Patent Application
20160362761
The present disclosure relates to a steel sheet for crown caps that is excellent in shape uniformity during forming of crown caps used for beer bottles etc., to a method for manufacturing the steel sheet, and to crown caps.
From the viewpoint of environmental load reduction and cost reduction in recent years, steel sheets for crown caps used for beer bottle caps etc. are being reduced in thickness. Generally, there are the following two types of steel sheets reduced in thickness. One is SR (Single Reduced) steel subjected to hot rolling, cold rolling, annealing, and then temper rolling, and the other is DR (Double Reduced) steel subjected to secondary cold rolling. As for steel sheets for crown caps, the demand for steel sheets having a thickness of 0.20 mm or less is increasing, and DR steel subjected to secondary cold rolling is preferred because work hardening can be utilized in order to compensate for a reduction in pressure capacity due to a reduction in thickness. However, the DR steel is generally harder than the SR steel, and a problem with the DR steel is its low formability.
When a crown cap is formed, a central portion is drawn to some extent at the initial stage of forming, and then an outer edge portion is formed into a fluted shape. The use of a steel sheet with low formability may cause a defective shape, i.e., non-uniform flute shapes. A crown cap with non-uniform flute shapes has a problem in that, after the crown cap is put on a bottle, pressure capacity is not achieved and leakage of the content occurs, so that the crown cap does not server as a cap. If the strength of the steel sheet of a crown cap is low, there is a risk that the crown cap comes off even when the shapes of the flutes are uniform.
One well-known steel sheet having good formability is ultra-low carbon IF (Interstitial Free) steel. For the purpose of achieving an improvement in formability and a reduction in thickness simultaneously, many studies have been conducted on DR steel that uses the ultra-low carbon steel (for example, Patent Literature 1 to Patent Literature 3).
PTL 1: Japanese Unexamined Patent Application Publication No. 7-11333
PTL 2: Japanese Unexamined Patent Application Publication No. 5-287445
PTL 3: Japanese Unexamined Patent Application Publication No. 2010-255021
However, even when the above conventional techniques are applied to crown caps, the crown caps have a problem in that the performance of the crown caps cannot be ensured.
It is an object of the present disclosure to provide a steel sheet for crown caps that is excellent in formability, a method for manufacturing the steel sheet, and a crown cap.
To solve the foregoing problem, the present inventors have conducted extensive studies. The present inventors have studied the chemical composition, hot rolling conditions, cold rolling conditions (primary and secondary), and continuous annealing conditions of ultra-low carbon steel used as a base material. The present inventors have found that, by increasing an average r value and controlling a YP to a suitable value, the rate of occurrence of defective shapes can be reduced and pressure capacity can be ensured.
Exemplary embodiments may include as follows.
[1] A steel sheet for crown caps, the steel sheet comprising, in mass %, C: 0.0005 to 0.0050%, Si: 0.020% or less, Mn: 0.10 to 0.60%, P: 0.020% or less, S: 0.020% or less, Al: 0.01 to 0.10% or less, N: 0.0050% or less, and Nb: 0.010 to 0.050%, the balance being Fe and inevitable impurities, the steel sheet having an average r value of 1.30 or more and a YP of 450 MPa or more and 650 MPa or less.
[2] The steel sheet for crown caps according to [1] above, wherein an elongation rate of ferrite is 4.2 or less.
[3] A method for manufacturing a steel sheet for crown caps, the method comprising: hot rolling a steel slab having a chemical composition according to [1] at a slab reheating temperature of 1,150° C. or higher and a finishing temperature of 870° C. or higher; coiling the hot rolled steel sheet at a coiling temperature of 600° C. or higher; performing pickling; then performing primary cold rolling; performing annealing at an annealing temperature of recrystallization temperature or more and 790° C. or less; and then performing secondary cold rolling at a rolling reduction of 10% or more and 50% or less.
[4] A crown cap obtained by forming the steel sheet for crown caps according to [1] or [2].
In the present disclosure, “%” indicating the percentage of each compositional component is percent by mass.
A steel sheet for crown caps that has an average r value or 1.30 or more and a YP of 450 MPa or more and 650 MPa or less and excellent in formability is obtained. The use of the steel sheet for crown caps of the disclosed embodiments can improve the shape uniformity of crown caps used for beer bottles etc. and allows sufficient pressure capacity to be achieved.
