JPH0464932B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a welded can body made of a tin-plated surface-treated steel sheet and having a coating film on at least the inner surface of the can body. [Prior Art and its Problems] In recent years, welded cans have come into use as food or beverage cans. Tin-plated steel sheets (tinplate materials) and stain-free steel sheets are used as materials for these welded cans, but among these, the tin-plated steel sheets that are conventionally used for solder cans are generally used. Yes, the amount of tin plating is 2.8 because it is necessary to maintain the so-called solderability as a solder can.
g/m ² or more. However, recently, the price of tin has been rising due to resource reasons, and welded cans in particular do not require as much tin plating as solder cans to ensure weldability. Research into using tin-plated steel sheets as a material for welded cans is actively underway. However, even in the case of welded cans, reducing the amount of tin plating inevitably leads to deterioration in weldability and corrosion resistance, and due to these problems, conventionally even if the amount of plating was reduced, it was only 1.8/ It was thought that the maximum amount of adhesion was about ² m2. In other words, in order to ensure weldability in tin-plated steel sheets, the presence of at least a certain amount of pure tin is required, but in the case of can materials, an alloy of tin and iron is formed in the paint baking process performed before welding. The tin-iron alloy layer increases and the amount of tin decreases. Since such a tin-iron alloy layer has a higher melting point than tin itself, it reduces weldability. Therefore, when trying to reduce the amount of tin plating and make the plating thinner, the tin-iron alloy layer has a higher melting point than tin itself. The ratio of the layers increases, especially in the case of extremely thin tin plating, to the extent that the entire amount of tin is alloyed, resulting in a marked deterioration of weldability. Furthermore, such thin plating of tin impairs the adhesion of the coating film, and as a result, it also has an adverse effect on the corrosion resistance. [Means for Solving the Problems] In order to solve these conventional problems, the present inventors have conducted various studies on the coating structure of a welded can body having a tin plating layer, and as a result, By identifying the adhesion structure of the tin layer made of unalloyed pure tin, specifically by distributing the tin layer discontinuously on the steel plate surface,
Weldability is improved rather than reduced by reducing the amount of tin plating, and furthermore, by adopting a specific coating structure that includes such a tin layer, excellent corrosion resistance can be obtained despite the reduction in the amount of tin plating. I found out that it can be done. Based on this knowledge, the present invention has adopted a coating structure based on the specification of the tin layer of the can body and the combination of this and a post-treatment coating on the tin plating, thereby achieving excellent corrosion resistance and excellent corrosion resistance even though the tin plating is extremely thin. The present invention provides a welded can body that ensures weldability. Therefore, in the present invention, a coating film is formed on at least the surface of the surface-treated steel sheet that is to be the inner surface of the can body, excluding the edges on both sides, and the edges on both sides of the coated can material are overlapped. In a welded can body in which the can body is formed by pressure seam welding, the surface-treated steel plate forming the can body is coated with at least a tin-iron alloy layer, a tin layer and an adhesion amount of 2 to 20 mg/m on the base steel plate. It has a chromium/chromate coating consisting of metallic chromium ⁽² ) and chromium oxide with a coating amount of 3 to 20mg/ ^m2 in terms of chromium, in this order, and the tin layer in the area excluding the welded area has a tin coating of 50mg/m2 ^or more. and is discontinuously distributed over a range of 10 to 60% of the surface of the steel plate. Such the present invention ensures that the tin layer of the steel plate, that is, the tin that remains without being alloyed, is at ^least 50 mg/m2,
Moreover, instead of covering the entire surface of the steel plate with the tin layer,
Discontinuous (island-like or patchy) within 60% of the area
It is manufactured by welding in a state in which the tin plating is distributed over the tin plating, and as a result, extremely excellent weldability can be obtained while suppressing the amount of tin plating deposited as a whole. In conventional plating technology, it is common sense to ensure plating uniformity in all processes, not just tin plating, and various studies have been conducted to ensure this uniformity. Conventionally, when discussing the limits of thin plating for tinplate materials, the only question raised was whether uniformity of the plating film could be ensured by thinning the plating. In response to such conventional technical common sense, the present inventors have discovered that even if the amount of tin plating itself is reduced, a predetermined amount or more of tin (free tin) remains at the stage after coating film formation and before welding, and By forming this tin discontinuously at a predetermined distribution ratio on the surface of the steel plate, the weldability of subsequent welding is improved compared to the conventional tin plate, which has the same amount of plating, regardless of the reduction in the amount of tin plating. We have found that there is a significant improvement even compared to The following points can be considered as reasons for such improvement in weldability. That is, one of the criteria for evaluating weldability is the width of the welding effective current range (hereinafter referred to as ACR), and the first reason for this invention is that this ACR can be widened. When welding tin-plated steel sheets, it is necessary to apply a fairly large welding current to ensure welding strength, but conversely, if the welding current becomes too large, it will cause splash and reduce the quality of the can. The above ACR is the range of welding current that allows welding to a predetermined welding strength without causing any welding.
