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United States Patent Application 20180112295
Kind Code A1
Kojima; Katsumi ;   et al. April 26, 2018

STEEL SHEET FOR CAN LID AND METHOD FOR PRODUCING THE SAME (AS AMENDED)

Abstract

A steel sheet for can lids contains, in mass %, 0.020-0.060% of C, 0.01-0.05% of Si, 0.20-0.60% of Mn, 0.001-0.100% of P, 0.008-0.020% of S, 0.0130-0.0190% of N, and from 0.005% to {-4.20.times.N+0.110}% of Al, with Mnf being 0.30% to 0.58% (inclusive), where Mnf=Mn-1.7.times.S, and with the balance made up of Fe and unavoidable impurities. The lower yield strength YP (N/mm.sup.2) and the yield point elongation YPEl (%) of this steel sheet for can lids after an aging treatment at 210.degree. C. for 10 minutes satisfy YP.gtoreq.355, YPEl.gtoreq.2, YPEl.gtoreq.(60/(YP-355))+2 and YP.ltoreq.4.09.times.YPEl+476. A method for producing this steel sheet for can lids.


Inventors: Kojima; Katsumi; (Fukuyama, JP) ; Hiraguchi; Tomonari; (Fukuyama, JP) ; Tanaka; Takumi; (Fukuyama, JP) ; Nakamaru; Hiroki; (Chiba, JP)
Applicant:
Name City State Country Type

JFE STEEL CORPORATION

Tokyo

JP
Assignee: JFE STEEL CORPORATION
Tokyo
JP

Family ID: 1000003092692
Appl. No.: 15/561842
Filed: March 28, 2016
PCT Filed: March 28, 2016
PCT NO: PCT/JP2016/001773
371 Date: September 26, 2017


Current U.S. Class: 1/1
Current CPC Class: C22C 38/06 20130101; C22C 38/02 20130101; C22C 38/04 20130101; C22C 38/001 20130101; C21D 9/46 20130101; C21D 8/0273 20130101; B21D 51/26 20130101
International Class: C22C 38/06 20060101 C22C038/06; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101 C22C038/04; C22C 38/00 20060101 C22C038/00; C21D 9/46 20060101 C21D009/46; C21D 8/02 20060101 C21D008/02

Foreign Application Data

DateCodeApplication Number
Mar 31, 2015JP2015-071165

Claims



1. A steel sheet for a can lid, comprising, in terms of % by mass: C: 0.020% to 0.060%; Si: 0.01% to 0.05%; Mn: 0.20% to 0.60%; P: 0.001% to 0.100%; S: 0.008% to 0.020%; N: 0.0130% to 0.0190%; and Al: 0.005% to {-4.20.times.N+0.110}% (where N in the formula represents an N content (% by mass) in steel), wherein, when Mnf=Mn-1.7.times.S (where Mn and S in the formula respectively represent an Mn content (% by mass) and a S content (% by mass) in the steel), Mnf is 0.30% or more and 0.58% or less, the balance is Fe and unavoidable impurities, and after 210.degree. C..times.10 minutes of an aging treatment, a lower yield strength YP (N/mm.sup.2) and a yield point elongation YPEl (%) satisfy YP.gtoreq.355, YPEl.gtoreq.2, YPEl.gtoreq.(60/(YP-355))+2, and YP.ltoreq.4.09.times.YPEl+476.

2. A method for producing the steel sheet for a can lid according to claim 1, comprising: re-heating a steel slab to a temperature of 1150.degree. C. or higher; hot-rolling the re-heated steel slab at a coiling temperature of 680.degree. C. or lower to produce a hot-rolled steel sheet; cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; recrystallization-annealing the cold-rolled steel sheet; and temper-rolling the resulting steel sheet at an elongation of 3% or less.
Description



CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is the U.S. National Phase Application of PCT/JP2016/001773, filed Mar. 28, 2016, which claims priority to Japanese Patent Application No. 2015-071165, filed Mar. 31, 2015, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention relates to a steel sheet for making lids of cans such as food cans and beverage cans, and to a method for producing the steel sheet.

BACKGROUND OF THE INVENTION

[0003] Food cans and beverage cans have their contents heat-sterilized during their manufacturing process. During the process, a pressure difference is generated between inside and outside of the cans, and the pressure acts on the cans. Cans are mainly constituted by can bodies and can lids. Of these, can bodies deform little under pressure since they have a cylindrical shape that easily disperses the stress. In contrast, can lids are mostly formed of flat parts and easily deform as the pressure is received by the flat surfaces. Excessive deformation of can lids is not preferable, and can lids that do not easily deform under pressure are preferably provided.

[0004] One of the methods for suppressing deformation of can lids against pressure is to increase the pressure resistance of the steel sheets for can lids. The technologies described in Patent Literatures 1 to 3 below are available as the technologies related to steel sheets for can lids in which pressure resistance is increased.

