EP0184115B1

EP0184115B1 - Surface-treated steel strip having improved weldability and process for making

Info

Publication number: EP0184115B1
Application number: EP85114989A
Authority: EP
Inventors: Hisatada Nakakouji; Kazuo Mochizuki; Toshio Ichida
Original assignee: Kawasaki Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1984-11-29
Filing date: 1985-11-26
Publication date: 1989-02-22
Also published as: AU5036685A; CN85109305A; CA1273600A; ZA859071B; EP0184115A1; DE3568356D1; KR860004165A; JPH052744B2; CN1006694B; JPS61130500A; AU559801B2; KR900001829B1

Description

Background of the invention

This invention relates to surface-treated steel strips or sheets having improved weldability, particularly improved seam weldability and sufficient corrosion resistance for use as can forming stock, and a process for making such surface-treated steel strips.
Among food can-forming materials there have been most widely used tin-coated steel strips generally called tin plates. In order to join the mating edges of a can body, conventional soldering techniques were previously used. Because of the toxicity of lead contained in conventional solder, pure tin solder has recently become prevalent. The pure tin solder, however, has a technical problem in making a joint because of inferior wetability during the soldering process and is so expensive as to create the economic problem of increased manufacture cost.
On the other hand, in recent years, food containers have enjoyed the development of inexpensive, competitive materials such as polyethylene, aluminum, glass, processed paper and the like, Despite their significantly improved corrosion resistance among other advantages, tin plate cans having expensive tin thickly coated thereon to a coating weight of as great as 2.8 to 11.2 g/m² require a relatively high cost of manufacture and have encountered severe competition.
In order to overcome the above-described drawbacks of tinplate cans, electric resistance welding of can bodies has recently replaced the conventional soldering technique and become widespread. There is a continuing need for can-forming steel compatible with electric resistance welding.
In addition to the tinplate discussed above, tin-free steel of chromium type is another typical example of conventional cam-forming steel (cf e.g. EP-A-121817). The tin-free steel is prepared by carrying out an electrolytic chromate treatment on steel to form metallic chromium and hydrated chromium oxide layers on the surface. Since the relatively thick hydrated chromium oxide layer at the top has a relatively high electric resistance, the chromated steel is ineffectively welded to form a weld of insufficient strength and thus unsuitable as welded can-forming steel despite its economic advantage.
Since other can-forming materials are also inadequate as welded can-forming material, a variety of proposals have been made. One example is nickel-plated steel, typically "Nickel-Lite" announced by National Steel Corporation of the U.S. which is prepared by plating a steel strip with nickel to a thickness of about 0.5 g/m² followed by a conventional chromate treatment. Inferior adhesion of lacquer and inferior weldability in high speed welding at 30 m/min. or higher have limited the spread of this nickel-plated steel.
Another example is "Tin Alloy" announced by Jones & Laughlin Steel Corporation of the U.S. This is prepared by thinly coating a steel strip with tin to a thickness of about 0.6 g/m² and effecting tin reflow or flow melting followed by a conventional chromate treatment. Unfortunately, rust resistance, lacquer adhesion and weldability are insufficient.
In general, can-forming steel sheets intended for electric resistance welding are required to exhibit improved weldability and corrosion resistance after lacquering. These requirements will be explained in detail. There must be an available welding electric current range within which a weld zone having sufficient weld strength is provided at the end of welding without any weld defects such as so-called "splashes". Since welded can are filled with foodstuffs after lacquer coating, the underlying steel must have sufficient adhesion to lacquer to take full advantage of the corrosion prevention of the lacquer film. Furthermore, despite defects unavoidably occurring in a lacquer film, the improved corrosion resistance of the underlying steel itself must prevent corrosion from proceeding.

