EP1518944B1

EP1518944B1 - Tin-plated steel plate and method for production thereof

Info

Publication number: EP1518944B1
Application number: EP03733259.0A
Authority: EP
Inventors: Hisatada I. P. D. Jfe Steel Corporation NAKAKOJI; Tomofumi I. P. D. Jfe Steel Corporation SHIGEKUNI; Takumi I. P. D. Jfe Steel Corporation TANAKA
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2002-06-05
Filing date: 2003-06-03
Publication date: 2014-05-14
Anticipated expiration: 2023-06-03
Also published as: WO2003104528A1; EP1518944A4; EP1518944A1

Description

FIELD OF THE INVENTION

The present invention relates to tin-plated steel sheets and methods for producing the same. More particularly, the present invention relates to a tin-plated steel sheet which requires solderability and a method for producing the same.

DESCRIPTION OF RELATED ARTS

Pb-Sn alloy solder has been used for bonding in household electric appliances, such as audio products and personal computers. However, because of the fact that Pb in the alloy solder is harmful to the human body, use of Pb has been restricted and conversion to Pb-free solder has been in progress. In the chassis and component cases of household electric appliances, Pb-Sn alloy-plated steel sheets which are suitable for conventional Pb-Sn soldering have been used. In order to meet the restriction of the use of Pb, novel steel sheets having excellent Pb-free solderability without using Pb have been demanded.
Furthermore, the surfaces of the conventional Pb-Sn alloy-plated steel sheets are subjected to chromate treatment. However, the consumer-electronics industry is moving toward prevention of the use of hazardous hexavalent chromium, and non-use of chromate treatment is required for newly produced steel sheets to be soldered.
Steel sheets to be subjected to Pb-free soldering are disclosed, for example, in Japanese Examined Patent Application Publications Nos. 6-99837 and 6-33466 , in which films mainly composed of Sn-Zn, Zn-Ni, Sn-Ni, and Fe-Ni are formed on a steel sheet, and a chromate film is further formed thereon.
However, the steel sheet disclosed in each of the Patent Application Publications has poor Pb-free solderability because of the use of Zn and is not acceptable to the consumer-electronics industry because of the presence of the chromate film.
Japanese Unexamined Patent Application Publication No. 2001-32085 discloses a surface-treated steel sheet in which a Cr-free post-treated film containing Si is formed on an Sn or Sn alloy-plating film. However, since an Fe-Sn alloy layer is not interposed between the steel sheet and the Sn-plating layer, adhesion between the steel sheet and the Sn-plating layer is poor, and Pb-free solderability is also unsatisfactory.
As the technique relating to conversion treatment which can replace chromate treatment for tin-plated steel sheets used for cans, for example, a surface treatment method for a tin-plated steel sheet is disclosed in Japanese Examined Patent Application Publication No. 55-24516 , in which a Cr-free chemical conversion coating is formed on a tin-plated steel sheet by DC electrolysis using the tin-plated steel sheet as a cathode in a phosphoric acid-based solution. Japanese Examined Patent Application Publication No. 1-32308 also discloses a tin electroplated sheet used for seamless cans, in which a Cr-free chemical conversion coating in which P alone or P and Al are incorporated is formed on the surface of the tin-plated sheet.
However, when properties, such as paint adhesion and corrosion resistance, are comprehensively evaluated, the chemical conversion coating in each of the Patent Application Publications described above is unsatisfactory compared to the conventional chromate film formed using a solution containing dichromic acid or chromic acid. Other documents that describe tin-plated steel sheets include EP1243668 and EP1270764 .

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a tin-plated steel sheet which does not include Pb and Cr that are environmentally undesirable and which has excellent solderability in Pb-free soldering, corrosion resistance, and whisker properties.
In order to achieve the object, the present invention provides a tin-plated steel sheet comprising a steel sheet having a surface roughness Ra of 1.5 µm or less, an Fe-Sn alloy layer disposed on a surface of the steel sheet, and a tin-plating layer disposed on the Fe-Sn alloy layer, the tin-plating layer having a coverage of more than 99% and a coating weight of 5 to 20 g/m². A chemical conversion coating containing P and Si is provided on the upper surface of the tin-plating layer. In the chemical conversion coating, the coating weight of P is 0.5 to 10 mg/m² and the coating weight of Si is 3 to 30 mg/m².
The surface roughness Ra is preferably 1 µm or less.
The Fe-Sn alloy layer is preferably formed by tin melting treatment.
The chemical conversion coating is preferably formed with a chemical conversion treatment solution containing P and a silane coupling agent. The silane coupling agent preferably contains an epoxy group.
Furthermore, the present invention provides a method for producing a tin-plated steel sheet, comprising the steps of (a) forming tin-containing plating layers on at least one surface of a steel sheet, (b) immersing the steel sheet provided with the plating layers in a chemical conversion treatment solution containing phosphate ions and a silane coupling agent or applying the chemical conversion treatment solution to the steel sheet, (c) heating the steel sheet to a temperature of 80°C to 200°C with the chemical conversion treatment solution being present on the plating layers to dry the steel, (d) washing the dried steel sheet with water, and (e) drying the water-washed steel sheet.
The chemical conversion treatment solution preferably contains a surfactant.