First, a description will be given of compositional components.
C is an element that increases the strength of the steel but decreases its formability. If the amount of solute C in the steel is large, yield elongation increases, and the large amount of solute C is likely to cause age hardening and stretcher strain during forming. Therefore, when a continuous annealing method is used, it is necessary to control and reduce the content of C as much as possible at the stage of steelmaking. If the amount of remaining solute C is large, the steel sheet becomes hard, and wrinkles easily occur at the initial stage of the forming of a crown cap, causing an increase in the rate of occurrence of defective shapes. C is also an element having an influence on recrystallization texture. As the amount of C decreases, accumulation in a crystal orientation group in which a <111> direction is parallel to the normal to the sheet surface increases in the texture of the annealed sheet, and the average r value increases. The increase in the average r value improves drawability, and the defective shape of the crown cap is improved. Therefore, the content of C is 0.0050% or less. To further improve the uniformity of the shape, the content of C is preferably 0.0035% or less and more preferably 0.0023% or less. However, since excessive decarburization leads to an increase in cost of steelmaking, the lower limit of the content of C is 0.0005%.
If a large amount of Si is added, the surface treatability of the steel sheet deteriorates, and the corrosion resistance of the steel sheet is reduced. Therefore, the amount of Si is 0.020% or less.
Mn is added for the purpose of preventing thermal embrittlement. Mn also has the effect of preventing a reduction in hot ductility caused by S contained in the steel. It is necessary that the amount of Mn added be 0.10% or more in order to obtain these effects. In a ladle analysis value specified in JIS G 3303 or a ladle analysis value specified by the American Society for Testing Materials (ASTM A623M-11), the upper limit of Mn in tin mill black plates used for general food containers is specified to be 0.60% or less. Therefore, the upper limit of Mn in the disclosed embodiments is 0.60% or less. From the viewpoint of formability, Mn is preferably 0.45% or less.
If a large amount of P is added, the steel becomes hard, and its formability deteriorates. In addition, the large amount of P causes a reduction in corrosion resistance. Therefore, the upper limit of P is 0.020%.
S is bonded to Fe in the steel to thereby form FeS, and this causes a reduction in the hot ductility of the steel. To prevent this reduction, S is 0.020% or less. If the amount of S is excessively small, the risk of the occurrence of pitting corrosion increases. Therefore, S is preferably 0.008% or more.
Al is an element added as a deoxidizer. In addition, since Al and N form AlN, Al has the effect of reducing the amount of solute N in the steel. If the content of Al is less than 0.01%, the deoxidation effect and the effect of reducing the amount of solute N are not sufficient. A content of Al exceeding 0.10% is not preferable because not only these effects are saturated but also the amount of inclusions such as alumina increases. Therefore, the content of Al is within the range of 0.01% or more and 0.10% or less.
If the amount of N is large, the steel becomes hard due to strain age hardening, and the formability deteriorates. In addition, it is necessary to increase the amount of an element added in order to fix solute N, and this leads to an increase in cost. Therefore, the upper limit of N is 0.0050% or less. It is difficult to stably reduce the amount of N to less than 0.0010%, and this causes the cost of manufacturing to increase. Therefore, N is preferably 0.0010% or more.
Nb is an element that fixes solute C in the steel sheet as NbC to thereby reduce the amount of the solute C, whereby the average r value can be increased. By increasing the average r value, drawability is improved, and this is effective to suppress a defective shape. If the amount of Nb is small, the effect of increasing the average r value is reduced. Therefore, the lower limit is set to 0.010%. If the amount of Nb added is large, recrystallization temperature becomes high, so that non-recrystallized crystals may be present after annealing. This may cause variations in material properties. Therefore, the amount of Nb is 0.050% or less.
The balance is Fe and inevitable impurities.
In addition, Cu, Ni, Cr, and Mo may be contained so long as the effects of the present disclosure are not impaired.
According to ASTM A623M-11, Cu is 0.2% or less, Ni is 0.15% or less, Cr is 0.10% or less, and Mo is 0.05% or less. Other elements are 0.02% or less.
Moreover, Sn may be contained so long as the effects of the present disclosure are not impaired.
