Improved method for producing can bodies for 3-piece CANS and apparatus for performing said method

Abstract

This invention relates to an improved method for welding chromium coated steel for 3-piece canmaking and to an apparatus for performing said method.

Description

[0001] IMPROVED METHOD FOR PRODUCING CAN BODIES FOR 3-PIECE CANS AND APPARATUS FOR PERFORMING SAID METHOD

[0002] Field of the invention

[0003] This invention relates to an improved method for welding chromium coated steel for 3-piece canmaking and to an apparatus for performing said method.

[0004] Background of the invention

[0005] Most of the cans produced today are one of two types: 3-piece cans consisting of three components: a bottom lid, a tubular body, and a top lid, and 2-piece cans consisting of two components: a body with an integrated bottom lid, and a top lid.

[0006] 3-piece can bodies are made of a rectangular blank formed into a tubular body and the edges are joined together by soldering or welding. Nowadays welded cans dominate the market. The rectangular blank is produced by cutting it from a steel strip or a steel sheet. 3-piece cans may be made in almost any practical combination of height and diameter.

[0007] After rolling the rectangular blank into a body the two edges slightly overlap and are subsequently welded together into a tube by mash seam welding. After necking and flanging the top end and bottom, the end of the tube can be closed with a lid.

[0008] 3-piece cans can be made of tinplate, which is a thin steel substrate coated with a very thin layer of tin, or of so-called tin-free steel (TFS). The latter used to be produced by applying electrolytic chromic acid (Cr6+) treatment over steel sheets so that they are coated by a thin film composed of a lower layer of metallic chromium and an upper layer of chromium hydroxide, e.g. as disclosed in JPH05287591A. This type of TFS is also referred to as ECCS (Electrolytic Chromium Coated Steel). However, the manufacturing of this material relies on the use of hexavalent chromium solutions which is increasingly being phased out and is soon to be banned in Europe in because of the toxic and carcinogenic character of hexavalent chromium solutions. Another issue with TFS is that it is significantly more difficult to weld than tinplate, which is a challenge for a more widespread use of TFS for 3-piece cans.

[0009] Tata Steel developed an alternative TFS which is known as TCCT® (Trivalent chromium-coating technology, registered trademark of Tata Steel) and which is based on harmless trivalent chromium technology. Chromium-chromium oxide layers can be safely deposited. TCCT® provided with a polymer coating layer (Protact®, registered trademark of Tata Steel) or lacquered is perfectly suitable for 3-piece can production. TCCT® is a proprietary technology developed by Tata Steel as disclosed for instance in WO2014202316-A1 and W02020173950-A1 and is based upon the deposition of the coating by electrodeposition from a trivalent chromium electrolyte. This process results in the deposition of a chromium oxide layer on top of a chromium metal layer (also comprising some Cr-carbides and Cr-sulphate) on a steel substrate (see figure 14).

[0010] In the context of this description and claims TCCT® is considered a tin-free steel (TFS), i.e. there is no tin present in the metallic coating. When producing the blanks for the 3-piece can bodies from a strip of tinplate there are two basic options: so-called C-grain and H-grain blanks. C-grain blanks are cut from the strip in such a way that the edge of the blank which is to be welded is parallel to the width of the coil. H-grain blanks are cut from the strip in such a way that the edge of the blank which is to be welded is perpendicular to the width of the coil (see figure 2). So, C-grain can bodies are 3-piece welded can bodies for which the weld (or welding direction) is perpendicular to the rolling direction of the steel sheet used for the can body. The C stands for circumferential - the rolling direction lies in the circumference of the can body (see the arrows in figure 2). H-grain can bodies are 3-piece welded can bodies for which the weld (or welding direction) is parallel to the rolling direction of the body sheet (see figure 2). The H stands for height - the rolling direction lies in the height of the can body.