Summary'of the invention

It is, therefore, an object of the present invention to provide a novel and improved surface-treated steel strip which can be seam welded into cans without the above-mentioned drawbacks and has improved weldability, corrosion resistance after lacquering, and lacquer adhesion.
Another object of the present invention is to provide a process for making such an improved can-forming stock in a simple and inexpensive manner.
According to a first aspect of the present invention, there is provided a surface-treated steel strip having improved weldability, comprising

a steel strip,
a metallic chromium layer deposited on the steel strip to a coating weight of 50 to 150 mg/m²,
a metallic tin layer deposited on the chromium layer to a coating weight of 50 to 400 mg/m², and
a chromate coating layer deposited on the tin layer and consisting essentially of 3 to 15 mg/m² of metallic chromium and 3 to 15 mg/m² of hydrated chromium oxide calculated as elemental chromium.

According to a second aspect of the present invention, there is provided a process for preparing a surface-treated steel strip having improved weldability. The surface-treated steel strip particularly suitable as can-forming stock is prepared by

cleaning at least one surface of a steel strip,
subjecting the cleaned strip surface to chromium electrodeposition to a coating weight of 50 to 150 mg/m2,
rinsing the chromium-deposited strip with water,
subjecting the strip to tin electrodeposition in an acidic bath containing 1.5 to 10 grams per liter of Sn ion at a pH of lower than 0.5, the tin being deposited to a coating weight of 50 to 400 mg/m²,
rinsing the tin-deposited strip with water, and
subjecting the strip to chromate treatment in a chromate bath containing up to 50 grams per liter of Cr0₃ and at least one member selected from the group consisting of sulfuric acid, sulfates, and fluorides at a current density of at least 10 A/dm², thereby forming a chromate coating layer consisting essentially of 3 to 15 mg/m² of metallic chromium and 3 to 15 mg/m² of hydrated chromium oxide calculated as elemental chromium.

In one preferred embodiment of the present invention, the acidic tin-plating bath contains hydrochloric acid and/or sulfuric acid and an insoluble anode is used to carry out the tin electrodeposition.

Brief description of the drawings

Figure 1 is a diagram showing the tin content in an alloy resulting from a baking treatment (210°C, 20 min.) as a function of the amount of underlying metallic chromium; and
Figure 2 is a diagram showing the corrosion resistance after lacquering as a function of the amount of metallic chromium on the tin layer and the amount of hydrated chromium oxide calculated as elemental chromium.