In this regard, in the present invention, since tin is discontinuously distributed in island-like shapes on the surface of the steel plate at a predetermined distribution ratio, it melts during welding to lower the contact resistance and weld with a relatively large welding current similar to that of ordinary tinplate. It is thought that the occurrence of splashes can be suppressed even in the case where the ACR is maintained properly. Second, it can prevent cracks from occurring in welded parts. Painted can stock before welding is not coated on both side edges, which will be the welded parts, but in reality, it is unavoidable that paint fumes will adhere to them. In conventional painted can bodies, the large amount of tin plating reduces the contact resistance between the plates, so a large current is required to obtain the desired heat generation state and the resulting welding strength. It is thought that a large current inevitably flows between the plate and the small-diameter cylinder roller, causing cracks to occur on its surface. That is, such a large current causes defects in the melting of the can material during welding, which appears as cracks in the welded area. In this regard, in the present invention, since the amount of tin plating is reduced and distributed in an island-like manner, the contact resistance between the plates is slightly higher, and in order to obtain the specified welding heat, a smaller current is required than with the tin plate. As a result, the occurrence of cracks on the welding surface can be appropriately suppressed. Reducing the amount of tin plating, especially with a coating structure in which the tin layer is discontinuously distributed as in the present invention, it is inevitable that the corrosion resistance will be inferior to a certain extent when only the tin layer is compared with a conventional tin-plated steel sheet. Through research conducted by the present inventors, it has been found that such corrosion resistance problems can be solved by forming a predetermined chromium/chromate film on the tin layer. Therefore, the present invention provides a coating structure in which a chromium/chromate coating consisting of metallic chromium and chromium oxide in a predetermined amount is formed on the tin layer. In addition, in the present invention, since the tin layer is distributed locally rather than over the entire surface of the steel plate, superior sulfur astenite resistance can be obtained compared to conventional tinplate materials. The present invention will be explained in detail below. The present invention has a structure in which the surface corresponding to the inner surface of the can body has at least a tin-iron alloy layer, a tin layer, and a chromium/chromate coating from the lower layer side, and a coating film is formed thereon. FIG. 1 shows the coating structure of the non-welded parts of the can body of the present invention, in which 1 is a base steel plate, 2 is a tin-iron alloy layer, 3 is a tin layer,
4 is a chromium/chromate film, and 5 is a coating film. The tin layer 3 needs to have a tin content of 50 mg/m ² or more, and if it is less than this, sufficient weldability cannot be ensured. Furthermore, this tin layer 3 needs to be distributed discontinuously on the steel plate surface, but the distribution ratio,
In other words, it accounts for 10 to 60% of the steel plate area.
regulated within the scope of. If this distribution ratio exceeds 60%, splash is likely to occur during welding, deteriorating weldability, and the amount of tin will inevitably increase, which is not preferable. On the other hand, if it is less than 10%, it is difficult to secure the above-mentioned tin amount of 50 mg/m ² or more, so the lower limit is set to 10%. A portion of the tin layer is alloyed when the steel sheet is heated, and therefore, in the stage before the coating process that involves heating the steel sheet, in other words, when it is simply a surface-treated steel sheet, the amount of tin is different from that in the painted can stock. Needless to say, it has. Therefore, in order to manufacture the can body of the present invention, it is necessary to secure the amount of tin in the pre-painting stage according to the target amount of tin in the coated can stock and the amount of tin alloyed during painting. There is. The chromium/chromate coating 4 is limited to a coating amount of 2 to 20 mg/m ² of metallic chromium and 3 to 20 mg/m ² of chromium oxide in terms of chromium. Although this chromium/chromate coating 4 is effective for corrosion resistance, it has the disadvantage that weldability deteriorates if the amount of coating is too large, and therefore it is necessary to set an upper limit as described above. Moreover, if it is less than the above-mentioned lower limit, a problem will arise in corrosion resistance. Furthermore, a base layer can be provided on the surface of the base steel plate, thereby improving corrosion resistance. As this base layer, a layer containing nickel is particularly preferable in order to ensure corrosion resistance. In this case, the nickel-containing layer with a nickel content of 5 to 200 mg/m ² is provided on the surface of the base steel plate and includes a nickel-plated layer, There are a nickel-containing layer formed by subjecting the nickel plating to annealing diffusion treatment and diffusing part or all of it into the base steel sheet, and a nickel-iron alloy plating layer provided on the surface of the base steel sheet. 