STEEL AEROSOL MONOBLOC

Abstract

A tin coated steel sheet is provided for manufacturing a drawn can having: a thickness inferior to 0.7 mm, a yield strength inferior to 400 MPa, an average grain aspect ratio below 1.5, a strain hardening coefficient below 1.5, a tin coating from 0.5 to 4.0 g.Math.m.sup.2 on a first face and from 2.8 to 11.2 g.Math.m.sup.2 on a second face, a chemical composition comprising in weight percent 0.002C0.09 and 0.0015B0.005 and a balance of Fe and unavoidable impurities, and the steel sheet having a ferritic microstructure with a mean grain size from 5 to 15 m.

Claims

1-22. (canceled)

23: A tin coated steel sheet for manufacturing a drawn can, the tin coated steel sheet comprising: a steel sheet having a first face and a second face, the steel sheet having a thickness inferior to 0.7 mm, a yield strength inferior to 400 MPa, an average grain aspect ratio below 1.5, a strain hardening coefficient below 1.5, and a chemical composition in weight percent comprising: 0.002C0.09; 0.05Mn0.6; 0.0015B0.005; N0.05; Ni0.2; S0.03;P50.02; Si0.03; Cr0.2; 0.01Al0.08; Cu0.2; Nb0.05; V0.02; Ti0.05, a balance consisting of Fe and unavoidable impurities, the steel sheet having a ferritic microstructure with a mean grain size from 5 to 15 m; and a tin coating from 0.5 to 4.0 g.Math.m2 on a first face and from 2.8 to 11.2 g.Math.m2 on a second face.

24: The tin coated steel sheet as recited in claim 23 wherein the mean grain size is from 7 m to 11 m.

25: The tin coated steel sheet as recited in claim 23 wherein the tin coated steel sheet respects the following ratio: BMASS %/NMASS %>0.6, wherein BMASS % is the weight percent of boron and NMASS % is the weight percent of nitrogen of the steel sheet.

26: The tin coated steel sheet as recited in claim 23 wherein the tin content on the second face is from 4.0 to 11.2 g.Math.m2.

27: A method for manufacturing the tin coated steel sheet as recited in claim 23, the method comprising the following successive steps: casting a steel to obtain a slab, said steel having the chemical composition; reheating the slab at a temperature Treheat from 1100 C. to 1300 C.; hot rolling the slab at a temperature from 1000 C. to 1300 C. to define a hot rolled steel sheet; coiling the hot rolled steel sheet at a coiling temperature of at least 600 C., cold rolling the hot rolled steel sheet until a thickness below 0.7 mm is obtained to define a cold rolled steel sheet; batch annealing the cold rolled sheet with a heating rate from 10 to 50 C. per hour, a soaking temperature from 600 to 700 C. and a soaking time of at least 1 hour to define an annealed sheet; temper rolling the annealed sheet at an elongation rate from 0 to 15% to define a temper rolled sheet, and tinning the temper rolled sheet.

28: The method as recited in claim 27 wherein the hot rolling is at a temperature from 1050 C. to 1150 C.

29: The method as recited in claim 27 wherein the coiling temperature is at least 625 C.

30: The method as recited in claim 27 wherein the cold rolling is at a rate from 80% to 90%.

31: The method as recited in claim 27 wherein the elongation rate is from 0 to 5%.

32: The method as recited in claim 27 wherein the elongation rate is from 3 to 15%.

33: A one-piece aerosol can manufactured by drawing, redrawing, wall-ironing, varnishing, necking and curling a tin coated steel strip as recited in claim 23, wherein the first face is located inside the aerosol can and the second face is located outside the aerosol can.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

[0035] FIG. 1 illustrates the process steps of a drawn can formation from a steel sheet.