Detailed description of the invention

Making investigations on the weldability of various can-forming stock materials by electric resistance welding, particularly wire seam welding as represented by the Soudronic welding process which is now widely and rapidly accepted as a can-forming welding process, we have found that the presence of metallic tin ensures satisfactory seam welding performance.
More specifically, metallic tin which has a relatively low melting point is readily melted during welding and spread under the welding pressure to increase the contact area between steel pieces and to facilitate mutual fusion of the steel pieces. "Splashes" due to local concentration of welding current are unlikely to occur. Formation of a strong welding joint provides a wide range of available welding current. Conventional #25 tin plate has a wide available welding current range because it contains about 2.2 g/m² of metallic tin.
A continuing study about the relationship of weldability and metallic tin amount has revealed that the presence of metallic tin in amounts of at least 50 mg/m², preferably at least 100 mg/m² provides a practically satisfactory wide range of available welding current even in high speed welding at 40 to 60 m/min.
Although it appears that applying at least 50 mg/m² of tin plating onto the steel surface results in satisfactory weldability, a problem arises because actually such plated strips are coated with lacquer before welding. A baking and hardening treatment is done after lacquer coating and such baking causes the tin to be alloyed with iron of the steel substrate. The usual baking temperature is 170 to 220°C and an iron-tin alloy in the form of FeSn₂ forms at the temperature. Since the resultant FeSn₂ alloy has a high melting point, the weldability improving effect the metallic tin inherently possesses is lost as a result of alloying. Then, to reserve satisfactory weldability, tin must be plated in an extra amount by taking into account the loss of tin by alloying during the baking treatment, resulting in an economic disadvantage.
Making investigations to prevent alloying between tin and substrate iron by baking treatment, we have found that the formation of an iron-tin alloy can be substantially controlled by interposing metallic chromium between tin and substrate iron. The content of tin in an alloy formed by a baking treatment at 210°C for 20 minutes is plotted in Figure 1 as a function of the amount of the underlying metallic chromium. As apparent from data plotted in Figure 1, the intervening metallic chromium is significantly effective in preventing an iron-tin alloy from forming during the baking. The metallic chromium which itself is a highly corrosion resistant metal has doulble functions of preventing formation of an iron-tin alloy and improving corrosion resistance at the same time.
By subjecting a steel strip to chromium plating and then to tin plating, not only expensive tin can be effectively utilized, but also corrosion resistance is improved.
Although forming metallic tin as an uppermost layer provides good weldability, lacquer baking forms tin oxide at the surface, resulting in insufficient lacquer adherence and hence, poor corrosion resistance after lacquering. When cans are filled with such contents as foods containing sulfur-bearing amino acids, so-called sulfide stain occurs, that is, tin sulfide having a black color forms to blacken the can inner surface. Prior art tinplate is cathodically treated in an electrolytic solution of sodium dichromate to form a coating of hydrated chromium oxides to mitigate the sulfide stain. The hydrated chromium oxide coating, however, does not perform well.
Unexpectedly, we have found that by forming a coating of double layers of metallic chromium and hydrated chromium oxide, not only lacquer adherence and corrosion resistance after lacquering are significantly improved, but sulfide stain is substantially eliminated. Particularly, metallic chromium is greatly effective in improving lacquer adherence and sulfide staining resistance. A significant effect is achieved with metallic chromium in as small amounts as 3 mg/m² or more. Corrosion resistance after lacquering is also improved.
Thus, the surface-treated steel strip of the present invention should have as an uppermost layer a chromate coating layer consisting essentially of 3 to 15 mg/m² of metallic chromium and 3 to 15 mg/m² of hydrated chromium oxide calculated as elemental chromium.
Since the steel strip of the present invention is intended for use in forming food cans, it is always coated with lacquer on one surface which becomes the inner surface of a can made therefrom. Most important among the requisite can properties are corrosion resistance after lacquering and sulfide staining resistance. Metallic chromium on the tin layer is essential to provide such resistances. Metallic chromium must be deposited in amounts of at least 3 mg/m^2. An excessive amount of metallic chromium can interfere with weldability as it is oxidized to form chromium oxide at elevated temperatures during welding. The upper limit of 15 mg/m² is thus imposed to the metallic chromium amount.
The hydrated chromium oxide is effective in improving lacquer adherence and corrosion resistance. Since hydrated chromium oxide itself is a high electric resistance material, it can interfere with weldability when present in excessive amounts. For this reason, the hydrated chromium oxide is limited to the range from 3 to 15 mg/m² calculated as elemental chromium.
The process for making a surface-treated steel strip according to the present invention will now be described in detail.
At the outset, a starting steel strip which may be one commonly used as the starting steel strip for tinplate or tin-free steel (TFS) strips is cleaned by any conventional techniques, for example, electrolytic degreasing and pickling.
