Method for producing a ring-pull top from a steel sheet provided with a protective layer and a ring-pull top produced thereby

Abstract

The invention relates to the use of a steel sheet provided with a protective layer for producing a ring-pull top or a can having a ring-pull top, where the steel sheet is made of an unalloyed or low-alloy steel having a carbon content of less than 0.1% by weight, and also an associated process. The problem proceeding from known steel sheets having a protective layer, namely to provide a steel sheet by means of which ring-pull tops which for a constant residual wall thickness of the notch line have a lower tear-off force can be produced by means of the ring-pull top, is solved by the steel sheet being recrystallizingly heat treated at a heating rate of more than 75 K/s and after the recrystallizing heat treatment cooled at a cooling rate of at least 100 K/s and then coated with the protective layer.

Claims

1. A method for producing a ring-pull top comprising: providing an uncoated, cold-rolled steel sheet made of an unalloyed or low-alloy steel having a carbon content of less than 0.1 wt %, a manganese content of less than 0.4 wt %, a silicon content of less than 0.04 wt %, an aluminum content of less than 0.1 wt %, and a chromium content of less than 0.1 wt %; heating the uncoated, cold-rolled steel sheet using electromagnetic induction at a heating rate of more than 75 K/s for recrystallization annealing; cooling the heated steel sheet at a cooling rate of at least 100 K/s; coating the cooled steel sheet with a protective layer; and stamping a top out of the steel sheet and scoring the top to produce the ring-pull top.

2. The method according to claim 1, further comprising, resulting from the recrystallization annealing and cooling, forming a multi-phase structure in the cooled steel sheet, the multi-phase structure including ferrite and at least one structural constituent selected from the group consisting of martensite, bainite, residual austenite, and combinations thereof.

3. The method according to claim 1, wherein the cooling includes cooling the heated steel sheet at a cooling rate higher than 500 K/s.

4. The method according to claim 1, wherein coating includes coating the cooled steel sheet with a protective layer of tin, chromium, aluminum, zinc, or zinc/nickel.

5. The method according to claim 1, where the providing an uncoated, cold-rolled steel sheet includes providing a low-alloy steel sheet having upper limits for weight proportions of alloying constituents as follows: N: 0.02%; Mn: 0.4%; Si 0.04%; Al: 0.1%; Cr: 0.1%; P: 0.03%; Cu: 0.1%; Ni: 0.1%; Sn: 0.04%; Mo: 0.04%; V: 0.04%; Ti: 0.05%; Nb: 0.05%; B: 0.005%; and other alloying constituents: 0.05%.

6. The method according to claim 5, wherein the upper limit for weight proportion of titanium (Ti) is less than 0.02% and the upper limit for weight proportion of niobium (Nb) is less than 0.02%.

7. A ring-pull top produced by the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An embodiment example of the invention is described in greater detail below with reference to the accompanying drawings. In these drawings:

(2) FIG. 1 shows a top view of a ring-pull top;

(3) FIG. 2 shows a section through the ring-pull top of FIG. 1 along the line A-A;

(4) FIG. 3 schematically shows a typical curve of the tear-off forces required for pulling open a ring-pull top along the notch line;

(5) FIG. 4 shows the dependence of the tear-off forces required for pulling open a ring-pull top on the tensile strength of the steel sheet used, and

(6) FIG. 5 schematically shows a typical annealing curve (temperature T of the steel sheet in dependence on the time t in seconds) for the recrystallizing heat treatment of the steel sheet used for the inventive ring-pull top.

DETAILED DESCRIPTION OF THE INVENTION

(7) Continuously cast and hot-rolled steel strips that were wound into coils and consisted of steels with the following composition were used for the production of a steel sheet from which ring-pull tops can be produced in accordance with the invention: C: max. 0.1%; N: max. 0.02%; Mn: max. 0.5%, preferably less than 0.4%; Si: max. 0.04%, preferably less than 0.02%; Al: max. 0.1%, preferably less than 0.05%; Cr: max. 0.1%, preferably less than 0.05%; P: max. 0.03%; Cu: max. 0.1%; Ni: max. 0.1%; Sn: max. 0.04%; Mo: max. 0.04%; V: max. 0.04%; Ti: max. 0.05%, preferably less than 0.02%; Nb: max. 0.05%, preferably less than 0.02%; B: max. 0.005%; other alloying constituents and impurities: max. 0.05%, remainder iron.