[Sn: Less than 0.0050%]
If the amount of Sn is large, the average r value becomes low. Therefore, the amount of Sn is preferably less than 0.0050%.
The structure of the steel sheet for crown caps of the disclosed embodiments is a recrystallized structure. This is because, if non-recrystallized crystals are present after annealing, the material properties become non-uniform, and this causes variations in mechanical properties. However, when the area ratio of the non-recrystallized crystals is 5% or less, the non-recrystallized crystals have almost no influence on the variations in material properties, and therefore the area ratio of 5% or less is allowable. The recrystallized structure is preferably a ferrite phase, and the amount of phases other than the ferrite phase is preferably less than 1.0%. From the viewpoint of suppressing anisotropy during secondary cold rolling, the elongation rate of the ferrite is preferably 4.2 or less. If the elongation rate of the ferrite grains in the steel sheet exceeds 4.2, it may be difficult to obtain uniformly-shaped flutes in a circumferential direction. The elongation rate of the ferrite can be set to 4.2 or less by setting the rolling reduction in the secondary cold rolling to 50% or less. The elongation rate of the ferrite can be measured by a method described in an Example described later.
Next, a description will be given of an example of a method for manufacturing the steel sheet for crown caps that is excellent in formability.
A slab having the above-described composition is subjected to hot rolling at a slab reheating temperature of 1,150° C. or higher and a finishing temperature of 870° C. or higher, coiled at a coiling temperature of 600° C. or higher, pickled, then subjected to primary cold rolling, annealed at an annealing temperature of the recrystallization temperature or more and 790° C. or less, and then subjected to secondary cold rolling at a rolling reduction of 10% or more and 50% or less, whereby a steel sheet for crown caps having excellent formability can be obtained.
If the slab reheating temperature before the hot rolling is excessively low, it is difficult to ensure final finishing rolling temperature. Therefore, the slab reheating temperature is 1,150° C. or higher. If the heating temperature is excessively high, problems such as defects on the surface of the product and an increase in energy cost occur. Therefore, the heating temperature is preferably 1,300° C. or lower.
If the hot rolling finishing temperature is excessively low, α grains (ferrite grains) in a surface layer of the steel sheet become coarse, and this causes variations in material properties. Therefore, the hot rolling finishing temperature is 870° C. or higher. If the hot rolling finishing temperature is excessively high, hot rolling scales are increased in thickness, and pickling properties deteriorate. Therefore, the hot rolling finishing temperature is preferably 910° C. or lower. In the disclosed embodiments, since the amount of a solute element is reduced by Nb added to form IF steel, it is unnecessary to perform treatment for precipitating carbide etc. before finishing rolling. Therefore, ordinary finishing rolling can be used.
[Coiling Temperature after Hot Rolling: 600° C. or Higher]
If the coiling temperature after the hot rolling is excessively low, a defective hot-rolled shape is formed. Therefore, the coiling temperature after the hot rolling is 600° C. or higher. In consideration of uniformity of the steel sheet, the coiling temperature is preferably higher than 700° C. However, if the coiling temperature is excessively high, the hot rolling scales are increased in thickness, and pickling properties deteriorate. Therefore, the coiling temperature after the hot rolling is preferably 730° C. or lower.
The conditions of the pickling are not particularly specified, so long as the scales in the surface layer can be removed. The pickling may be performed by any commonly used method. The picking has been described as an example of the method for removing scales. However, any method other than the pickling may be used so long as the scales can be removed. For example, the scales may be removed mechanically.
If the rolling reduction in the primary cold rolling is excessively high, an excessively large load is applied to reduction rolls during the rolling, and this causes a large burden on the facility. If the rolling reduction is excessively low, it is necessary to reduce the thickness of the produced hot-rolled steel sheet accordingly. In this case, it is difficult to control the material properties. Therefore, the rolling reduction in the primary cold rolling is preferably 86 to 89%.
From the viewpoint of uniformity in material properties and productivity, the annealing method is preferably a continuous annealing method. The annealing temperature during continuous annealing must be the recrystallization temperature or more. However, if the annealing temperature is excessively high, the crystal grains become coarse, and the strength of the steel sheet is reduced. In this case, the YP may be outside the range defined in the disclosed embodiments. In thin sheets, the risk of fracture in a furnace and buckling increases. Therefore, the annealing temperature is 790° C. or lower. The soaking time during the annealing is preferably 10 seconds or more and 90 seconds or less from the viewpoint of productivity.