[0011] Many of the 3-piece cans are made from DR (double-cold rolled) steel qualities because the secondary reduction of the recrystallisation annealed cold rolled steel strip provides the steel with additional strength. The difference in mechanical properties with respect to the rolling direction (anisotropy), which is intrinsic to DR materials, favours the C-grain cans. A smaller risk of flange cracks and a larger welding range for C-grain cans are the main reasons to stay with the C-grain can bodies. The welding range (also referred to as welding latitude) is expressed as the electrical current (in Amperes) required to produce weld nuggets in the weld that are neither too cold nor too hot. Too cold welds reduce weld strength, and too hot welds increases risk of molten metal splashing.

[0012] However, H-grain cans are advantageous for polymer coated steel cans. For instance, in patent document W02019 / 110616 it is disclosed how H-grain blanks can be produced on the basis of polymer coated steel strip of which strips or certain areas are left uncoated, so that polymer coated steel blanks can be produced with bare edges to facilitate the welding after forming the can body from the blank. This method is not suitable for producing can bodies from C-grain blanks. In those cases (i.e. in C-grain blanks) the parallel edges to be welded together of each individual blank must be made bare (i.e. the removal of the polymer coating) by a much more laborious procedure, e.g. using mechanical (e.g. scouring), chemical (e.g. dissolution) or optical (e.g. laser) means.

[0013] One important advantage of H-grain bodies is of a commercial nature. The height of a can body is determined by the blank size. It is much easier to change the can height for a fixed diameter of an H-grain can body because the blanks are cut from the strip in the rolling direction, and the width of the strip is tailored to the number of blanks that are taken from the width of the strip. The number of coil specs is reduced which leads to cost reduction for the canmaker.

[0014] Tinplate is generally speaking easier to weld than chromium coated steels. So, any issues with welding H-grain tinplate are present in chromium coated steels even more strongly. Because of the different coating (tin-based versus chromium-based) the welding of can bodies of the TCCT® material is even more challenging than the welding of can bodies from tinplate. JPH05287591A discloses a method to produce a tin-free steel sheet for the welded can with an improved brightness of the formed can by forming a granular metallic Cr layer on one surface of the sheet and forming a flat metallic Cr layer on the opposite surface by cathodising a cold-rolled steel sheet in a CrOs-containing electrolyte to plate both surfaces with Cr. It is noted that CrOs-containing electrolyte is a hexavalent (Cr6+) electrolyte. Only one surface is anodized in the same electrolyte to dissolve off the deposited Cr while leaving > = 10mg / m2of Cr, and the amount of Cr to be dissolved on the opposite surface is controlled to < 10%. The plated steel sheet is then electrolyzed in an aqueous C Os containing solution with the sheet as a cathode, hence 40-150 mg / m2of Cr layer is formed on the unanodized surface, 5-25 mg / m2of hydrous Cr oxide layer, expressed in terms of Cr, is formed thereon, and a protrusion with the maximum diameter of a part of the Cr layer controlled to 20-250 nm, the height from the base to > 10nm and the abundance to lxlO12to lxlO15units / m2is formed. The unanodized surface has 40-150 mg / m2of Cr layer and 2.5-25 mg / m2of hydrous Cr oxide layer, expressed in terms of Cr, thereon, the height of the protrusion is controlled to < 100 nm, and the protrusions of > = 10nm are controlled to < lxlO14units / m2. It is clear that the Cr layers on one side of the strip are drastically different from the one on the other side, both in dimensions, composition and morphology.

[0015] Objectives of the invention

[0016] It is an objective of this invention to provide an improved method for welding chromium coated steel sheets into can bodies.

[0017] It is an objective of this invention to provide an improved method for welding chromium coated steel sheets into can bodies with a larger welding range.

[0018] It is also an objective of this invention to provide an apparatus capable of performing the improved method for welding chromium coated steel sheets.