[0005] The technology described in Patent Literature 1 relates to a steel sheet for making aerosol can bottoms, and this steel sheet achieves formability since the product of the lower yield strength and the sheet thickness is limited and achieves pressure resistance since the product of the upper yield strength after aging and the square of the sheet thickness is limited. Patent Literature 1 discloses a steel sheet for an aerosol can bottom having high pressure resistance and excellent formability, the steel sheet having a composition that includes, in terms of % by mass, C: 0.020% or more and 0.090% or less, Si: 0.01% or more and 0.05% or less, Mn: 0.05% or more and 0.60% or less, P: 0.001% or more and 0.100% or less, S: 0.001% or more and 0.025% or less, N: 0.0010% or more and less than 0.0070%, Al: 0.010% or more and {-4.2.times.N content (%)+0.11}% or less, and the balance being Fe and unavoidable, impurities, in which the sheet thickness is 0.350 mm or less, the product of the lower yield strength (N/mm.sup.2) and the sheet thickness (mm) is 195 (N/mm) or less, and the product of the square of the sheet thickness (mm) and the upper yield strength (N/mm.sup.2) which is observed after performing an aging treatment at room temperature under condition of a temperature of 25.degree. C. and a duration of 10 days after giving a tensile pre-strain of 10% is 52.0 N or more. The literature also discloses a method for making this steel sheet.

[0006] The technology described in Patent Literature 2 involves intentionally adding 0.0075% to 0.013% by mass of N for solid solution strengthening in order to increase the pressure resistance. Patent Literature 2 discloses a steel sheet for making an aerosol can lid and a method for producing the steel sheet, in which the steel sheet has a chemical composition that includes, in terms of % by mass, C: 0.025% to 0.065%, Mn: 0.10% to 0.28%, P: 0.005% to 0.03%, Al: 0.01% to 0.04%, N: 0.0075% to 0.013%, Si: 0.05% or less, S: 0.009% or less, and the balance being Fe and unavoidable impurities, a yield strength YP in the rolling direction after an aging treatment is in the range of 460 to 540 MPa, a total elongation in the rolling direction after the aging treatment is 15% or more, a yield point elongation EL.sub.YP in the rolling direction after the aging treatment is 6% or less, and a sheet thickness t in terms of millimeter, a yield strength YP in terms of MPa in the rolling direction after the aging treatment, and a yield point elongation EL.sub.YP in terms of % in the rolling direction after the aging treatment satisfy 130.ltoreq.t.times.YP.times.(1-EL.sub.YP/100). A method for producing the steel sheet is also disclosed.

[0007] Whereas Patent Literature 1 and Patent Literature 2 obtain high strength at a relatively high elongation, the technology described in Patent Literature 3 involves adding 0.007% to 0.025% of N, which is greater than the amount added in the technology described in Patent Literature 2, to induce strain-age hardening and thereby increase the pressure resistance. Patent Literature 3 discloses a high-pressure-resistance-strength, excellent workability steel sheet for making an aerosol can bottom, in which: the steel sheet contains, in terms of % by mass, C: 0.02% to 0.10%, Si: 0.01% to 0.5%, P: 0.001% to 0.100%, S: 0.001% to 0.020%, N: 0.007% to 0.025%, and Al: 0.01% to {-4.2.times.N (%)+0.11}%; when Mnf=Mn-1.71.times.S (in the formula, the Mn content and the S content are the Mn content (% by mass) and the S content (% by mass) in the steel), Mnf is 0.10% or more and less than 0.30%; and the balance is Fe and unavoidable impurities. This steel sheet has a sheet thickness of 0.35 (mm) or less, the product of the lower yield strength (N/mm.sup.2) of the steel sheet and the sheet thickness (mm) is 160 (N/mm) or less, and the product of the square of the sheet thickness (mm) and an upper yield strength (N/mm.sup.2) which is observed after performing an aging treatment at room temperature under condition of a temperature of 25.degree. C. and a duration of 10 days after giving a tensile pre-strain of 10% is 52.0 (N) or more. A method for producing the steel sheet is also disclosed.

CITATION LIST

Patent Literature

[0008] PTL 1: Japanese Unexamined Patent Application Publication No. 2013-147744

[0009] PTL 2: International Publication No. 2012/077628

[0010] PTL 3: Japanese Unexamined Patent Application Publication No. 2012-207305

SUMMARY OF INVENTION

Technical Problem

[0011] However, the technologies described in Patent Literatures 1 to 3 all relate to steel sheets for use in aerosol cans. The bottom lids and bottoms of these aerosol cans have a dome shape protruding toward the can inner surface side in order to achieve high pressure resistance strength. The technologies described in Patent Literatures 1 to 3 do not address deformations caused by a pressure difference between inside and outside of the can, which are likely to occur in the flat can lids of cans such as food cans.

[0012] There has been no established technology for suppressing deformations caused by the pressure difference between inside and outside of the can, which are likely to occur in the flat can lids mostly constituted by flat parts such as those used in food cans etc. The present invention has been made under such circumstances and aims to provide a steel sheet for a can lid capable of suppressing deformation caused by a pressure difference between inside and outside of the can, and to provide a method for producing the steel sheet.

Solution to Problem

[0013] The inventors of the present invention have examined the influence of the mechanical properties of the steel sheet on the pressure resistance of a can lid. As a result, the inventors have found that when the lower yield strength YP and the yield point elongation YPEl are appropriately controlled, excellent pressure resistance can be obtained even in a flat can lid mostly constituted by flat parts.