The cleaned surface of steel strip is subjected to electrodeposition of metallic chromium to a coating weight of 50 to 150 mg/m². The underlying metallic chromium is plated for the purposes of preventing formation of an iron-tin alloy during lacquer baking and improving corrosion resistance. Metallic chromium is effective in preventing iron-tin alloy even in as small amounts as 5 mg/m^2. However, metallic chromium is desirably deposited in a coating weight of at least 50 mg/m² in order to effectively prevent alloying of expensive tin. The larger the amount of metallic chromium, the greater are the iron-tin alloying prevention and the corrosion resistance. Metallic chromium amounts in excess of 150 mg/m² are uneconomic because the iron-tin alloying prevention and the corrosion resistance improvement reach plateau at such a level. Excessive metallic chromium is also undesirable because such a thick chromium plating is liable to cracks due to electrodeposition stress.
Metallic chromium may be deposited on the steel strip by any well-known methods, for example, by a well-known electrodeposition method as by carrying out cathodic electrolysis in an aqueous solution containing a major proportion of chromic acid anhydride and an additive amount of sulfate (SO₄ ^--) and fluoride (F-). Upon completion of chromium electrodeposition, the steel strip is rinsed with water.
Next, tin plating is applied onto the metallic chromium layer on the steel strip. When tin electrodeposition is performed on the electrodeposited metallic chromium, a conventional tin electrodeposition process fails to achieve satisfactory tin plating because hydrated chromium oxide is present on the metallic chromium surface. Illustratively, the ordinary industrial chromium electrodeposition is to electrochemically reduce hexavelent chromium ion (Cr") to metallic chromium. Since Crs+ ion is reduced to metallic Cr through the hydrated oxide of trivalent chromium as is well known in the art, hydrated chromium oxide is always present on the plating surface. This hydrated chromium oxide interferes with tin electrodeposition. When tin electrodeposition is effected in the presence of hydrated chromium oxide on the metallic chromium surface, there is obtained a deposit having poor bond strength.
In order to effectively electrodeposit tin, the hydrated chromium oxide must be previously removed from the electrodeposited chromium surface. Removal of hydrated chromium oxide may be effected by a variety of well-known methods, for example, by dissolving away in a hot aqueous alkaline solution or by dissolving away in an aqueous solution such as sodium hydroxide solution, phosphate buffer solution, and borate buffer solution through anodic electrolysis.
In the process of dissolving away in hot aqueous alkaline solution, chromium oxide which is insoluble in the alkaline solution is left to impede satisfactory tin deposition. In the anodic electrolysis, metallic chromium is preferentially dissolved and hydrated chromium oxide is retained until the metallic chromium has been dissolved away. Thus the anodic electrolysis cannot be applied to the production of the Sn/Cr double layer deposited steel strip according to the present invention.
Also known as a special tin plating pretreatment is a method for depositing various metals onto the surface of chromium and chromium alloys as disclosed in Japanese Patent Publication No. 33-1455. The surface activating treatment with caustic alkali as disclosed therein fails to provide a tin deposit having a practically acceptable bond strength.
Another method is disclosed in Japanese Patent Publication No. 48-35136 entitled "Chromium-tin double layer plating method". The immersion in a chromium plating solution as disclosed therein cannot completely remove the hydrated chromium oxide from the electrodeposited chromium surface, failing to achieve a satisfactory bond strength or adherence. This method is difficult to carry out in a commercial installation because it requires a striking treatment at a very high current density.
A further method is disclosed in Japanese Patent Application Kokai No. 60-190597 entitled "Surface-treated steel strip for use in welded cans and method for making". There are disclosed a pretreatment by cathodic electrolysis in an acid solution at pH 0.5 to 2 and tin electrodeposition from a tin plating bath having a low tin ion concentration and pH 0.5 to 3. Removal of hydrated chromium oxide is still insufficient to ensure satisfactory tin deposition.
Making a series of experiments to search for effective removal of the hydrated chromium oxide, we have found that the use of a strongly acidic tin plating bath containing 1.5 to 10 grams per liter of tin ion and having a pH of lower than 0.5 can effectively remove the hydrated chromium oxide and conduct tin deposition at the same time.
The process in such a specific bath according to the present invention carries out tin deposition and generates a great volume of active H₂ gas at the same time. Evolution of a great volume of active hydrogen gas on the deposited chromium surface in a strongly acidic solution can completely remove the hydrated chromium oxide which could not otherwise completely be removed from the deposited chromium surface. Tin deposition proceeds concurrently. As a result, there is deposited a tin layer having a high bond strength to the chromium layer.
The tin electrodeposition may be carried out at a bath temperature of 25 to 65°C and a cathodic current density of 15 to 50 amperes per square decimeter (A/dm²).
The tin plating bath used herein contains tin ion at a concentration of 1.5 to 10 grams per liter. Tin ion concentrations of less than 1.5 g/I do not allow tin to deposit on the chromium layer whereas high concentrations in excess of 10 g/I result in a coarse deposit of powder or dendrite structure having too poor appearance to meet practical application.