[0019] Description of the invention

[0020] One or more objectives of the invention is reached by a method for producing a welded can body for a 3-piece can, the method comprising the steps of:

[0021] • Producing a rectangular blank from a sheet of steel strip coated on one or both sides with a chromium coating layer and a chromium oxide layer or chromium hydroxide layer on top of the chromium coating layer (tin-free steel), from blackplate, or from a low-tin sheet or tinplate, wherein the rectangular blank comprises a top edge, a bottom edge and two parallel outside edges;

[0022] • Forming the rectangular blank, e.g. by roll forming, into a tube by bringing the outside edges together wherein the edges overlap

[0023] • Welding the outside parallel edges together to form a tubular welded can body by mash seam welding the overlapping edges together in a body welder by resistance welding and producing a continuous longitudinal weld along the overlapping edges wherein the resistance welding is achieved by presenting the overlapping edges to a nip between a conductive rotating upper welding wheel and a conductive rotating lower welding wheel, wherein the overlapping edges are propelled by the upper and lower welding wheel while being welded together by resistance welding as a result of a welding current being sent through the upper and lower welding wheel;

[0024] • Wherein the upper and lower welding wheel during the resistance welding are forced towards each other thereby exerting a welding force on the overlapping edges of the outside parallel edges of the tube wherein the welding force is at least 60 daN;

[0025] • Removing the tubular welded can body from the welder.

[0026] Tinplate is defined in EN20202:2022, particularly in § 7.1. Blackplate is a defined as a cold-rolled steel that has not been provided with a coating (except optionally oiling or DOS-oiling to protect against rust). This material is used in applications where the tin or chromium coating characteristics (such as corrosion prevention or surface characteristics) are of little relevance.

[0027] In a preferred embodiment the welding force is at most 150 daN. Preferably the welding force is at least 60 daN, more preferably at least 65 daN, or even at least 70 daN. Preferably the welding force is at most 120 daN, more preferably at most 100 daN, more preferably at most 90 daN. The inventors found that a suitable range in welding force is between 60 and 90 daN.

[0028] In an embodiment the welding range is at least 100 A, preferably at least 200 A, more preferably at least 250 A and even more preferably at least 275 A.

[0029] In an embodiment the rectangular blank, which is to be shaped into a tube in the form of a cylinder or any other suitable shape and welded to form open ended tubular can bodies, is cut from the sheet of steel strip such that the weld seam that closes the can body is parallel to the rolling direction of the steel strip.

[0030] In an embodiment the rectangular blank, which is to be shaped into a tube in the form of a cylinder or any other suitable shape and welded to form open ended tubular bodies, is cut from the sheet of steel strip such that the weld seam that closes the can body is perpendicular to the rolling direction of the laminated tinplate.

[0031] In an embodiment a cylindrical can body with constant diameter is produced, which cylindrical can body is subjected to subsequent forming steps such as flanging, beading, necking, forming into a shaped can body, e.g. by expanding or blow forming.

[0032] This method is particularly advantageous for production of steel food or beverage containers (also referred to as cans or tins). Welding current is continuous and higher frequencies are used for welding thinner materials. Welding speeds can be over 70 m / min when this process is applied to substrates having a thickness of between 0.10 to 0.40 mm. The substrate thickness is preferably between 0.12 and 0.30 mm.

[0033] The invention is not restricted to particular steel substrates. A large variety of steel grades can be used for packaging purposes, see e.g. Table 4 of EN10202:2001 in Figure 13.

[0034] A good quality weld consists of intermittently repeated overlapping weld nuggets, which create a continuous gas tight welded seam along the total height of a can body. To ensure good weldability with high quality joints, a welding parameter range is determined, within which the materials are welded. This operating window is bound by a lower limit, above which good mechanical behaviour is established and an upper limit, below which no material ejection (splashing) is encountered during welding. A can body welder is at the heart of any 3-piece can production line. It forms the body blanks into their basic shape and welds the seam overlap. Optimum control of the welding current combined with precision-matched pressure on the overlap guarantees a mashed (=deformed) weld seam that is thinner than 2 times the thickness (t) of the sheet metal to be welded together in the seam. The active face of the welding wheel is wider than this contact width and during welding the material is compressed to a final thickness typically of 1.4-t or smaller. As known in the art a mashed seam is a weld seam wherein the material is not only melted at the point of contact between the overlapping edges but also deformed and forged to join the materials together. The smaller the overlap, the thinner the weld can be made. The overlap is usually chosen in the range of 0.25 to 0.80 mm, preferably at most 0.60 mm, and more preferably at most 0.50 mm, and most preferably at most 0.40 mm.