[0014] The inventors have also found that when the N content is increased, when the Al, Mn, and S contents are adjusted to particular levels, and when the manufacturing conditions, namely, the slab heating temperature, the coiling temperature of hot rolling, and the elongation in temper rolling, are adjusted within particular ranges, mechanical properties that satisfy the particular conditions described above are obtained.

[0015] The present invention is based on such findings and can be summarized as follows.

[0016] [1] A steel sheet for a can lid, containing, in terms of % by mass,

C: 0.020% to 0.060%,

Si: 0.01% to 0.05%,

Mn: 0.20% to 0.60%

P: 0.001% to 0.100%,

S: 0.008% to 0.020%,

N: 0.0130% to 0.0190%, and

[0017] Al: 0.005% to {-4.20.times.N+0.110}% (where N in the formula represents an N content (% by mass) in the steel), wherein, when Mnf=Mn-1.7.times.S (where. Mn and S in the formula respectively represent a Mn content (% by mass) and a S content (% by mass) in the steel), Mnf is 0.30% or more and 0.58% or less, the balance is Fe and unavoidable impurities, and after 210.degree. C..times.10 minutes of an aging treatment, a lower yield strength YP (N/mm.sup.2) and a yield point elongation YPEl (%) satisfy YP.gtoreq.355, YPEl.gtoreq.2, YPEl.gtoreq.(60/(YP-355))+2, and YP.ltoreq.4.09.times.YPEl+476. [2] A method for producing the steel sheet for a can lid described in [1], including

[0018] re-heating a steel slab to a temperature of 1150.degree. C. or higher,

[0019] hot-rolling the re-heated steel slab at a coiling temperature of 680.degree. C. or lower to produce a hot-rolled steel sheet,

[0020] cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet,

[0021] recrystallization-annealing the cold-rolled steel sheet, and

[0022] temper-rolling the resulting steel sheet at an elongation of 3% or less.

Advantageous Effects of Invention

[0023] According to the present invention, a steel sheet for a can lid, with which deformation caused by a pressure difference between inside and outside of the can is suppressed, can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

[0024] FIG. 1 is a diagram showing the appearance of a can constituted by a can body and a can lid.

[0025] FIG. 2 includes (a) a plan view showing the shape of the can lid and (b) a sectional view taken along A-A in (a).

[0026] FIG. 3 is a graph showing the results of evaluating the influence of the lower yield strength YP and the yield point elongation YPEl of the steel sheet for a can lid, on deformation caused by a pressure difference between inside and outside of the can lid.

DESCRIPTION OF EMBODIMENTS

[0027] A steel sheet for a can lid according to the present invention has a composition that contains C: 0.020% to 0.060%, Si: 0.01% to 0.05%, Mn: 0.20% to 0.60%, P: 0.001% to 0.100%, S: 0.008% to 0.020%, N: 0.0130% to 0.0190%, Al: 0.005% to {-4.20.times.N+0.110}% (wherein N in the formula represents a N content (% by mass) in steel), and the balance being Fe and unavoidable impurities, in which when Mnf=Mn-1.7.times.S (where Mn and S in the formula respectively represent a Mn content (% by mass) and a S content (% by mass) in the steel), Mnf is 0.30% or more and 0.58% or less. After 210.degree. C..times.10 minutes of an aging treatment, a lower yield strength YP (N/mm.sup.2) and a yield point elongation YPEl (%) satisfy YP.gtoreq.355, YPEl.gtoreq.2, YPEl.gtoreq.(60/(YP-355))+2, and YP.ltoreq.4.09.times.YPEl+476. The steel sheet for a can lid and a method for producing the steel sheet according to the present invention are described in detail below.

[0028] First, the composition of the steel sheet for a can lid according to the present invention is described. All of the contents are in terms of % by mass.

[0029] C: 0.020% to 0.060%

[0030] The steel sheet according to the present invention is a steel sheet produced through the steps of hot rolling, cold rolling, recrystallization annealing, and temper rolling, and is required to have the mechanical properties described above. In order to satisfy these properties, it is important that the steel sheet according to the present invention contain C as a solid solution strengthening element. The lower limit of the C content is to be 0.020%. At a C content less than 0.020%, the mechanical properties specified in the present invention are not obtained. Preferably, the C content is 0.030% or more. However, at a C content exceeding 0.060%, the steel sheet becomes excessively hard, and thus during forming the lid, the contact face pressure between the steel sheet and a forming die becomes high, thus damaging the organic coating covering the surface of the steel sheet. Thus, the upper limit of the C content is to be 0.060%. Preferably, the C content is 0.050% or less.

[0031] Si: 0.01% to 0.05%

[0032] Silicon (Si) is effective for solid solution strengthening but deteriorates corrosion resistance of the steel sheet if contained in a large quantity. Silicon (Si) is contained in large quantities in iron ore, the raw material for the steel sheet. Thus, the Si content is adjusted by removing Si in the refining process. In the present invention, rather than the contribution of Si to the solid solution strengthening, elimination of the effect of Si of deteriorating corrosion resistance is more preferable. Thus, the Si content is to be 0.05% or less, which is the level at which the effect on corrosion resistance does not appear. Preferably, the Si content is 0.03% or less. From the viewpoint of corrosion resistance, the Si content is preferably minimized. However, since operation to excessively decrease the Si content increases the workload during refining, the lower limit is to be 0.01%.