The tin plating bath should have a hydrogen ion concentration in excess of 0.32 mols per liter, that is, lower than 0.5 in pH. Baths having a hydrogen ion concentration of 0.32 mol/I or lower, that is, at least pH 0.5 are insufficient to impart a strongly acidic environment necessary to remove the hydrated chromium oxide and to generate a great volume of active hydrogen gas, so that the hydrated chromium oxide is not completely removed and a satisfactory tin deposit is not obtained. It is preferable to add hydrochloric acid and/or sulfuric acid to the acidic bath to provide a pH value of lower than 0.5.
When a soluble tin anode is used in the strongly acidic tin plating bath according to the present invention, the tin deposition efficiency at the cathode (steel strip) is lower than the tin dissolving efficiency at the anode (tin anode) because of the high acidity. If tin electrodeposition is continued under such conditions for an extended period of time, the tin ion concentration of the bath is gradually increased to eventually exceed the critical level of 10 g/I, resulting in a coarse deposit.
The source for supplying tin ion to the tin plating bath according to the present invention may be any tin compounds such as tin sulfate, tin chloride, tin borofluoride, tin pyrophosphate, tin fluoride, and similar tin compounds. The acid added to the tin plating bath may be any one or mixtures of inorganic acids such as sulfuric acid, hydrochloric acid, hydrofluoric acid, borofluoric acid, etc. and organic acids such as phonolsulfonic acid, cresolsulfonic acid, etc.
When a sulfuric acid-acidified tin plating bath is prepared by using tin sulfate as the tin ion source and sulfuric acid as the acid, an insoluble anode formed of Pb, Pb alloys, Pt, etc. may conveniently be used to eliminate the above-mentioned problem of rising tin ion concentration. Jhe tin ion may be made up simply by dissolving tin sulfate or tin oxide or finely divided metallic tin into the plating solution.
Since tin is electrodeposited for the purpose of achieving excellent weldability, the amount of tin deposited may be determined in conjunction with the underlying metallic chromium amount such that at least 50 mg/m², preferably at least 100 mg/m² of metallic tin is left at the end of lacquer baking. Excessive amounts of tin deposited have no detrimental effect, but the tin coating weight may desirably be up to 400 mg/m² for economic reasons. The preferred amount of tin deposited thus ranges from 50 to 400 mg/m².
The tin plating bath used in the practice of the present process may further contain a surface-active agent having an oxyethylene chain -[CH2CH20-]n-, for example, ethylene glycol, polyethylene glycol, a polyethylene glycol alkyl ether such as ethylene glycol monomethyl ether, etc., an aromatic ethylene oxide such as ethoxylated a-naphthol, etc., and a polyethylene glycol-fatty acid ester in amounts of 0.1 to 10 grams per liter. The addition of such a surface-active agent results in a tin electrodeposit having excellent appearance and tone on the chromium electrodeposit. Concentrations of oxyethylene chain-bearing surface-active agent of lower than 0.1 g/I are too low to improve the appearance and tone whereas the effect is saturated beyond the concentration of 10 g/I. The use of such an expensive surface-active agent in an unnecessarily excessive amount is, of course, not economical. Since the oxyethylene chain-bearing surface-active agent is expensive, it is desired to add it only when a particularly good appearance and tone is required. The tin deposited strip is rinsed with water to be ready for subsequent treatment.
According to the present invention, a chromate coating layer consisting essentially of metallic chromium and hydrated chromium oxide is further deposited on the tin layer for the purpose of improving lacquer adherence, corrosion resistance after lacquering, and sulfide staining resistance. The amount of metallic chromium should be at least 3 mg/m² to achieve significant improvements in lacquer adherence, corrosion resistance after lacquering, and sulfide staining resistance. Generally, the larger the amount of metallic chromium, the greater are improved the lacquer adherence, corrosion resistance after lacquering, and sulfide staining resistance. Too large amounts of metallic chromium in excess of 15 mg/m² tend to be oxidized at elevated temperatures during subsequent welding into chromium oxide to detract from weldability, and are thus undesirable.
To provide sufficient lacquer adherence and corrosion resistance after lacquering, the amount of hydrated chromium oxide should be at least 3 mg/m² calculated as elemental chromium. Since the hydrated chromium oxide itself is a high electric resistance matrial, it impedes weldability when present in large amounts. The amount of hydrated chromium oxide should preferably be limited up to 15 mg/m² calculated as elemental Cr.
The metallic chromium and hydrated chromium oxide may be formed by carrying out cathodic electrolysis in a bath containing at least 50 grams per liter of Cr0₃ and an effective amount of one member selected from sulfuric acid, sulfate salts, fluorides, and mixtures thereof at a current density of at least 10 A/dm². The metallic Cr and hydrated Cr oxide may be formed any desired combination by properly selecting cathodic electrolysis conditions including current density, bath temperature, bath concentration, and the like.
Chromate baths containing CrO₃ in concentrations of higher than 50 g/I undesirably attack and etch the tin layer whereas low current densities of lower than 10 A/dm² are difficult to produce metallic chromium.