DESCRIPTION OF THE EMBODIMENTS

Tin-plated steel sheet

Pb-Sn alloy solder has a low melting point, for example, 37%Pb-Sn alloy solder has a melting point of 184°C. However, Sn-3.5%Ag-0.75%Cu alloy solder, which is predominantly used as Pb-free solder, has a high melting point at 219°C. Because of its high melting point, the Pb-free solder has lower soldering performance compared with the Pb-Sn alloy solder. Therefore, steel sheets to be soldered must have higher solderability.
Corrosion resistance and whisker resistance are also required for steel sheets to be soldered. Accordingly, the present inventors have conducted thorough research to overcome the problems described above based on tin plating mainly composed of Sn, which is the principal component of Pb-free solder. As a result, it has been found that all the properties described above can be satisfied when a tin-plating layer with a predetermined coating weight is formed on a steel sheet having a surface roughness Ra of 1.5 µm or less with an Fe-Sn alloy layer therebetween, the Fe-Sn alloy layer being formed by tin melting treatment, and a chemical conversion coating containing P and Si is formed on the tin-plating layer.
More specifically, by forming a chemical conversion coating containing proper amounts of P and Si on the tin-plating layer, preferably using a chemical conversion treatment solution containing P and a silane coupling agent, excellent solderability with Pb-free solder can be exhibited. In particular, since the chemical conversion coating functions as an effective protective film to prevent degradation with time, excellent solderability with Pb-free solder is ensured even after an accelerated degradation test. In addition, it has also been found that the chemical conversion coating provides excellent corrosion resistance and whisker resistance.
A tin-plated steel sheet of the present invention includes a steel sheet having a surface roughness Ra of 1.5 µm or less, an Fe-Sn alloy layer formed by tin melting treatment on a surface of the steel sheet, a tin-plating layer having a coverage of more than 99% on the Fe-Sn alloy layer, and a chemical conversion coating containing P and Si formed on the tin-plating layer using a chemical conversion treatment solution containing P and a silane coupling agent. In the chemical conversion coating, the coating weight of P is set in the range of 0.5 to 10 mg/m² and the coating weight of Si is set in the range of 3 to 30 mg/m². Preferably, the ratio Si/P (by mass) in the chemical conversion coating is set in the range of 5 to 30.
In the tin-plated steel sheet, after tin plating is performed on the steel sheet by a known tin electroplating method, the tin plating is melted by tin melting treatment (reflow treatment) so that an Fe-Sn alloy layer is formed as an intermediate layer at the interface with the steel sheet. The coating weight of metallic Sn is preferably 5.0 to 20.0 g/m² after the formation of the Fe-Sn alloy layer.
More preferably, the silane coupling agent contains an epoxy group.
The construction of the present invention will be described in detail below.
In a tin-plated steel sheet of the present invention, a tin-plating layer is formed on the surface of a steel sheet having a surface roughness Ra of 1.5 µm or less with an Fe-Sn alloy layer therebetween, the Fe-Sn alloy layer being formed by tin melting treatment. The tin-plating layer is formed so as to cover substantially the entire surface of the substrate, and more specifically, to have a coverage of more than 99%. If the ratio of covering the Fe-Sn alloy layer by the tin-plating layer, i.e., the coverage at the surface area, is 99% or less, satisfactory solderability cannot be achieved, and also corrosion resistance becomes insufficient.
In the present invention, after tin plating is performed on the steel sheet by a known tin electroplating method, by heating the steel sheet at a temperature not less than the melting point of Sn, the tin plating is melted by tin melting treatment (also referred to as "reflow treatment"). In the tin layer as-electroplated, stress of electrodeposits is present, and acicular crystals called whiskers grow from the surface of the tin layer due to energy that tries to liberate the stress of electrodeposits. Since whiskers cause short circuits in electric circuits, no whisker growth is required. When the Sn layer formed by electroplating is melted, stress of electrodeposits is liberated, and thus whiskers do not substantially occur. For this reason, tin melting treatment is essential in the present invention.
During the tin melting treatment, since electroplated tin becomes molten and fluid, tin flows into concave portions of the rough surface of the steel sheet. The tin content increases in the concave portions and decreases in the convex portions. Consequently, in the convex portions of the rough surface of the steel sheet, corrosion resistance is degraded as the tin content decreases, and rust is likely to start therefrom. This tendency becomes remarkable with the increase in the surface roughness of the steel sheet. The present inventors have newly found that, with respect to the coating weight of Sn in the present invention, if the surface roughness of the steel sheet is set at 1.5 µm or less in terms of centerline average (Ra), degradation of corrosion resistance in the convex portions is negligible. Therefore, the surface roughness Ra of the steel sheet is set at 1.5 µm or less.
The Fe-Sn alloy layer is formed at the interface between the steel sheet and the tin layer. The Fe-Sn alloy layer is extremely important because it improves the adhesion between the steel sheet and the tin-plating layer, thus preventing the tin layer from peeling off during working, and also ensures solderability between the steel sheet and solder when the tin layer is melted in a solder bath during soldering. Consequently, in the present invention, interposition of the Fe-Sn alloy layer between the steel sheet and the tin layer is essential. In order to exhibit the above advantage, the amount of the Fe-Sn layer formed is preferably 0.05 g/m² in terms of the coating weight. Since the alloy layer is harder than the tin-plating layer, the alloy layer degrades workability. Therefore, it is necessary to suppress the amount of the alloy layer formed, and the coating weight of the Fe-Sn alloy layer is preferably 1 g/m² or less and more preferably 0.7 g/m² or less.
When a steel sheet subjected to Ni-based pretreatment, such as Ni flash plating or Ni diffusion, is used, the amount the alloy formed during the tin melting treatment is suppressed, and thus such Ni-based pretreatment may be used appropriately.
The coating weight of the tin-plating layer which is not alloyed after the tin melting treatment is preferably 5 to 20.0 g/m². If the coating weight of the tin-plating layer is less than 5.0 g/m², it is not possible to achieve satisfactory solderability with Pb-free solder and also corrosion resistance becomes insufficient. If the coating weight exceeds 20.0 g/m², although satisfactory solderability and corrosion resistance are achieved, the cost is increased, which is undesirable. Additionally, the coating weight of Sn can be measured by coulometry or surface analysis using fluorescent X-rays.
The major feature of the present invention is that a chemical conversion coating containing P and Si is formed on the tin-plating layer, preferably by using a chemical conversion treatment solution containing P and a silane coupling agent, and the coating weights of P and Si in the chemical conversion coating are set in the ranges of 0.5 to 10.0 mg/m² and 3 to 30 mg/m², respectively.