[0035] A resistance welding machine comprises an upper welding wheel, a lower welding wheel and optionally a copper electrode wire. Both the upper and lower welding wheel are mounted on a mechanism that allows the exertion of a pressure on any material that passes between the welding wheels. The mechanism is often provided in the form of an arm at the end of which the wheel is rotatably mounted. Since the welding wheels become hotter as a result of the welding operation, the wheels need to be cooled. A current can be passed through the wheels if the overlapping edges of the future can body pass between the rolls. During the resistance seam welding the steel substrate is heated because the current, usually an alternating current, passes through the steel substrate. Because of all the resistances (contact resistance, material resistance, etc.) heat is produced within the metal sheet. Depending on the magnitude of the current and the speed with which the overlapping edges move between the welding wheels the steel will be heated up to a temperature, which usually will be slightly below the melting point of steel. In combination with the welding force the viscous steel will join together, and a strong continuous longitudinal weld will be formed.

[0036] In an embodiment a first copper electrode wire is used between the upper wheel and the upper parallel edge and / or wherein a second copper electrode wire is used between the lower wheel and the lower parallel edge.

[0037] A copper electrode wire-feeding system can be used to overcome problems of electrode degradation, for instance as a result of pick-up of particles on the electrode wheels during use of the can body welder, the copper wire being placed between the electrodes and the sheet material to be welded. The welding electrode then does not touch the can body, only the copper electrode wire contacts the steel of the can body. The copper electrode wire moves at the same speed as the rotational speed of the welding wheel and the speed of the can body. This wire is recycled after use. If the electrode wheels are not polluted, then the copper wire may not be needed. The use of the copper wire prevents impurities on the weld from accumulating on the welding wheel.

[0038] The main factors that affect the quality of a resistance weld are:

[0039] 1. Welding current: The welding current is too small, and the weld is not strong.

[0040] If the current is too large, the weld will become brittle and easy to break. In addition, when the current is too large, the temperature is too high, the metal is melted, forming a large molten metal splash point. These metal splash points may adhere to the inner surface of the can body and may negatively impact the protective coating of the can body.

[0041] 2. Welding pressure: Too little welding pressure leads to an increased contact resistance, higher welding temperature, but uneven welding. If the welding pressure is too high, then the contact resistance will decrease, the welding current will increase, and the weld will be wider due to squeezing.

[0042] 3. Welding resistance: The welding resistance is related to the thickness, material, overlap width and pressure of the can body plate. When the welding resistance increases, the heat generated by the current is concentrated at the weld. One of the contributors to the welding resistance is the presence of an oxide layer on the metals that have to be welded together.

[0043] In accordance with the invention the improvement of control of these factors also contributed to a more consistent welding range and therefore a more consistent welding operation.

[0044] Where reference has been made to can bodies herein above it should be noted that these can bodies usually have a circular cross-section resulting in a cylindrical tube or can body, but they may also have a different cross section, such as oval, square, rectangular or the like. The invention as described herein is equally applicable to these less conventional can body shapes. Blow-moulded shaped cans are usually formed by blow forming a cylindrical weld tube into the shaped can body after welding. This imposes even more strict quality requirements to the weld.

[0045] The TCCT® substrate based on a steel substrate in accordance with figure 14 contains a chromium layer consisting of a chromium metal layer and an oxide layer on top. The minimum total chromium weight is 50 mg / m2, and the maximum is 250 mg / m2of which between 2 and 35 mg / m2of that chromium is present in the oxide layer as oxides and hydroxides. The total chromium is the sum of metallic chromium and chromium present as oxides and hydroxides.

[0046] In an embodiment a sheet of steel strip is provided which is coated on both sides with a chromium coating layer and a chromium oxide layer or chromium hydroxide layer on top of the chromium coating layer (tin-free steel) deposited from a trivalent chromium electrolyte, preferably wherein the chromium layer consists of a chromium metal layer and an oxide layer on top with the minimum total chromium weight being 50 mg / m2and the maximum being 250 mg / m2of which between 2 and 35 mg / m2of that chromium is present in the oxide layer as oxides and hydroxides. Preferably the layers on both sides are identical in morphology. Preferably the layers on both sides are identical in terms of the total layer thickness and of the individual chromium metal and chromium oxide layer thicknesses.