Examples

Example 1

A steel stock strip generally intended for conventional tinplate or TFS was electrolytically degreased, pickled, and then subjected to chromium electrodeposition in a bath containing 200 g/I of Cr0₃ and 2 g/I of H₂SO₄ at a temperature of 50°C and a current density of 40 A/dm². The Cr deposited strip was rinsed with water and then subjected to tin electrodeposition in a tin plating bath containing 4 g/I of SnCl₂ 2H₂0 and adjusted to pH 0.3 with HCI at a temperature of 50°C and a current density of 20 A/dm^2. The Sn deposited strip was rinsed with water and then subjected to chromate treatment in a bath containing 20 g/I of CrO₃ and 0.2 g/I of H₂SO₄ at a temperature of 50°C and a current density of 15 A/dm², thereby forming metallic chromium and hydrated chromium oxide as a topcoat.

Example 2

A steel stock strip generally intended for conventional tinplate or TFS was electrolytically degreased, pickled, and then subjected to chromium electrodeposition in a bath containing 180 g/I of Cr0₃, 6 g/I of Na₂SiF₆, and 0.75 g/I of H₂S0₄ at a temperature of 50°C and a current density of 50 A/dm^l. The Cr deposited strip was rinsed with water and then subjected to tin electrodeposition in a tin plating bath containing 6 g/I of SnS0₄ and adjusted to pH 0.48 with H₂S0₄ at a temperature of 40°C and a current density of 30 A/dm². The Sn deposited strip was rinsed with water and then subjected to chromate treatment in a bath containing 15 g/1 of Cr0₃ and 1.5 g/I of NH₄F at a temperature of 45°C and a current density of 15 A/dm², thereby forming metallic chromium and hydrated chromium oxide as a topcoat.

Example 3

A steel stock strip generally intended for conventional tinplate or TFS was electrolytically degreased, pickled, and then subjected to chromium electrodeposition in a bath containing 150 g/I of Cr0₃ and 7.5 g/I of NaF at a temperature of 50°C and a current density of 35 A/dm². The Cr deposited strip was rinsed with water and then subjected to tin electrodeposition in a tin plating bath containing 8 g/I of SnS0₄ and adjusted to pH 0.2 with H₂SO₄ at a temperature of 45°C and a current density of 25 A/dm² using an insoluble anode of Pb-Sn alloy. The Sn deposited strip was rinsed with water and then subjected to chromate treatment in a bath containing 15 g/I of Cr0₃ and 0.12 g/I of H₂S0₄ at a temperature of 50°C and a current density of 20 A/dm², thereby forming metallic chromium and hydrated chromium oxide as a topcoat.

Example 4

A steel stock strip generally intended for conventional tinplate or TFS was electrolytically degreased, pickled, and then subjected to chromium electrodeposition in a bath containing 250 g/I of CrO₃, 5 g/I of Na₂SiF₆, and 1.5 g/I of H₂SO₄ at a temperature of 55°C and a current density of 50 A/dm². The Cr deposited strip was rinsed with water and then subjected to tin electrodeposition in a tin plating bath containing 4 g/I of SnS0₄ and adjusted to pH 0.40 with H₂SO₄ at a temperature of 30°C and a current density of 20 A/dm² using an insoluble anode of platinum. The Sn deposited strip was rinsed with water and then subjected to chromate treatment in a bath containing 30 g/I of Cr0₃ and 0.27 g/I of H2S04 at a temperature of 40°C and a current density of 25 A/dm², thereby forming metallic chromium and hydrated chromium oxide as a topcoat.