[0033] Mn: 0.20% to 0.60%

[0034] Manganese (Mn) is an element effective for adjusting the strength of the steel sheet. At a Mn content less than 0.20%, this effect is not obtained. At a Mn content exceeding 0.60%, however, the strength of the steel sheet increases excessively. Thus, the Mn content is to be 0.20% or more and 0.60% or less. The lower limit of the Mn content is preferably 0.25% or more. The upper limit of the Mn content is preferably 0.55% or less.

[0035] P: 0.001% to 0.100%

[0036] Phosphorus (P) is an element that has a high solid solution strengthening ability. However, at a P content exceeding 0.100%, corrosion resistance is significantly deteriorated. Thus, the upper limit of the P content is to be 0.100%. Preferably, the P content is 0.020% or less. However, decreasing the P content to less than 0.001% requires a large dephosphorization cost. Thus, the lower limit of the P content is to be 0.001%.

[0037] S: 0.008% to 0.020%

[0038] Sulfur (S) bonds with Mn in the steel to generate MnS. At a S content exceeding 0.020%, MnS precipitates in grain boundaries at high temperature and causes embrittlement. Thus, the upper limit of the S content is to be 0.020%. However, decreasing the S content to less than 0.008% requires a large desulfurization cost. Thus, the lower limit of the S content is to be 0.008%.

[0039] N: 0.0130% to 0.0190%

[0040] Nitrogen (N) is an element that contributes to solid solution strengthening and ensuring the yield point elongation YPEl described below. In order to obtain these effects, the N content must be 0.0130% or more. At a N content exceeding 0.0190%, the effect of strain age hardening is saturated. Not only N does not act effectively but also N deteriorates the hot-ductility. Thus, the upper limit of the N content is to be 0.0190%. The lower limit of the N content is preferably 0.0135% or more. The upper limit is preferably 0.0175% or less.

[0041] Al: 0.005% to {-4.20.times.N+0.110}% (where N in the formula represents a N content (% by mass) in the steel)

[0042] Aluminum (Al) is an element that acts as a deoxidizer and is needed to increase the cleanliness of the steel sheet. In the present invention, solid solution N is used to obtain the mechanical properties. However, Al bonds with N in the steel to form AlN. Thus, excessive precipitation of AlN must be suppressed and the upper limit of the Al content needs to be limited. At an Al content exceeding {-4.20.times.N content (%)+0.110}%, excessive precipitation of AlN occurs and a problem of shortage of the solid solution N arises. Meanwhile, a steel having an Al content less than 0.005% exhibits deteriorated cleanliness due to insufficient deoxidation. Thus, the lower limit of the Al content is to be 0.005%. In the present invention, Al is acid soluble Al.

[0043] When Mnf=Mn-1.7.times.S (where Mn and S in the formula respectively represent the Mn content (% by mass) and the S content (% by mass) in the steel), Mnf is 0.30% or more and 0.58% or less

[0044] Manganese (Mn) increases the strength of the steel sheet through solid solution strengthening and crystal grain refinement. However, since Mn bonds with S to form MnS, the amount of Mn contributing to solid solution strengthening is assumed to be the difference obtained by subtracting the amount of Mn capable of forming MnS from the Mn content. Considering the atomic weight ratio of Mn and S, the amount of Mn contributing to the solid solution strengthening can be expressed as Mnf=Mn-1.7.times.S. At Mnf exceeding 0.58%, the effect of crystal grain refinement becomes extensive, and excessive hardening results. Thus, Mnf is to be 0.58% or less. Preferably, Mnf is 0.53% or less. However, at Mnf less than 0.30%, softening occurs and the required pressure resistance is not obtained. Thus, Mnf is to be 0.30% or more. Preferably, Mnf is 0.33% or more.

[0045] The balance is Fe and unavoidable impurities.

[0046] The steel sheet according to the present invention preferably has a microstructure free of any pearlite structure. A pearlite structure is a microstructure in which a ferrite phase and a cementite phase precipitate in layers. If a coarse pearlite structure is present, it may serve as a starting point of cracking caused by stress concentration during deformation. When a can lid is attached to a can body through seaming and when such a starting point of cracking exists, the seamed portion may crack. Thus, the steel sheet according to the present invention is preferably free of any pearlite structure. A microstructure free of any pearlite structure can be obtained by adjusting the cold rolling reduction to 80% or more and by adjusting the annealing temperature for recrystallization annealing, which is performed after cold rolling, to a temperature of lower than the Ac.sub.1 transformation point.

[0047] Next, the mechanical properties that the steel sheet according to the present invention desirably has are described. The inventors of the present invention examined the influence of the mechanical properties of the steel sheet on the pressure resistance of the can lid. As a result, it has been found that when the lower yield strength YP and the yield point elongation YPEl are appropriately controlled, a can lid having excellent pressure resistance can be obtained even in the case of a flat lid mostly formed of flat parts.

[0048] Specifically, the steel sheet according to the present invention after 210.degree. C..times.10 minutes of an aging treatment exhibits a lower yield strength YP (N/mm.sup.2) and a yield point elongation YPEl (%) that satisfy YP.gtoreq.355, YPEl.gtoreq.2, YPEl.gtoreq.60/(YP-355)+2, and YP.ltoreq.4.09.times.YPEl+476.