[0047] In an embodiment one or both of the two parallel outside edges which are to be welded together, are treated with a plasma treatment to modify the surface of the chromium coated steel strip to improve the weldability of the edges in the mash seam welding process. In a preferable embodiment the plasma treatment is an atmospheric plasma treatment, a vacuum plasma treatment or a reducing plasma treatment. Preferably the plasma treatment activates the chromium layer at the location of the one or both of the two parallel outside edges to facilitate the mash seam weldability of the overlapping outside edges of the blank: the plasma treatment removes part of the chromium oxide layer or the entire existing chromium oxide layer from the one or both of the two parallel outside edges of the blank to facilitate the mash seam weldability of the overlapping outside edges of the blank after forming the can body from the blank. By removing the top Cr-oxide layer (and contaminations) by means of a plasma treatment the chemistry of the surface is changed from an insulating one to a more conductive oriented surface. This allows the welding of the blank to be performed at more preferred parameters. In addition, the plasma treatment will also cause physical changes in the surface microstructure which might also be beneficial for the welding process.

[0048] This embodiment alters the surface condition of the blank at the location of the one or both parallel edges where the weld is to be made. The plasma treatment may be performed at one or both parallel edges, and on one or both sides of said parallel edges, and preferably on both parallel edges. It is believed that the plasma treatment removes the surface chromium(III)oxide layer and thereby lowers and / or equalises the resistivity of the chromium / chromium oxide coating on the steel substrate along the parallel edges. In addition to an increased welding force this results in a further improvement of the consistency of welding process and the quality of the resulting welds.

[0049] A plasma is an ionised gas, i.e. the atoms lose one or more electrons. The total of the ionised gas is neutral as being composed of the positive ions and the negatively charged electrons. If enough molecules are ionised to affect the overall electrical characteristics of the gas the resulting state of matter is called a plasma. Plasmas are therefore often referred to as the fourth state of matter.

[0050] A plasma contains positive ions, electrons, neutral gas atoms or molecules, UV light and also excited gas atoms and molecules, which can carry a large amount of internal energy. All of these components can interact with the surface during plasma treatment. By choosing the gas mixture, power, pressure, the effects of the plasma treatment can be tailored to the needs.

[0051] Plasma cleaning is performed on steel surfaces with a plasma cleaning apparatus. In this description and the claims this term plasma cleaning is used interchangeable with plasma sputter cleaning and sputter cleaning. In a plasma cleaning apparatus the fed gas is ionised by a voltage difference. The gas in the apparatus is inert, preferably Argon gas (Ar) or Argon based gas because of its high atomic mass. The sputter cleaning process is based on the impact of the gas flow containing the charged gas atoms (ions), electrons and neutral atoms, therefore it is obvious to use the heavy Ar gas. Furthermore, one can consider adding hydrogen (or other reactive species) to the gas mixture to increase the reduction effect. On the other hand, hydrogen might induce hydrogen embrittlement so there might be a trade-off and gas composition can be optimised. For example, a high-density atmospheric pressure plasma, which allows a high-rate cleaning, can be created with a direct current (de), or alternated current (ac) arc discharges. However, the main disadvantage is the high thermal load of the substrate. Therefore, often dielectric barrier discharge (DBD) is used which is a low frequency and high voltage discharge where an electrical insulator (dielectric barrier) is placed between two electrodes to suppress arc discharges. However other configurations can be considered to create an atmospheric plasma treatment of the substrate surface.

[0052] By directing a plasma towards the parallel edges of the blanks or, before cutting of the blanks, towards the locations in the strip where the edges of the future blanks will be, the properties of the surface of these edges will be improved. The change of the properties may be the result of activating the surface or of removing a thin layer of the substrate surface. In case of a metal coated substrate, e.g. with tin or chromium, the layer may even be completely removed until the substrate is bare, or the removal may be limited to only the oxide layer. In the latter case it is possible that also a very small layer of the metal underneath the oxide layer is removed, but the substrate itself is still provided with a remaining tin or chromium layer.