(8) Steel sheets of this type were initially subjected to a thickness reduction of 50% to 96% by being cold-rolled to a final thickness of about 0.5 mm and subsequently annealed in a recrystallizing fashion by means of inductive heating in an induction furnace.

(9) In this case, an induction coil with a power of 50 kW at a frequency of f=200 kHz was used, e.g., for a sample size of 2030. A typical annealing curve is illustrated in FIG. 5. According to the annealing curve in FIG. 5, the steel strip was heated to a maximum temperature T.sub.max above the A.sub.1 temperature (T(A.sub.1)725 C.) within a very short heating time t.sub.A that typically lies between approximately 0.5 s and 10 s. The maximum temperature T.sub.max lies above the phase transition temperature T.sub.f of the ferromagnetic phase transition (T.sub.f770 C.). The temperature of the steel strip was then maintained at a temperature value above the A.sub.1 temperature for an annealing period t.sub.G of approximately 0.75-1 second. During this annealing period t.sub.G, the steel strip, slightly cooled from its maximum temperature T.sub.max, e.g., of 750 C. to the A.sub.1 temperature (approximately 725 C.). Subsequently, the steel strip was cooled to room temperature (approximately 23 C.) within a cooling interval of approximately 0.25 seconds by means of a fluid cooling process that may be carried out, for example, with the aid of a water-cooling system or an air-cooling system. If so required, an additional cold-rolling step with a thickness reduction of up to 40% may be carried out after the cooling process.

(10) The thus treated steel sheet was subsequently examined with respect to its strength and its percentage elongation at fracture. Comparative tests showed that the percentage elongation at fracture was in all instances higher than 6% and usually higher than 10%, and that the tensile strength amounted to at least 500 MPa and in many instances even exceeded 650 MPa.

(11) Color etching according to Klemm made it possible to confirm that the steel sheets treated in accordance with the invention have an alloying structure that features ferrite as soft phase and martensite and, if applicable, bainite as hard phase.

(12) It was furthermore determined in comparative tests that the best results with respect to strength and ductility are achieved if the heating rate during the recrystallizing inductive annealing lies between 200 K/s and 1200 K/s, and if the steel strip annealed in a recrystallizing fashion is subsequently cooled at a cooling rate of more than 100 K/s. With respect to the cooling system, cooling rates between 350 K/s and 1000 K/s are advantageous because an elaborate water-cooling system is not required in this case and the cooling process can be carried out by means of a cooling gas such as air. However, the best results with respect to the material properties were achieved by using a water-cooling system with cooling rates in excess of 1000 K/s.

(13) The steel sheet of the invention is superbly suitable for use as packaging steel. For example, ring-pull tops for preserved food or beverage cans with ring-pull tops can be produced from the heat-treated steel sheets. Since the requirements with respect to the corrosion resistance of packages are particularly strict in the food industry, it is advantageous to provide the steel sheet produced in accordance with the invention with a metallic and corrosion-resistant coating, for example, by means of electrolytic tin plating or chromium plating, after the heat treatment and, if applicable, after an ensuing cold-rolling step. However, it can also be conceived to use other coating processes such as, e.g., galvanizing or lacquering or even laminating on a plastic film. In this case, the coating may be applied on one or both sides depending on the respective requirements.

(14) With respect to its strength and ductility, the steel sheet used for the inventive production of ring-pull tops is comparable to the dual phase steels known from the automobile industry. In comparison with the dual phase steels known from the automobile industry, however, the steel sheet used for the inventive production of ring-pull tops is characterized, in particular, by significantly lower production costs and the advantage that a steel with lower alloying concentration and few alloying constituents is used such that contaminations of the packaged foods due to diffusion of the alloying constituents can be prevented.

(15) Ring-pull tops for preserved food or beverage cans were produced from the steel strips or plate-shaped steel sheets produced and heat-treated in the above-described fashion. Subsequently, the steel strip or steel plate was coated with a protective surface layer on one or both sides. The protective layer may be applied, for example, by means of lacquering or galvanizing. The protective layer may consist of a metallic coating, for example of tin or chromium, that can be applied, e.g., with an electrolytic coating process. However, it may also consist of one or more coats of lacquer or of a plastic film that is applied onto the surface of the steel sheet on one or both sides by means of a laminating process.