[0049] In general, in order to suppress deformation of a can lid against pressure, it is effective to increase the stiffness of the can lid by increasing the stiffness of the steel sheet itself, for example. In contrast, the present invention has focused on the residual stress in the can lid.

[0050] FIG. 1 is a diagram showing the appearance of a can 10 having a can lid formed by using the steel sheet according to the present invention. As shown in FIG. 1, the can 10 is mainly constituted by a can body 1 and a can lid 2. FIG. 2 includes part (a) showing a plan view showing the shape of the can lid 2, and part (b) showing a section taken along A-A in (a). As shown in FIG. 2, the can lid 2 of a food can, a beverage can, or the like, which is targeted by the present invention, includes an expanding ring near the outer circumference (see reference symbol x in FIG. 2(b)). When the expanding ring is formed, springback is generated, and residual stress can be generated in the can lid 2.

[0051] In order to enhance springback, it is effective to increase the lower yield strength YP of the steel sheet. The can lid 2 of a food can or a beverage can targeted by the present invention has, as shown in FIG. 2, a substantially flat region at the center portion. In order to generate residual stress in this portion, it is effective to improve the yield point elongation YPEl. In other words, discontinuous deformation is induced in the flat portion of the can lid 2 by improving the yield point elongation YPEl, and this causes deformed portions and un-deformed portions to coexist. As a result, residual stress is generated inside the can lid 2.

[0052] In order to induce the residual stress, the lower yield strength YP and the yield point elongation YPEl must be appropriately adjusted. FIG. 3 is a graph showing the results of evaluating the influence of the lower yield strength YP and the yield point elongation YPEl of the steel sheet for a can lid, on deformation caused by pressure difference between inside and outside of the can lid. According to the evaluation shown in FIG. 3, a can lid having a sheet thickness of 0.251 mm to 0.277 mm and a nominal diameter of 603 (diameter is about 6 and 3/16 inches) was formed, and the deformation of the lid caused by pressure difference between inside and outside of the can lid was investigated by using a pressure resistance tester. Specifically, after the formed can lid was seamed onto a can body at an atmospheric pressure, compressed air was injected into the inside of the can to create a pressure difference of 50 kPa between the inside and outside of the can. The height difference between the height of the center portion of the can lid and the height of the apex of the seamed portion under this condition was measured. Samples with a difference of 4 mm or less were rated acceptable and samples with a difference exceeding 4 mm were rated unacceptable. Acceptable and unacceptable samples are indicated by circular marks and cross marks, respectively.

[0053] In order to induce residual stress in the can lid, the lower yield strength YP and the yield point elongation YPEl are preferably large. In the region where YPEl.gtoreq.60/(YP-355)+2, acceptable lid deformation is achieved. However, at an excessively high lower yield strength YP, acceptable lid deformation is no longer achieved. Thus, YP.ltoreq.4.09.times.YPEl+476 is required. The mechanism behind this is not currently clear but it is possible that excessively high lower yield strength YP increases the springback, makes the shape of the expanding ring uneven, and thus makes the shape of the lid unstable, for example.

[0054] The tensile test in the present invention can be performed in accordance with JIS Z 2241 "Metallic materials--Tensile testing" by using a No. 5 specimen specified in JIS Z 2201 "Tension Test Pieces for Metallic Materials".

[0055] The yield point elongation YPEl is the elongation with reference to a gauge length of 50 mm. The tensile direction in the tensile test is set to the rolling direction of the steel sheet. In general, the lower yield strength YP of a steel sheet is lowest in the rolling direction. When a can lid is deformed under pressure, deformation starts from the rolling direction where the lower yield strength YP is lowest. When a pressure resistance behavior of a can lid is considered, the tensile direction is set to the rolling direction of the steel sheet since the lower limit of the pressure resistance strength is given by considering the direction in which the lower yield strength YP is lowest.

[0056] The lower yield strength YP can be adjusted by controlling the composition and the production conditions to be within appropriate ranges. In particular, controlling the Mn content and the S content and controlling the reduction of temper rolling are important. The yield point elongation YPEl can be controlled by controlling the composition and the production conditions to be within the appropriate ranges. In particular, controlling the Al content and the N content and controlling the slab heating temperature and the coiling temperature for hot rolling are important.

[0057] In the present invention, the limitations regarding the lower yield strength YP (N/mm.sup.2) and the yield point elongation YPEl (%) are determined from the experimental results conducted on can lids having a nominal diameter of 603. Since deformation of the can lid against pressure decreases with the decrease in diameter of the can lid, the evaluation reference described above can be applied to can lids having smaller diameters than the can lid having a nominal diameter of 603.

[0058] Next, one example of a method for producing a steel sheet for a can lid according to the present invention is described.

[0059] The steel sheet according to the present invention is produced through the steps of hot rolling, cold rolling, recrystallization annealing, temper rolling, and, if needed, surface treatment. The details are described below.