6 in FIG. 1 shows a nickel-containing layer formed in the steel sheet by diffusion treatment after nickel plating. Note that the lower portion of the tin-iron alloy layer 2 is an alloy layer that necessarily contains nickel. The coated can stock, which is the raw material for the can bodies of the present invention, is made by forming the coating film 5 on at least the surface of the steel plate that is to become the inner surface of the can body of a surface-treated steel plate having a predetermined coating structure. This coating film 5 is not formed on both side edges of the steel plate which are to be welded parts. When the can body is used as a food can or a beverage can, this coating film 5 protects the can material from the contents to be filled into the can, and also prevents discoloration of the contents and deterioration of flavor due to elution of the can material. However, in the present invention, which aims to reduce the amount of tin plating deposited, it is preferable to give consideration to the coating film components. As this paint, a thermosetting epoxy phenol resin paint is suitable, especially a phenol resin containing 65% by weight or more of a resol type phenol resin formed from bisphenol A, and a bisphenol A resin having a number average molecular weight of 1,400 to 7,000. A suitable example is a thermosetting epoxy phenol resin paint containing epoxy resin in a weight ratio of 50/50 to 5/95. It is preferable to form the following. After the painted can stock is cut to the size of a can body blank, it is rolled up and the edges on both sides are overlapped.
The can body is manufactured by pressure seam welding the portions using a welding machine. The can body as a final product is further coated and corrected on the exposed iron surface of the welded joint and its vicinity, then a flange is applied to the end of the can body, a lid is seamed to one end of the can body, and the can body is further sealed as necessary. It can be obtained by applying correction coating to the inner surface of the body. The above-mentioned coating structure, especially the tin layer distributed discontinuously on the steel sheet surface, originates from the coating structure of the original surface-treated steel sheet,
As a method for obtaining the discontinuous (island-shaped) tin layer mentioned above, tin is applied to the steel plate (or base layer) during tin plating.
A method in which the reflow process after tin plating is performed under specific conditions. Alternatively, a method using a combination of both methods may be considered.
As a method for uniformly depositing the above-mentioned medium tin plating, there is a method in which a plating process in which the current density of electroplating is too high or too low is incorporated into a part of a series of plating processes, and a method is performed in a reflow process. There are methods such as stopping the usual immersion of the steel plate in the crack liquid and melting the tin in a non-uniform state by reflow. [Example] Welded can bodies as examples of the present invention and comparative examples were manufactured under the manufacturing conditions shown in Table 1, and the weldability and corrosion resistance of the cans were investigated. The results are also shown in Table 1. The test conditions for each characteristic test are as described in the notes to Table 1. Furthermore, the contents of paints (a) and (b) in Table 1 are as follows. ●Paint (a) 15 parts of resol type phenol resin obtained by reacting a mixed phenol of 75 parts of P-cresol and 25 parts of m-cresol with formaldehyde in the presence of an ammonia catalyst and a number average molecular weight of approximately 3000.
Solid content: 30%, viscosity (Fc ₄ #, 25°C): 40% by dissolving 85 parts of bisphenol A type epoxy resin in a mixed solvent consisting of alcohol, ketone, ester, or aromatic organic solvent.
Second epoxy/phenolic resin paint ●Paint (b) 20 parts of resol type phenolic resin obtained by reacting bisphenol A with formaldehyde in the presence of an ammonia catalyst and a number average molecular weight of approx.
Solid content obtained by dissolving 80 parts of bisphenol A type epoxy resin of 3410 in a mixed solvent: 29
%, viscosity (Fc ₄ #, 25℃) 40 seconds thermosetting epoxy phenolic paint

【table】

【table】

〔Effect of the invention〕

According to the present invention described above, it is possible to provide a welded can body that can achieve excellent weldability in can manufacturing welding while reducing the amount of tin plating deposited compared to conventional tin-plated steel sheets, and also has excellent corrosion resistance. This has the effect of providing excellent quality welded cans at low cost.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing an example of the coating structure of the welded can body of the present invention.

Claims (1)

[Claims] 1. A coating film is formed on at least the surface of the surface-treated steel sheet that is to be the inner surface of the can body, excluding the edges on both sides, and the edges on both sides of the coated can material are overlapped. In a welded can body formed by pressurized seam welding, the surface-treated steel sheet forming the can body has at least a tin-iron alloy layer, a tin layer, and an adhesion amount of 2 to 20 mg/min on the base steel sheet. It has a chromium/chromate coating consisting of metallic chromium ( ^m2 ) and chromium oxide (3 to 20 mg/ ^m2 in terms of chromium) in this order, and the tin layer in the area excluding the weld has a tin deposit of 50 mg/ ^m2. A welded can body having the above properties and distributed over a range of 10 to 60% of the surface of the steel plate. 2. The welded can body according to claim 1, which has a base layer containing 5 to 200 mg/m ² of nickel on the surface of the base steel plate.