[0053] When removing the oxide layer immediately before the welding of the can body then the oxide layer will be thinner and preferably even absent. The (re)formation of oxide can be further prevented if the plasma treatment and subsequent welding is performed in a non-oxidising atmosphere, at least between removing the oxide layer and the welding operation. For instance, a reducing plasma treatment could be used. This can for instance be achieved by introducing H2 into the plasma thus making it into a reducing plasma.

[0054] According to a second aspect the invention is also embodied in an apparatus for executing the method according to the invention capable to apply a welding force of at least 60 daN.

[0055] In an embodiment the apparatus is capable of applying a welding force of at most 150 daN. Preferably the welding force is at least 65 daN, or more preferably at least 70 daN. Preferably the welding force is at most 120 daN, more preferably at most 100 daN, more preferably at most 90 daN. The inventors found that a suitable range in welding force is at from 65 to 90 daN.

[0056] In an embodiment the apparatus is provided with a plasma treatment device, wherein the plasma treatment device is an atmospheric plasma treatment or a vacuum plasma treatment for modifying the surface of the one or both of the two parallel outside edges of the blank to improve the weldability of the edges in the mash seam welding process, preferably wherein the mash seam welding immediately follows the plasma treatment to prevent recontamination of the edges to be welded together.

[0057] The plasma treatment device may be mounted in the welding machine prior to the welding operation, or prior to or after roll-forming the can body. Alternatively, the plasma treatment may also be executed on the blank.

[0058] Examples

[0059] The steel substrate used in these examples is a TS275 steel grade (EN 10202:2001) having a yield strength of 275 MPa. Experiments were performed with tinplate (C-grain) as the benchmark of a very easily weldable substrate. This is at least partly related to the very low electrical resistance of tinplate compared to a chromium coated, Typical values are 0.03-0.7 mQ (depending on the thickness of the tin layers) and 22 mQ respectively. The main cause of the higher resistance of the chromium is attributed to the oxide layer on top of the metallic chromium.

[0060] The invention is not restricted to particular steel substrates. A large variety of steel grades can be used for packaging purposes. See e.g. Table 4 of EN10202:2001 in Figure 13.

[0061] The substrate of the examples was coated with a chromium / chromium oxide coating based on the TCCT® technology. TCCT® is a proprietary technology developed by Tata Steel as disclosed for instance in W02020173950-A1 and is based upon the deposition of the coating by electrodeposition from a trivalent chromium electrolyte. To improve the welding quality during the welding of a can body for a 3-piece can from a rectangular blank the edges to be welded together were subjected to a vacuum based plasma treatment which removed the oxide layer of the chromium / chromium oxide coating. After this plasma treatment the surface composition was investigated with XPS.

[0062] The coating was also studied with a Kratos Axis Ultra DLD XPS (X-ray Photoelectron Spectrometer) using a focused using monochromatic Al K-a X-rays of 1486.6 eV (~7 nm penetration depth) at a background pressure of 1010mbar. Depth information (0 - 100 nm) is obtained by combining XPS measurements with an in-situ Ar-ion sputter gun (4keV beam energy, 2x2 mm sputter crater, 2.6 nm / min sputter rate, IO-8mbar background pressure). A 700x300 pm XPS spot-size is used.

[0063] The figures 3 and 4 show the profiles of the as-received TCCT® coated substrate. The XPS results of the surface are presented in Table 1.

[0064] The figures 5 and 6 show the profiles of the plasma treated substrate. The XPS results of the surface are presented in Table 2. About 8 nm coating is removed by the plasma treatment, mainly Cr(III) oxide and some metallic chromium.

[0065] The figures 7 and 8 show the profiles of the material that has been removed from the as-received TCCT® coated substrate by the plasma treatment.