[0060] First, a steel slab having the composition described above can be obtained by refining and continuous casting. In continuous casting, a slab is preferably produced by a vertical curved continuous caster or a curved continuous caster, and the surface temperature of a corner portion in a region where bending or bending-back deformation occurs in the slab is preferably adjusted to 800.degree. C. or lower or 900.degree. C. or higher. As a result, cracking in corner portions at the long sides and the short sides in a slab horizontal section can be avoided.

[0061] The steel slab is re-heated at 1150.degree. C. or higher. When the slab is re-heated at a temperature of 1150.degree. C. or higher, AlN that has precipitated during the course of slab cooling can be dissolved. In order to prevent excessive oxidation due to heating, the slab heating temperature is preferably 1300.degree. C. or lower. In the present invention, the slab temperature is the surface temperature of the slab.

[0062] Next, the slab is hot-rolled. In this step, the finishing temperature of hot rolling is preferably a temperature equal to or higher than the Ara point. The coiling temperature is to be 680.degree. C. or lower, preferably lower than 680.degree. C., and more preferably 600.degree. C. or lower. When the coiling temperature after finish rolling exceeds 680.degree. C., AlN precipitates and the effect of N expected in the present invention is not obtained. In order to avoid excessive hardening of the steel sheet, the coiling temperature is preferably 540.degree. C. or higher. The coiling temperature is the steel sheet surface temperature.

[0063] After hot rolling, the cooled hot-rolled steel sheet (hot rolled steel strip) is preferably descaled. The descaling method may be any of various methods. For example, chemical descaling such as pickling, physical descaling, and other methods can be adopted. For example, pickling can be conducted by a common procedure such as a sulfuric acid method or a hydrochloric acid method.

[0064] Next, cold rolling is performed. Cold rolling is preferably performed at a reduction of 80% or more. When the cold rolling reduction is 80% or more, the pearlite structure generated after the hot rolling can be destroyed. When the cold rolling reduction is less than 80%, there is a possibility that the pearlite structure would remain. The upper limit of the cold rolling reduction is preferably 95% to avoid the increase in load on the rolling machine due to an excessively high reduction and the resulting occurrence of rolling failure.

[0065] Next, recrystallization annealing is performed after cold rolling. Recrystallization annealing is preferably continuous annealing. In box annealing, solid solution N precipitates as AlN and thus room-temperature strain age hardening may not be achieved. The annealing temperature is preferably lower than the Ac.sub.1 transformation point. When the annealing temperature is equal to or higher than the Ac.sub.1 transformation point, the austenite phase occurs during annealing, and a pearlite structure that can serve as a starting point of cracking during can-lid working is sometimes formed. In the present invention, the Ac.sub.1 transformation point (.degree. C.) can be determined by differential thermal analysis. The annealing temperature is a steel sheet surface temperature.

[0066] After annealing, temper rolling is conducted so that the steel sheet has particular mechanical properties and surface roughness. The larger the elongation during temper rolling, the higher the lower yield strength YP and the lower the yield point elongation YPEl. In order to obtain a balance between the lower yield strength and the yield point elongation according to the present invention, the elongation is to be 3% or less. In order to obtain a particular surface roughness, the elongation is preferably 0.8% or more.

[0067] A steel sheet for a can lid according to the present invention is produced through the above-described process.

[0068] The steel sheet produced as described above is used as a base sheet for making a surface-treated steel sheet. The effects of the present invention are not affected by the type of the surface treatment, and thus any surface treatment may be employed. Representative examples of the surface treatment for cans include coating treatments that use metals, such as tin plating (tin) or chromium plating (tin-free steel), metal oxides, metal hydroxides, inorganic salts, etc., and coating treatments, such as a lamination treatment, that form organic resin films on top of the coating formed by the foregoing coating treatment. In performing the surface treatment, the steel sheet is heat-treated in some cases and ages as a result. Also during storage before the steel sheet is formed into can lids, the steel sheet ages according to the storage temperature and storage period. The steel sheet also ages when lacquering is performed on the steel sheet. However, it has been confirmed that aging in the state of the base sheet rarely influences the effects of the present invention.

Examples

[0069] Examples of the present invention will now be described. First, each of steels having compositions shown in Table 1 was casted into a slab. The slab was heated at a slab heating temperature shown in Tables 2 to 4 and hot-rolled at a coiling temperature shown in Tables 2 to 4. After the hot-rolled sheet was cold rolled, recrystallization annealing was conducted and temper rolling was conducted at an elongation shown in Tables 2 to 4.

[0070] In Table 1, steel K was unused. In Tables 2 to 4, numbers 34 to 37 were unused.