After the annealing, secondary cold rolling is performed in order to reduce the thickness of the steel sheet and increase the strength of the steel sheet. The secondary cold rolling is a particularly important manufacturing condition in the disclosed embodiments. If the rolling reduction exceeds 50%, the steel sheet is hardened excessively, and the formability is reduced. In addition, the average r value is reduced, and a Δr value increases. Therefore, the rolling reduction in the secondary cold rolling is 50% or less. To ensure pressure capacity, the secondary rolling is performed at a rolling reduction of 10% or more. To further ensure the pressure capacity, the rolling reduction is preferably more than 30%.
Preferably, the cold-rolled steel sheet obtained in the manner described above is subjected to the following surface treatment before it is formed into a crown cap. The steel sheet subjected to the following surface treatment is also the steel sheet for crown caps of the disclosed embodiments.
The surface of the steel sheet subjected to the secondary cold rolling described above may be subjected to various types of surface treatment. Examples of the surface treatment include general plating methods such as electroplating. Such a method is used to form at least one of tin plating, chromium plating, and nickel plating.
The thickness of the plating etc. formed by the surface treatment is sufficiently smaller than the thickness of the steel sheet. Therefore, the influence of the plating on the mechanical properties of the steel sheet for crown caps is negligible.
Next, the material properties of the steel sheet for crown caps of the disclosed embodiments will be described.
The defective shape of a crown cap is caused by the occurrence of wrinkles during drawing at the initial stage of the forming of the crown cap. To avoid the occurrence of wrinkles, it is necessary to increase drawability, i.e., to achieve a high average r value. If the average r value is small, the drawability is low, and wrinkles are formed at the initial stage of the forming of a crown cap, causing a defective shape. Therefore, the average r value is 1.30 or more. To improve the drawability at the initial stage of the forming, the average r value is preferably 1.40 or more. The practical upper limit of the average r value is 2.00.
[|Δr|≦0.5 (Preferred Condition)]
To form flutes uniformly in the circumferential direction when a crown cap is formed, it is preferable that |Δr|≦0.5 holds. It is more preferable that |Δr|≦0.4 holds, and it is still more preferable that |Δr|≦0.3 holds. The Δr (in-plane anisotropy) can be measured using a natural oscillation method specified in JIS Z 2254 Appendix JA. Specifically, the resonance frequency of a steel sheet is measured in directions 0°, 45°, and 95° with respect to the rolling direction to compute the anisotropy ΔE of the Young's modulus, and an experimental formula representing the correlation between the Δr and ΔE is used to compute the value of the Δr.
The pressure capacity of a container is proportional to the YP of a lid material. If the strength of the steel sheet is insufficient, a sufficient pressure capacity is not obtained. Therefore, the lower limit of the YP is 450 MPa. If the YP is excessively high, circumferential compressive stress in the flute portions of the crown cap becomes high and exceeds the critical buckling stress at the initial stage of the forming of the crown cap, so that wrinkles are easily formed. To prevent such a defective shape, the upper limit is 650 MPa. A tensile test is performed according to JIS Z 2241 using a JIS No. 5 tensile test piece. The rolling direction (L direction) is used as the tensile direction.
The pressure capacity of a container is proportional to the square of the thickness of a lid material. If the thickness is excessively small, the pressure capacity becomes low, and the lid cannot play its role. Therefore, the thickness is preferably 0.13 mm or more and more preferably 0.16 mm or more. From the viewpoint of a reduction in thickness of the steel sheet for crown caps for the purpose of resource conservation, a reduction in environmental load, and a reduction in material cost, it is preferable that the thickness of the steel sheet is smaller than 0.22 mm, which is the thickness of the existing steel sheets for crown caps. To achieve the above effects, the thickness is preferably 0.18 mm or less.
The steel sheet for crown caps is excellent in formability is obtained in the manner described above.