[0066] Table 1: As-received

[0067] Table 2: After plasma treatment

[0068] * These values were calculated from the C-carbide and S-sulphate signals. The figures 7 and 8 show that the material that was removed from the as-received TCCT® coated substrate by the vacuum plasma treatment consisted almost entirely of Cr(III) oxide. Negligible amounts of Cr-metal, Cr-carbide, and Cr-sulphate were detected, proving that the plasma treatment removed the oxide layer and a minute amount of the Cr-layer. The plasma-treated surface therefore consists of Cr-metal. Any Cr-oxide present has formed after the plasma-treatment. No hexavalent chromium is detected. The plasma treatment therefore does not cause formation of this undesired and toxic compound. When removing the oxide layer immediately before the welding of the can body then the Cr-oxide layer will be thinner and preferably even absent. The formation of Cr-oxide can be further prevented if the plasma treatment and subsequent welding is performed in a non-oxidising atmosphere, at least between removing the oxide layer and the welding. For instance, a reducing plasma treatment could be used.

[0069] The welding experiments on the Soudronic welding machine (type AFB1080) proved that the weldability and the welding window improved after removal of the oxide layer by the plasma treatment and welds of good quality could be made with a sufficiently large welding range. To avoid contamination during the testing a copper wire electrode was employed.

[0070] Table 3 - Welding ranges

[0071] The conclusion from these examples is that the welding range for TCCT is smaller, consistent with earlier observations that chromium coated steels are harder to weld than tinplate. However, the inventors found that a good welding range can be found by increasing the force with which the parallel edges are pressed together during seam welding. The ranges as presented herein above exceed the manufacturer's recommended range, and consequently this may require some straightforward modification to the body welder itself, but it is evident that the method according to the invention results in a workable welding range for both C and H grain TCCT grade material.

[0072] Brief description of the drawings

[0073] The invention will now be explained by means of the following, non-limiting figures.

[0074] Figure 1 shows a schematic overview of the production of SR and DR steel suitable for 3-piece can, starting from a suitable hot-rolled steel strip with a suitable composition and suitable dimensions.

[0075] Figure 2 shows a cold-rolled and coated strip on which the blanks for the 3-piece can bodies are projected that, after cutting, enable production of H-grain can bodies on the top of the figure and a for a C-grain can body on the bottom.

[0076] Figure 3 shows a schematic of resistance mash seam welding with on the lefthand side a front view and on the right-hand side a side view. The radii of the copper electrode wheels are different in size for the various can body diameters and a copper wire electrode moves between the electrode wheels and sheet material to prevent problems of electrode contamination and degradation.

[0077] Figure 4 shows a graphical representation of the results presented in table 3.

[0078] Figure 5 to 10 show the results of the XPS measurements. Figures 5 and 6 show the elemental depth profile of the main elements and the Cr and Fe compound depth profile respectively of the as-received TCCT material. Figures 7 and 8 show the elemental depth profile of the main elements and the Cr and Fe compound depth profile respectively of the plasma treated TCCT material. Figures 9 and 10 show the elemental depth profile of the main elements and the Cr and Fe compound depth profile respectively of the collected material that was removed by the plasma-treatment.

[0079] Figure 11 shows a comparison of figure 5 and 7 and shows that about 8 nm of material was removed by the plasma treatment which according to figure 10 consisted mainly of Cr(III)oxide. The thickness of the removed layer was determined on the basis of the position of the cross-over point of the Cr and Fe curves. The fact that the compounds do not add up to 100% is explained by the fact that the oxygen and carbon compounds are not shown.

[0080] Figure 12 shows the result of a good weld, a cold weld where the base metal was not melted, and a too hot weld where metal splatter occurred.

[0081] Figure 13 shows a non-exhaustive list of various substrates for tin mill products relevant for this invention.

[0082] Figure 14 shows the schematic layer structure of the steel substrate coated with a chromium coating layer and a chromium oxide layer on top (not to scale).