TABLE-US-00001 TABLE 1 Composition (mass %) Mnf Steel C Si Mn P S Al N (=Mn - 1.7 .times. S).sup.(*.sup.1) -4.20 .times. N + 0.110.sup.(*.sup.2) Note A 0.045 0.02 0.24 0.008 0.010 0.035 0.0110 0.22 0.064 Comparative Steel B 0.045 0.02 0.30 0.008 0.010 0.067 0.0028 0.28 0.098 Comparative Steel C 0.040 0.02 0.35 0.009 0.014 0.025 0.0142 0.33 0.050 Compliant Steel D 0.040 0.02 0.18 0.008 0.010 0.038 0.0020 0.16 0.102 Comparative Steel E 0.020 0.02 0.35 0.006 0.010 0.042 0.0150 0.33 0.047 Compliant Steel F 0.071 0.02 0.55 0.010 0.012 0.055 0.0045 0.53 0.091 Comparative Steel G 0.030 0.02 0.40 0.010 0.012 0.02 0.0155 0.38 0.045 Compliant Steel H 0.058 0.01 0.37 0.008 0.015 0.045 0.0050 0.34 0.089 Comparative Steel I 0.038 0.01 0.36 0.010 0.010 0.015 0.0136 0.34 0.053 Compliant Steel J 0.059 0.02 0.35 0.006 0.010 0.049 0.0131 0.33 0.055 Compliant Steel L 0.065 0.01 0.33 0.010 0.010 0.022 0.0160 0.31 0.043 Comparative Steel M 0.018 0.01 0.33 0.010 0.010 0.022 0.0136 0.31 0.053 Comparative Steel N 0.039 0.02 0.55 0.010 0.010 0.030 0.0150 0.53 0.047 Compliant Steel O 0.042 0.02 0.65 0.010 0.010 0.018 0.0147 0.63 0.048 Comparative Steel P 0.035 0.01 0.30 0.010 0.010 0.018 0.0200 0.28 0.026 Comparative Steel Q 0.035 0.01 0.25 0.010 0.010 0.051 0.0150 0.23 0.047 Comparative Steel The balance of the composition: Fe and unavoidable impurities .sup.(*.sup.1)In the formula, Mn and S respectively represent the Mn content (mass %) and the S content (mass %) in the steel. .sup.(*.sup.2)In the formula, N represents the N content (mass %) in the steel.

TABLE-US-00002 TABLE 2 Slab heating Coiling Judgement Judgement temperature temperature Elongation YP YPEI condition condition Pressure No. Steel (.degree. C.) (.degree. C.) (%) (N/mm.sup.2) (%) 1.sup.(*.sup.1) 2.sup.(*.sup.2) resistance Note 1 A 1200 590 1.5 395 2.3 X .largecircle. X Comparative Example 2 B 1200 590 1.5 351 3.8 X .largecircle. X Comparative Example 3 C 1200 590 1.5 403 4.5 .largecircle. .largecircle. .largecircle. Example 4 D 1200 560 1.5 308 0.8 .largecircle. X X Comparative Example 5 E 1230 560 1.0 430 5.9 .largecircle. .largecircle. .largecircle. Example 6 F 1200 600 2.0 500 4.5 .largecircle. X X Comparative Example 7 F 1200 600 2.5 505 5.5 .largecircle. X X Comparative Example 8 F 1200 620 1.5 380 3.8 X .largecircle. X Comparative Example 9 F 1200 620 1.5 430 2.7 X .largecircle. X Comparative Example 10 G 1200 680 1.5 416 4.4 .largecircle. .largecircle. .largecircle. Example 11 G 1200 590 2.9 464 4.4 .largecircle. .largecircle. .largecircle. Example 12 G 1200 560 1.2 409 6.6 .largecircle. .largecircle. .largecircle. Example 13 G 1200 560 2.5 467 5.3 .largecircle. .largecircle. .largecircle. Example 14 H 1200 590 1.2 508 2.0 X X X Comparative Example 15 H 1200 590 3.3 487 1.7 X X X Comparative Example 16 I 1150 600 1.5 472 3.7 .largecircle. .largecircle. .largecircle. Example 17 I 1160 600 1.5 478 5.2 .largecircle. .largecircle. .largecircle. Example 18 I 1170 600 1.5 470 3.9 .largecircle. .largecircle. .largecircle. Example .sup.(*.sup.1)Judgement condition 1: After 210.degree. C. .times. 10 minutes of artificial aging treatment, YP (N/mm.sup.2) .gtoreq. 355, YPEI (%) .gtoreq. 2, and YPEI .gtoreq. (60/(YP - 355)) + 2 .sup.(*.sup.2)Judgement condition 2: After 210.degree. C. .times. 10 minutes of artificial aging treatment, YP .ltoreq. 4.09 .times. YPEI + 476

TABLE-US-00003 TABLE 3 Slab heating Coiling Judgement Judgement temperature temperature Elongation YP YPEI condition condition Pressure No. Steel (.degree. C.) (.degree. C.) (%) (N/mm.sup.2) (%) 1.sup.(*.sup.1) 2.sup.(*.sup.2) resistance Note 19 I 1200 620 1.5 474 5.3 .largecircle. .largecircle. .largecircle. Example 20 I 1200 650 2.0 473 3.9 .largecircle. .largecircle. .largecircle. Example 21 I 1100 560 1.2 390 3.3 X .largecircle. X Comparative Example 22 I 1100 590 2.2 410 2.8 X .largecircle. X Comparative Example 23 I 1230 700 2.5 484 2.3 X .largecircle. X Comparative Example 24 I 1230 720 1.5 440 2.2 X .largecircle. X Comparative Example 25 I 1230 600 2.0 490 4.9 .largecircle. .largecircle. .largecircle. Example 26 I 1230 600 1.5 490 3.9 .largecircle. .largecircle. .largecircle. Example 27 J 1200 650 2.2 466 6.4 .largecircle. .largecircle. .largecircle. Example 28 J 1200 560 1.4 431 3.1 .largecircle. .largecircle. .largecircle. Example 29 J 1200 590 1.5 427 3.3 .largecircle. .largecircle. .largecircle. Example 30 J 1200 560 1.8 446 4.2 .largecircle. .largecircle. .largecircle. Example 31 J 1230 650 2.0 463 4.1 .largecircle. .largecircle. .largecircle. Example 32 J 1230 650 2.0 476 3.5 .largecircle. .largecircle. .largecircle. Example 33 J 1230 650 2.0 472 4.0 .largecircle. .largecircle. .largecircle. Example .sup.(*.sup.1)Judgement condition 1: After 210.degree. C. .times. 10 minutes of artificial aging treatment, YP (N/mm.sup.2) .gtoreq. 355, YPEI (%) .gtoreq. 2, and YPEI .gtoreq. (60/(YP - 355)) + 2 .sup.(*.sup.2)Judgement condition 2: After 210.degree. C. .times. 10 minutes of artificial aging treatment, YP .ltoreq. 4.09 .times. YPEI + 476