Example 5

A steel stock strip generally intended for conventional tinplate or TFS was electrolytically degreased, pickled, and then subjected to chromium electrodeposition in a bath containing 200 g/I of Cr0₃, 5.5 g/I of Na₂SiF₆, and 1.0 g/I of H₂S0₄ at a temperature of 40°C and a current density of 55 A/dm². The Cr deposited strip was rinsed with water and then subjected to tin electrodeposition in a tin plating bath containing 5 g/I of SnS0₄ and adjusted to pH 0.45 with H₂S0₄ at a temperature of 35°C and a current density of 40 A/dm2. The Sn deposited strip was rinsed with water and then subjected to chromate treatment in a bath containing 20 g/I of CrO₃ and 3 g/I of NH₄F at a temperature of 45°C and a current density of 30 A/dm2, thereby forming metallic chronium and hydrated chromium oxide as a topcoat.

Example 6

A steel stock strip generally intended for conventional tinplate or TFS was electrolytically degreased, pickled, and then subjected to chromium electrodeposition in a bath containing 180 g/I of Cr0₃, 6 g/I of Na₂SiF₆, and 0.75 g/I of H₂SO₄ at a temperature of 55°C and a current density of 50 A/dm². The Cr deposited strip was rinsed with water and then subjected to tin electrodeposition in a tin plating bath containing 6 g/I of SnS0₄ and adjusted to pH 0.46 with H₂S0₄ at a temperature of 35°C and a current density of 35 A/dm². The Sn deposited strip was rinsed with water and then subjected to chromate treatment in a bath containing 24 g/I of Cr0₃ and 2 g/I of NaF at a temperature of 40°C and a current density of 35 A/dm², thereby forming metallic chromium and hydrated chromium oxide as a topcoat.

Comparative Example 1

Cr electrodeposition and Sn electrodeposition were carried out under the same conditions as in Example 1. The Sn deposited strip was rinsed with water and then only hydrated chromium oxide was formed thereon in a bath containing 30 g/I of Na₂Cr₂O₇. 2H₂0 at a temperature of 40°C and a current density of 5 A/dm².

Comparative Example 2

Cr electrodeposition and Sn electrodeposition were carried out under the same conditions as in Example 2. The Sn deposited strip was rinsed with water and then subjected to chromate treatment in a bath containing 40 g/I of Cr0₃ and 0.1 g/I of H₂S0₄ at a temperature of 50°C and a current density of 3 Aldm² to form hydrated chromium oxide containing a minor amount of metallic chromium.

Comparative Example 3

Cr electrodeposition and Sn electrodeposition were carried out under the same conditions as in Example 5. The Sn deposited strip was rinsed with water and then subjected to chromate treatment in a bath containing 60 g/I of CrO₃ and 3 g/I of NaF at a temperature of 45°C and a current density of 40 A/dm² to form metallic chromium and hydrated chromium oxide.
The thus treated steel strip samples were evaluated for various properties by the following procedures.

Weldability

Sample pieces overlapped a distance of 0.4 mm were seam welded by means of an electric resistance seam welding machine using a copper wire as the electrode at a welding speed of 40 m/min. and a welding force or can body joining force of 40 kg - f to determine the available welding current range (ACR) within which a sound weld having a sufficient strength was accomplished without generating a splash.
Before the welding, the sample was subjected to a heat treatment at 210°C for 20 minutes which was the. expected lacquer baking.

Lacquer adherence

Two sample pieces (5 mm x 100 mm) were coated with an epoxy-phenol lacquer to a coating weight of 50 mg/dm², overlapped a distance of 90 mm from the edge, and heat bonded to each other with a nylon adhesive. The unjoined portions were outwardly bent over an angle of 90° to form a T shape and oppositely pulled at a pulling speed of 200 mm/min. to determine tensile strength. The tensile strength at which the pieces were peeled off is evaluated as lacquer adherence and called T-peel strength as expressed in kg/5 mm.