DETAILED DESCRIPTION

[0036] The invention relates to a tin coated steel sheet for manufacturing a drawn can having: [0037] a thickness inferior to 0.7 mm, [0038] a yield strength inferior to 400 MPa, [0039] an average grain aspect ratio below 1.5, [0040] a strain hardening coefficient below 1.5, [0041] a tin coating from 0.5 to 4.0 g.Math.m.sup.2 on a first face and from 2.8 to 11.2 g.Math.m.sup.2 on a second face, [0042] a chemical composition of the steel sheet in weight percent comprising: 0.002C0.09; 0.05Mn0.6;0.0015B0.005;N0.05;Ni0.2;S0.03;P0.02;Si0.03; Cr0.2; 0.01Al0.08; Cu0.2; Nb0.05; V0.02; Ti0.05; and a balance consisting of Fe and unavoidable impurities and said steel sheet having a ferritic microstructure with a mean grain size from 5 to 15 m.

[0043] The manufacture of one-piece aerosol leads to necking rates from 40 to 60%. The necking rate is equal to: (D.sub.Booy-D.sub.ApERTuRE)/(D.sub.Booy), wherein D.sub.BODY is the dimeter of the body and D.sub.APERTURE is the diameter of the aperture.

[0044] The steel sheet comprises from 0.002 to 0.090 weight percent of carbon. If the carbon content exceeds 0.090 weight percent, the yield stress after being temper rolled would exceed 400 MPa and the planar anisotropy would be too high leading to a waste of too much metal during the trimming step. Preferably, the carbon content is of at least 0.02.

[0045] The manganese content of the steel strip is from 0.05 to 0.6 weight percent. If the manganese content is above 0.6 weight percent, the steels becomes too hard and negatively affect the formability but if its content is below 0.05 weight percent, surface cracks might form.

[0046] The nitrogen content of the steel strip is below 0.05 weight percent. If the nitrogen content exceeds 0.05 weight percent, a too high boron content would be necessary to avoid having nitrogen in solid solution after hot rolling.

[0047] The sulphur content of the steel strip is below 0.03 weight percent. If the S content is above, cracks can appear during the curling operation due to a lower local ductility.

[0048] The silicon content of the steel strip is below 0.03 weight percent. If it is higher, the renders the hot rolling difficult and the steel becomes too hard.

[0049] The aluminium content of the steel strip is from 0.01 weight percent. If the aluminium content exceeds 0.08 weight percent, the risk of having inclusions of alumina in the steel becomes too high.

[0050] The boron content of the steel strip is from 0.0015 to 0.005 weight percent. Such a boron content has a positive impact on the homogeneity and the strain hardening coefficient. This boron content permits a decrease of the impact of the coiling temperature on carbide size and an increase of the homogeneous precipitation of boron nitride in the austenite which favour the formation of equiaxed grains after annealing. Furthermore, it lowers the strain hardening coefficient which improves the formability.

[0051] The microstructure is 100% ferritic. However, it can comprise precipitates of cementite. It does not comprise martensite, nor bainite, nor austenite.

[0052] After being temper rolled at an elongation up to 15%, the yield strength is inferior to 400 MPa, and preferably ranges from 180 to 400 MPa. Preferably, the temper rolling is performed at an elongation rate from 3% to 15%. Such a low value of the yield strength of the steel sheet, permits to generally have a yield strength value inferior to 600 MPa of the top wall of the aerosol being formed after an ironing of 50% and before the necking and curling steps. Such a yield strength permits to reduce the shear-compression stress applied by the tools during the necking and curling steps and thus reduce the wrinkling risk. Consequently, the varnish is preserved, especially in the neck area, and forming issues and defects can be prevented. Such a range also increases the reproducibility of the stresses acting on the coating during the forming and thus of the forming process.

[0053] The average grain aspect ratio is below 1.5. It permits to have homogeneous mechanical properties which increases reproducibility of the forming process.

[0054] The mean grain size is from 5 to 15 m. This mean grain size can be considered as a small one. This range permits to lower the deformation roughness which is particularly key in the neck area wherein the deformation is the greatest. It permits preservation of the varnish, especially in the neck area. Preferably, said mean grain size is from 7 to 11 m. Such a range permits one to increase even more the homogeneity of the mechanical properties and to preserve even better the varnish during the forming steps.