TABLE-US-00004 TABLE 4 Slab heating Coiling Judgement Judgement temperature temperature Elongation YP YPEI condition condition Pressure No. Steel (.degree. C.) (.degree. C.) (%) (N/mm.sup.2) (%) 1.sup.(*.sup.1) 2.sup.(*.sup.2) resistance Note 38 L 1200 600 2.9 516 4.1 .largecircle. X X Comparative Example 39 M 1230 590 2.0 371 5.1 X .largecircle. X Comparative Example 40 N 1180 600 1.5 438 3.9 .largecircle. .largecircle. .largecircle. Example 41 N 1180 600 1.5 437 4.2 .largecircle. .largecircle. .largecircle. Example 42 N 1180 640 1.5 441 3.6 .largecircle. .largecircle. .largecircle. Example 43 N 1200 620 1.5 478 2.9 .largecircle. .largecircle. .largecircle. Example 44 N 1200 620 2.0 465 3.0 .largecircle. .largecircle. .largecircle. Example 45 N 1200 620 2.5 485 3.0 .largecircle. .largecircle. .largecircle. Example 46 N 1230 620 1.8 415 5.2 .largecircle. .largecircle. .largecircle. Example 47 N 1230 620 1.8 386 4.9 .largecircle. .largecircle. .largecircle. Example 48 N 1230 620 1.8 436 4.8 .largecircle. .largecircle. .largecircle. Example 49 N 1230 620 1.8 435 4.4 .largecircle. .largecircle. .largecircle. Example 50 O 1180 590 1.5 508 3.6 .largecircle. X X Comparative Example 51 P 1230 560 4.0 506 2.1 X X X Comparative Example 52 P 1230 560 2.0 532 4.2 .largecircle. X X Comparative Example 53 Q 1200 620 1.8 457 2.3 X .largecircle. X Comparative Example .sup.(*.sup.1)Judgement condition 1: After 210.degree. C. .times. 10 minutes of artificial aging treatment, YP (N/mm.sup.2) .gtoreq. 355, YPEI (%) .gtoreq. 2, and YPEI .gtoreq. (60/(YP - 355)) + 2 .sup.(*.sup.2)Judgement condition 2: After 210.degree. C. .times. 10 minutes of artificial aging treatment, YP .ltoreq. 4.09 .times. YPEI + 476

[0071] A 210.degree. C..times.10 minutes artificial aging treatment was then conducted. The lower yield strength (YP) and the yield point elongation (YPEl) of the steel sheets obtained as described above were measured by a tensile test in accordance with JIS Z 2241 "Metallic materials--Tensile testing" using No. 5 specimens prescribed in JIS Z 2201 "Tension Test Pieces for Metallic Materials". Whether the results were acceptable or unacceptable was judged according to the judgment condition 1 and the judgment condition 2. Here, samples that conformed to the judgment condition are indicated by circular marks and samples that did not conform to the condition are indicated by cross marks.

[0072] Each of the obtained steel sheets was formed into a 603 diameter lid and seamed onto a can body. The pressure inside the can was increased to 50 kPa. The height of the can lid center portion with reference to the seamed portion was measured. When the measured height was 4 mm or less, the sample was rated acceptable (circle), and its pressure resistance was evaluated.

[0073] As indicated in Tables 2 to 4, samples that pass the judgment condition 1 and the judgment condition 2 also pass the pressure resistance. According to Examples, a steel sheet for a can lid, with which deformation against pressure can be suppressed, was obtained.

[0074] Judgement Condition 1

[0075] After a 210.degree. C..times.10 minutes artificial aging treatment, the lower yield strength YP (N/mm.sup.2) and the yield point elongation YPEl (%) satisfy YP.gtoreq.355, YPEl.gtoreq.2, and YPEl.gtoreq.(60/(YP-355))+2.

[0076] Judgement Condition 2

[0077] After a 210.degree. C..times.10 minutes artificial aging treatment, the lower yield strength YP (N/mm.sup.2) and the yield point elongation YPEl (%) satisfy YP.ltoreq.4.09.times.YPEl+476.

REFERENCE SIGNS LIST

[0078] 1 can body [0079] 2 can lid [0080] 10 can

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