Corrosion resistance after lacquering

A sample piece (40 mmx80 mm) was coated with an epoxy-phenol lacquer to a coating weight of 50 mg/dm², inserted into boiled tomato juice in a glass container such that the lower half of the sample was immersed, and held in the closed container at 55°C for 18 days. The sample was taken out of the container and observed for blister occurrence. The criterion of evaluation is shown below.

0: No blister occurred.
0: A few blisters occurred.
X: Numerous blisters occurred.

Sulfide staining resistance

A sample piece was coated with an epoxy-phenol lacquer to a coating weight of 50 mg/dm², drawn by a distance of 5 mm by means of an Erichsen machine, immersed in a test solution containing 1% Na₂S and adjusted to pH 7 with lactic acid (pH 3.5), and heat treated at 110°C for 60 minutes. The processed and unprocessed (flat) portions of the sample were observed for sulfide staining. The criterion of evaluation is shown below.

0: No staining.
1: Slight staining in only the processed portion.
2: Moderate staining in both the. processed and flat portions.
3: Severe staining in both the processed and flat portions.

All the samples of Examples and Comparative Examples were evaluated for the above properties. The results are shown in Table 1. The data in Table 1 shows that the steel strips treated according to the present invention are significantly improved in weldability, lacquer adherence, corrosion resistance after lacquering, and sulfide staining resistance.
Figure 1 graphically illustrates the tin content in an alloy resulting from a baking treatment (210°C, 20 min.) as a function of the amount of underlying metallic chromium. Figure 2 graphically illustrates the corrosion resistance after lacquering as a function of the amount of metallic chromium on the tin layer and the amount of hydrated chromium oxide calculated as elemental chromium.

Benefits of the invention

The surface-treated steel strip of the present invention which has a surface coating consisting essentially of a Cr layer, Sn layer, metallic Cr layei, and hydrated Cr layer in specific coating weights displays improved weldability, lacquer adherence, corrosion resistance after lacquering, and sulfide staining resistance.
The present process enables surface treatments of a steel strip such that the Cr layer, Sn layer, metallic Cr layer, and hydrated Cr oxide layer are formed to the specific coating weights, and thus ensures the efficient production of surface-treated steel strips meeting the requirements for can-forming stock material.

Claims

1. A surface-treated steel strip having improved weldability, comprising

a steel strip,

a metallic chromium layer deposited on the steel strip to a coating weight of 50 to 150 mg/m²,

a metallic tin layer deposited on the chromium layer to a coating weight of 50 to 400 mg/m², and

a chromate coating layer deposited on the tin layer and consisting essentially of 3 to 15 mg/m of metallic chromium and 3 to 15 mg/m² of hydrated chromium oxide calculated as elemental chromium.

2. A process for preparing a surface-treated steel strip having improved weldability, comprising the steps of

cleaning at least one surface of a steel strip,

subjecting the cleaned strip surface to chromium electrodeposition to a coating weight of 50 to 150 mg/m2,

rinsing the chromium-deposited strip with water,

subjecting the strip to tin electrodeposition in an acidic bath containing 1.5 to 10 grams per liter of Sn ion at a pH of lower than 0.5, the tin being deposited to a coating weight of 50 to 400 mg/m²,

rinsing the tin-deposited strip with water, and

subjecting the strip to chromate treatment in a chromate bath containing up to 50 grams per liter of Cr0₃ and at least one member selected from the group consisting of sulfuric acid, sulfates, and fluorides at a current density of at least 10 A/dm², thereby forming a chromate coating layer consisting essentially of 3 to 15 mg/m² of metallic chromium and 3 to 15 mg/m² of hydrated chromium oxide calculated as elemental chromium.

3. A process for preparing a surface-treated steel strip having improved weldability as set forth in claim 2 wherein the acidic tin-plating bath contains Sn ion and sulfuric acid and an insoluble anode is used to carry out the tin electrodeposition.