[0055] Preferably, said tin coated steel sheet respects the following ratio: B.sub.MASS%/N.sub.MASS%>0.6, wherein B.sub.MASS is the weight percent of boron and N.sub.MASS is the weight percent of nitrogen of the steel sheet. Such a content permits to reduce the quantity of nitrogen in solid solution. Even more preferably, said tin coated steel sheet respects the following ratio: B.sub.MASS%/N.sub.MASS%>0.8.

[0056] The tin coating is not necessarily the same on both faces of the steel strip. The tin coating is from 0.5 to 4.0 g.Math.m.sup.2 on a first face and from 2.8 to 11.2 g.Math.m.sup.2 on a second face. The tin coating is preferably not reflown. This first face is intended to be used as the interior face of a drawn can because such a tin content improves the varnish adherence. On the contrary, the second face is intended to be used as an exterior face of a drawn can because such a tin content increases the shine. Preferably, said tin content on said second face is from 4.0 to 11.2 g.Math.m.sup.2. The tin coating can be done by electroplating where electrodes are used to attract tin ions onto the steel strip. Usually, each strip side is faced by at least an electrode. The greater the intensity of the electrode, the greater will be the tin content. So, in order to have a difference of tin content between the two strip faces, each strip side should be faced by electrodes having different intensity.

[0057] Thanks to the features of the claimed steel, it has been observed that the claimed steel is able to preserve the varnish and ease the forming of one-piece aerosol in the neck and curl area. Moreover, thanks to the homogeneous mechanical properties within the coil and from one coil to another, the reproducibility of the forming process of one-piece aerosol is improved.

[0058] The invention also relates to a method for manufacturing the tin coated steel sheet, said method comprising the following successive steps: [0059] casting a steel to obtain a slab, said steel having the above chemical composition, [0060] reheating the slab at a temperature T.sub.reheat comprised from 1100 C. to 1300 C., [0061] hot rolling said slab at a temperature from 1000 C. to 1300 C. [0062] coiling the resulting hot rolled steel sheet at a coiling temperature of at least 600 C., [0063] cold rolling said sheet until a thickness below 0.7 mm is obtained, [0064] batch annealing said cold rolled sheet with a heating rate from 10 to 50 C. per hour, a soaking temperature from 600 to 700 C. and a soaking time of at least 1 hour, [0065] temper rolling said annealed sheet at an elongation rate from 0 to 15%, [0066] tinning.

[0067] The steel strip temperature at the end of the hot rolling is above the Ar3 temperature of the steel sheet. Preferably, said steel strip is hot rolled from 1050 C. to 1150 C.

[0068] The coiling temperature of at least 600 C. and the claimed boron content favours the formation of a steel sheet having a mean grain size from 5 to 15 m, e.g. the formation of equiaxed grains, and prevents the precipitation of AlN, before the steel recrystallisation, during the annealing. Indeed, the claimed boron content leads to a homogeneous precipitation of boron nitride in the austenite during the hot rolling while the claimed coiling temperature lead to the precipitation of the residual nitrogen, forming AlN. Avoiding nitrogen in solid solution with the precipitation of boron nitride is preferred than with the precipitation of aluminium nitride because the first precipitates homogeneously while the latter precipitates heterogeneously. Preferably, said steel strip is coiled at a coiling temperature of at least 625 C. Such a coiling temperature permits to ensure that the non-precipitated nitrogen will form AlN during the coiling.

[0069] This synergetic effect, of the boron content and of the coiling temperature, is improved with the claimed boron/nitrogen ratio.

[0070] Preferably, the soaking time is from 20 to 50 hours. Even more preferably, the soaking time is from 30 to 50 hours.

[0071] Preferably, said steel strip is cold at a reduction rate from 80% to 90%. Such a reduction rate favours a good planar anisotropy r.

[0072] Preferably, said steel strip is tempered rolled at an elongation rate from 3 to 15%.

[0073] Preferably, said steel strip is tempered rolled at an elongation rate from 0 to 5%. Such a range reduces the hardening of the steel strip and permits to keep a low yield strength.

[0074] Performing the annealing step with a batch annealing permits to manufacture a steel sheet being non-ageing. The batch annealing, contrary to the continuous annealing, lowers the presence of carbon in solid solution. Apparently, the carbon in solid solution precipitates to form cementite.