Claims

CLAIMS1. Method for producing a welded can body for a 3-piece can, the method comprising the steps of:Producing a rectangular blank from : i. a sheet of steel strip coated on one or both sides with a chromium coating layer and a chromium oxide layer or chromium hydroxide layer on top of the chromium coating layer (tin-free steel) deposited from a trivalent chromium electrolyte, or from ii. blackplate sheet, or from iii. a low-tin sheet or tinplate, wherein the rectangular blank comprises a top edge, a bottom edge and two parallel outside edges;Forming the rectangular blank, e.g. by roll forming, into a tube by bringing the outside edges together wherein the edges overlap;Welding the outside parallel edges together to form a tubular welded can body by mash seam welding the overlapping edges together in a body welder by resistance welding and producing a continuous longitudinal weld along the overlapping edges wherein the resistance welding is achieved by presenting the overlapping edges to a nip between a conductive rotating upper welding wheel and a conductive rotating lower welding wheel, wherein the overlapping edges are propelled by the upper and lower welding wheel while being welded together by resistance welding as a result of a welding current being sent through the upper and lower welding wheel;Wherein the upper and lower welding wheel during the resistance welding are forced towards each other thereby exerting a welding force on the overlapping edges of the outside parallel edges of the tube wherein the welding force is at least 60 daN and at most 150 daN;Removing the tubular welded can body from the welder.

2. Method according to claim 1 wherein the welding force is between 65 and 90 daN.

3. Method according to claim 1 or 2 wherein the welding range is at least 100 A, preferably at least 200 A, more preferably at least 250 A and even more preferably at least 275 A.

4. Method according to any one of claims 1 to 3 wherein the rectangular blank, that is to be shaped into a tube in the form of a cylinder or any other suitable shape and welded to form open ended tubular can bodies, is cut from the sheet of steelstrip such that the weld seam that closes the can body is parallel to the rolling direction of the steel strip.

5. Method according to any one of claims 1 to 3 wherein the rectangular blank, that is to be shaped into a tube in the form of a cylinder or any other suitable shape and welded to form open ended tubular bodies, is cut from the sheet of steel strip such that the weld seam that closes the can body is perpendicular to the rolling direction of the laminated tinplate.

6. Method according to any one of the preceding claims wherein a cylindrical can body with constant diameter is produced, which cylindrical can body is subjected to subsequent forming steps such as flanging, beading, necking, forming into a shaped can body, e.g. by expanding or blow forming.

7. Method according to any one of the preceding claims wherein one or both of the two parallel outside edges which are to be welded together, are treated with a plasma treatment to modify the surface of the chromium coated steel strip to improve the weldability of the edges in the mash seam welding process.

8. Method according to claim 7 wherein the plasma treatment is an atmospheric plasma treatment, a vacuum plasma treatment or a reducing plasma treatment.

9. Method according to claim 7 or 8 wherein the plasma treatment activates the chromium layer at the location of the one or both of the two parallel outside edges to facilitate the mash seam weldability of the overlapping outside edges of the blank.

10. Method according to any one of claims 7 to 9 wherein the plasma treatment removes part of the chromium oxide layer or the entire existing chromium oxide layer from the one or both of the two parallel outside edges of the blank to facilitate the mash seam weldability of the overlapping outside edges of the blank after forming the can body from the blank.

11. Method according to any one of the preceding claims wherein a first copper electrode wire is used between the upper wheel and the upper parallel edge and / or wherein a second copper electrode wire is used between the lower wheel and the lower parallel edge.

12. Method according to any one of the preceding claims wherein the chromium coating layer and a chromium oxide layer or chromium hydroxide layer on top of the chromium coating layer (tin-free steel) deposited from a trivalent chromium electrolyte is identical on both sides of the blank produced from a sheet of steel strip .

13. Apparatus for executing the method of any one of claims 1 to 12 capable to apply a welding force of at least 60 daN.

14. Apparatus according to claim 13 wherein the welding force is at most 150 daN, preferably wherein the welding force is between 60 and 90 daN.

15. Apparatus according to claim 13 or 14 provided with a plasma treatment device, wherein the plasma treatment device is an atmospheric plasma treatment or a vacuum plasma treatment for modifying the surface of the one or both of the two parallel outside edges of the blank to improve the weldability of the edges in the mash seam welding process, preferably wherein the mash seam welding immediately follows the plasma treatment to prevent recontamination of the edges to be welded together.

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