate is processed, for example, dents will be generated in the processing of the can neck. Therefore, the reduction ratio of the temper rolling is set to 3.0% or less, preferably 1.6% or less. On the other hand, the temper rolling has the function of imparting surface roughness to the steel plate. In order to impart uniform surface roughness to the steel plate and set the upper yield strength to 550 MPa or above, the reduction ratio of the temper rolling needs to be set to 0.1% or above. It should be noted that the temper rolling process can be carried out in the annealing apparatus or in a separate rolling process.

[0097] The can steel sheet of this embodiment can be obtained as described above. It should be noted that, in the present invention, various further processes may be performed after temper rolling. For example, the can steel sheet of the present invention may have a coating on the surface of the steel sheet. Examples of the coating include Sn coating, Cr coating such as tin-free coating, Ni coating, and Sn-Ni coating. Furthermore, processes such as coating and sintering treatment and film lamination may be performed. It should be noted that the thickness of the coating, laminated film, etc. is sufficiently small relative to the plate thickness, so the effect on the mechanical properties of the can steel sheet can be negligible.

[0098] Example

[0099] Steel containing the components shown in Table 1, with the remainder consisting of Fe and inevitable impurities, was smelted in a converter and continuously cast to obtain a steel slab. The steel slab was then hot rolled under the hot rolling conditions shown in Tables 2 and 3, and pickled after hot rolling. The steel slab was then cold rolled at the reduction ratio shown in Tables 2 and 3, and continuously annealed under the annealing conditions shown in Tables 2 and 3, followed by temper rolling at the reduction ratio shown in Tables 2 and 3, thereby obtaining a steel plate. The steel plate was continuously subjected to conventional Sn plating to obtain a single-sided Sn coating of 11.2 g / m 2 Then, the Sn-plated steel sheets subjected to heat treatment corresponding to a coating baking treatment at 210° C. for 10 minutes were subjected to the following evaluations.

[0100] <Tensile test>

[0101] Tensile testing was conducted according to the metal material tensile testing method specified in "JIS Z 2241:2011." Specifically, a JIS No. 5 tensile test piece (JIS Z 2201) was prepared with the tensile direction perpendicular to the rolling direction. A 50 mm (L) mark was placed on the parallel portion of the tensile test piece. The tensile test was conducted at a tensile speed of 10 mm / min in accordance with JIS Z 2241 until the tensile test piece broke. The upper yield strength was then measured. The results are shown in Tables 2 and 3.

[0102] <Investigation of Metal Structure>

[0103] After grinding the cross section in the thickness direction parallel to the rolling direction of the Sn-plated steel sheet, it was corroded using a corrosive solution (3% by volume nitric alcohol). Then, an optical microscope was used to observe the area from the depth position of 1 / 4 of the thickness (the position of 1 / 4 of the thickness of the above-mentioned cross section along the thickness direction from the surface) to the position of 1 / 2 of the thickness in 10 fields of view at a magnification of 400 times. Then, using the tissue photographs taken using an optical microscope, the unrecrystallized ferrite occupied in the metal structure was determined by visual judgment, and the area ratio of the unrecrystallized ferrite was obtained by image analysis. Here, the unrecrystallized ferrite is a metal structure in a shape elongated along the rolling direction in the optical microscope observation at a magnification of 400 times. Then, the area ratio of the unrecrystallized ferrite in each field of view was obtained, and the value obtained by averaging the area ratios of 10 fields of view was used as the proportion of the unrecrystallized ferrite in the metal structure. It should be noted that image analysis uses image analysis software (particle analysis system of Nippon Steel & Sumitomo Metal Corporation). The survey results are shown in Tables 2 and 3.

[0104] <Corrosion resistance>

[0105] Sn-plated steel sheets were observed using an optical microscope at 50x magnification, measuring an area of ​​2.7 mm². The number of areas where the Sn plating was thinned and holes was measured. Pores with fewer than 20 holes were rated as positive, those with 20 to 25 holes were rated as negative, and those with more than 25 holes were rated as negative. The observation results are shown in Tables 2 and 3.

[0106] <Whether there are dents>

[0107] A square steel plate was processed sequentially through roller processing, seam welding, and necking to form a can body. The neck of the resulting can body was visually inspected at eight locations along the circumference to determine the presence of dents. The evaluation results are shown in Tables 2 and 3. A dent occurring at one of the eight circumferential locations was evaluated as "Dent: Present," while a dent occurring at none of the eight locations was evaluated as "Dent: None."

[0108] [Table 1]

[0109]

[0110]

[0111] [Table 2]

[0112]

[0113]

[0114] [Table 3]

[0115]

[0116]

[0117] Industrial applicability

[0118] The present invention provides high-strength steel sheet for cans, particularly with sufficiently high processing accuracy for can neck stock. Furthermore, the present invention provides high uniform deformability, enabling the production of can products with high processing accuracy, for example, when processing can bodies. Furthermore, the present invention is optimal for use as steel sheet for cans, particularly for three-piece cans with highly processed bodies, two-piece cans with several-percent bottom processing, and can lids.

Claims

1. A steel plate for a can having the following composition and structure, wherein the upper yield strength is 550 MPa to 620 MPa, wherein the composition comprises, in mass%, C: 0.010% to 0.130%, Si: 0.04% or less, Mn: 0.10% to 1.00%, P: 0.007% to 0.100%, S: 0.0005% to 0.0090%, Al: 0.001% to 0.100%, N : 0.0050% or less, Ti: 0.025% to 0.1000%, B: 0.0005% or more and less than 0.0020%, and Cr: 0.08% or less, and when Ti*=Ti-1.5S, the relationship of 0.005≤(Ti* / 48) / (C / 12)≤0.700 is satisfied, the remainder is Fe and unavoidable impurities, and the proportion of unrecrystallized ferrite in the structure is 3% or less.

2. The can steel plate according to claim 1, wherein The above-mentioned component composition further contains, in mass%, one or more selected from the group consisting of Nb: 0.0050% to 0.0500%, Mo: 0.0050% to 0.0500%, and V: 0.0050% to 0.0500%.

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