[0075] Performing the annealing step with a batch annealing in combination with the claimed boron content permits manufacture of a steel sheet having a yield strength lower than 400 MPa. This is due to a slight increase of the grain size and to the precipitation of carbide in the matrix instead of the grain boundaries before the temper mill.

[0076] Performing the annealing step with a batch annealing in combination with an excess of boron compared to the nitrogen content, permits to suppress or a least lower the presence of nitrogen in solid solution. The following equation permits a determination if the boron is in excess compared to the nitrogen: B*=B(11/14) N, wherein B* is the quantity of boron in excess (also called free boron), B is the quantity of boron and N the quantity of nitrogen. It is due to the formation of boron nitride.

[0077] Consequently, contrary to what is recommended in the state of the art, as in EP 2 098 312 B1 for example, the annealing is done by a batch annealing in the present invention. Indeed, it is believed in the state of the art that the batch annealing, having a low heating rate, is harmful to the varnish preservation and for the homogeneity of the mechanical properties for two reasons because it favours the formation of elongated grains and thus the inhomogeneity of the steel sheet mechanical properties and microstructure and because the nitrogen in solution tends to precipitate and to form AlN before the steel recrystallisation.

[0078] The present application also relates to a one-piece aerosol manufactured by drawing, redrawing, wall-ironing, varnishing, necking and curling the tin coated steel strip, wherein said first face is located inside the aerosol and the second face is located outside the aerosol.

[0079] Experimental Results

[0080] In order to assess the impact of the boron content on the steel sheet properties, two samples (S1 and S2) have been produced. Si is an embodiment of the claimed steel strip while S2 differs from the claimed steel strip because its boron content is outside the claimed range, their compositions are described in the Table 1. During their manufacturing, both samples have been reheated at a temperature of 1100 C., hot-rolled at a temperature above Ar3 and coiled at a temperature of 640 C. Then they have been cold rolled with a thickness reduction ratio of 85%. Then the cold rolled steel strips underwent a batch annealing, wherein they have been heated, with a heating rate of 35 C.h.sup.1 to 600 C., then they have been maintained at a temperature from 600 C. to 650 C. for 35 hours and finally cooled with a cooling rate of 15 C.h.sup.1. Then they have been temper rolled with an elongation rate of 3% and tinned.

[0081] S1 has a thickness of 0.457 mm and S2 has a thickness of 0.459 mm. Si has a tin coating of 3.01 g.Math.m.sup.2 on a first face and of 4.62 g.Math.m.sup.2 on a second face while S2 has a tin coating of 2.95 g.Math.m.sup.2 on a first face and of 4.42 g.Math.m.sup.2 on a second face. S1 and S2 have a 100 percent ferritic microstructure.

TABLE-US-00001 TABLE 1 Element (w %) C N Al Mn B P Cr Ni Cu Ti Si S1 0.035 0.0035 0.02 0.185 0.0022 0.009 0.018 0.017 0.022 0.001 0.007 S2 0.029 0.006 0.058 0.22 0.0003 0.01 0.039 0.019 0.021 0.0015 0.006

[0082] Then, the yield strength, measured according to the norm ISO6892-1:2016, and the strain hardening coefficient (r) have been measured using a sample according to ISO 2080. The average grain size measured according to the norm ASTM E112-10, and the average aspect ratio of the grains, which is the grain length divided by the grain height, have also been measured. The results are summed up in Table 2.

TABLE-US-00002 TABLE 2 yield strength (MPa) r grain size (m) Aspect ratio S1 205 1.1 8.0 1.3 S2 226 1.7 16.1 2.0

[0083] It can clearly be seen that the anisotropy ratio is much lower (around 4 times lower) for Si than for S2 and that the grain size for Si is half the one for S2. Such properties indicate that the steel sheet according to the present invention is much more homogeneous than one not comprising the claimed boron content. Moreover, the strain hardening coefficient is much lower for Si than for S2 which improves the formability of the steel sheet. Furthermore, the grain aspect ratio is much closer to 1 for S1 than for S2 which indicates that even if the annealing is done in a batch annealing, the claimed boron content permits favoring of the formation of equiaxed grains.