By forming the steel sheet for crown caps of the disclosed embodiments, a crown cap excellent in shape uniformity and having sufficient pressure capacity is obtained. The crown cap is a lid material used for, for example, a drink bottle and has flute-like protrusions on a side surface of the crown cap (the number of flutes is generally 21), and the content of the bottle is sealed by crimping the flute-like grooves onto the tap of the bottle. A packing is provided on the inner surface of the crown cap in order to increase the sealing performance. The material used for the packing is a cork sheet, PVC (polyvinyl chloride), PE (polyethylene), etc.
Molten steel containing compositional components shown in Table 1 with the balance being Fe and inevitable impurities was produced, and then a steel slab was obtained. The amount of Sn was confirmed to be less than 0.0050% at every level. Each of the steel slabs obtained was reheated at a temperature shown in Table 2 and then hot rolled at a finishing rolling temperature and a coiling temperature shown in Table 2. The resultant slab was pickled and then subjected to primary cold rolling at a rolling reduction shown in Table 2. The obtained thin-steel sheet was annealed in a continuous annealing furnace at an annealing temperature (recrystallization temperature) shown in Table 2 and then subjected to secondary cold rolling at a rolling reduction shown in Table 2 to thereby manufacture a thin-steel sheet having a final finishing thickness shown in Table 2.
The structure of each of the steel sheets obtained by the above-described manufacturing method was observed.
The structure observation was performed according to “JIS G 0551.” Specifically, nital was used to allow ferrite grains to appear, and a photograph was taken at 400× using an optical microscope. The presence or absence of non-crystallized crystals was checked visually under the optical microscope, and grains not undergoing recrystallization were judged to be non-crystallized crystals. The photograph taken using the optical microscope was subjected to image processing to distinguish non-crystallized portions and recrystallization-completed portions from each other, and the area ratio of the non-crystallized grains was computed. An “AA” rating was assigned when the area ratio of the non-crystallized crystals was 0%. An “A” rating was assigned when the area ratio of the non-crystallized crystals was more than 0% and 5% or less. A “C” rating was assigned when the area ratio of the non-crystallized crystals was more than 5%. The elongation rate of the ferrite grains was computed using a method shown in “JIS G 0202.”
Each of the steel sheets obtained by the above-described manufacturing method was subjected to surface treatment, i.e., chromium (tin-free) plating, and coated with paint (baking conditions: heat treatment at 210° C. for 20 minutes), and pressed into the shape of a crown cap. Mechanical properties and formability were examined under the following test conditions.
The natural oscillation method specified in JIS Z 2254 Appendix JA was used for the average r value (average plastic strain ratio). Specifically, the resonance frequencies of the steel sheet in directions 0°, 45°, and 95° with respect to the rolling direction and the average Young's modulus were determined, and the average r value was computed. The natural oscillation method specified in JIS Z 2254 Appendix JA was used for the Δr (in-plane anisotropy). Specifically, the resonance frequencies of the steel sheet in directions 0°, 45°, and 95° with respect to the rolling direction were measured to compute the anisotropy ΔE of the Young's modulus, and an experimental formula representing the correlation between the Δr and ΔE was used to compute the value of Δr.
A tensile test for YP measurement was performed according to JIS Z 2241 using a JIS No. 5 tensile test piece. The rolling direction (L direction) was used as the tensile direction.
Crown caps were formed, and the uniformity of the flute shape of each of the crown caps was evaluated. A crown cap ruptured during forming was evaluated as fail (“F” in Table 3). For each of crown caps formed without any rupture, the lengths (L in
As for the pressure resistance (pressure capacity) of a crown cap, a pressure test was performed according to JIS S 9017 after the crown cap was put on a bottle. A crown cap with a pressure capacity of 115 PSI or more was evaluated as pass (“P” in Table 3), and a crown cap with a pressure capacity of less than 115 PSI was evaluated as fail (“F” in Table 3).
The results are shown in Table 3. Crown caps with poor shape uniformity could not be put on bottles, so the pressure test was not performed.
As can be seen from Table 3, in Inventive Examples, the average r value is 1.30 or more, and the YP is 450 MPa or more and 650 MPa or less. Non-recrystallized crystals that may cause variations in material properties are not present, and shape uniformity and pressure capacity are high.
However, in Comparative Examples, at least one of the shape uniformity and the pressure capacity is poor, or non-recrystallized crystals that may cause variations in material properties are present at an area ratio of more than 5%.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-033851 | Feb 2014 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2015/000684 | 2/13/2015 | WO | 00 |