(16) After the application of the protective layer, tops were stamped out of the steel sheet and scored (i.e. provided with a score line) in order to obtain a top of the type illustrated in FIGS. 1 and 2.

(17) The circular top 1 illustrated in FIGS. 1 and 2 comprises a flanged edge region 2 that serves for fixing the top on the cylindrical body of a can by means of lock-seaming. The flanged edge region 2 is adjoined by an annular transition region 3 that essentially extends horizontally and is adjoined by a section 4 that is bent vertically downward. The section 4 ends in a groove-shaped bead 5 adjoined by a central region 6 that essentially extends horizontally. The central region 6 is surrounded by a score line 7 that continuously extends over the entire circumference. The score line consists of a material-thinning notch that advantageously has a triangular or trapezoidal cross section with a straight or oblique notch root and with a residual wall thickness in the range between 50 and 100 m. The score line 7 serves for tearing off the ring-pull top by separating the central region 6 from the outer region 2, 3, 4 of the top 1 along the score line 7.

(18) A pull-tab 8 with a pulling ring 12 is fixed in the central region 6 by means of a rivet 9, wherein the rivet is formed from the top (i.e., drawn from the material of the top). A depression 10 is provided in the central region 6 in order to take hold of the pull-tab 8. The pull-tab 8 can be manually pulled up on its pulling ring 12, wherein the pointed end 11 of the pull-tab 8 that lies opposite the pulling ring 12 punctures the notch line 7 in order to initially produce a local slot in the notch line. The notch line 7 is ultimately opened along its entire circular circumference by pulling on the pulling ring 12 of the pull-tab 8 such that the central region 6 of the ring-pull top is separated from the outer region and an opening is exposed in the ring-pull top.

(19) FIG. 3 shows a typical curve of the aforementioned forces F (in Newtons) for tearing off a full ring-pull top, wherein said forces are plotted as a function of the tear-off distance D along the notch line. The force required for puncturing a local slot into the notch line 7 when the pull-tab 8 is initially lifted is referred to as score breaking force or opening force A (score brake). The force required for completely opening the notch line 7 by pulling on the pull-tab 8 is referred to as tearing force or score tearing force B (score tear). In order to separate the central region 6 from the outer edge region, a force referred to as tear-off force C (tear-off) is ultimately also required for completely exposing the opening in the ring-pull top by pulling off the central region 6.

(20) Until now, it was assumed that the forces required for opening a ring-pull top decrease as the tensile strength increases if the residual wall thickness in the region of the notch line is constant. FIG. 4 shows a typical curve of the (maximum) score tearing force B (maximum score tear) in dependence on the tensile strength of the steel sheet used for the production of the ring-pull top for two different residual wall thicknesses SR (SR=75 m and SR=60 m) of a cover with a diameter of 73 mm and a steel sheet thickness of 0.22 mm.

(21) During the course of comparative tests that were carried out with the ring-pull tops produced in accordance with the invention, it was determined that there also exist other influencing variables for the score tearing forces. For example, the score tearing force is also highly dependent on the carbon content of the steel sheet used. The lower the carbon content of the steel sheet used, the higher the required score tearing force. It was furthermore determined that the forces required for opening a ring-pull top, particularly the maximum score tearing force, is lower if a steel sheet that was heat-treated in accordance with the invention is used for producing the ring-pull top. The steel sheets used for producing the ring-pull tops in accordance with the invention have a multi-phase structure that comprises at least martensite as hard structural phase. It is assumed that this hard martensite phase initiates an early material failure when the ring-pull top is opened and in this way significantly reduces the score tearing forces. At a residual wall thickness of 60 m and a tensile strength of the steel sheet used of 500 MPa, ring-pull tops produced in accordance with the invention have maximum score tearing forces, e.g., in the range of 40 N or less.

(22) The invention is not limited to the embodiment example illustrated in the figures, wherein said illustrations were merely provided for describing the invention in greater detail. The steel sheet proposed for producing ring-pull tops in accordance with the invention is equally suitable for the production of ring-pull tops with a different design, as well as for the production of cans with a ring-pull top. For example, it would also be possible to accordingly produce ring-pull tops according to the invention in which the tear-off part is not completely removed from the top, but merely pushed into the interior of the can with the pull-tab. Furthermore, the ring-pull tops may also have a different shape, e.g. an oval shape, and the notch line may likewise have a different shape, e.g. an oval or helical or spiral shape.