(1) To set P content in chemical conversion coating in range of 0.5 to 10.0 mg/m² in terms of coating weight
A chemical conversion coating which covers the Sn surface as a phosphate salt and functions as a binder between Sn and the Si compound is formed. In order to achieve the binder effect which is not easily influenced by the surface structure of the steel sheet and which is not greatly changed depending on the size of the surface roughness, regardless of the surface roughness, the P content in the chemical conversion coating must be in the range of 0.5 to 10.0 mg/m² in terms of the coating weight. If the P content is less than 0.5 mg/m², coverage of the chemical conversion coating is insufficient and tin oxides grow on the Sn surface with time, resulting in a degradation in solderability. If the P content exceeds 10.0 mg/m², contact between solder and the tin layer is inhibited, resulting in a degradation in solderability. The coating weight of P is measured by surface analysis using fluorescent X-rays.
(2) To set Si content in chemical conversion coating in range of 3 to 30 mg/m² in terms of coating weight
As described above, the tin melting treatment is essential in the present invention. During the tin melting treatment, electroplated tin becomes molten and fluid, and tin flows into concave portions of the rough surface of the steel sheet which is a mother sheet to be plated. The tin content increases in the concave portions and decreases in the convex portions. If the surface roughness is increased, the surface area of the mother sheet becomes larger compared with the case in which the surface roughness is low. Consequently, in order to achieve satisfactory solderability and corrosion resistance, the Si content in the chemical conversion coating must be large enough to cover the convex portions having the low Sn content. Within the surface roughness range (about 0.1 to 5.0 µm in terms of Ra) of the industrially manufactured steel sheets (mother sheets to be plated), if the Si content in the chemical conversion coating is 30 mg/m² or more, satisfactory solderability and corrosion resistance can be achieved even in the present invention in which tin melting treatment is performed.

On the other hand, from the economical point of view, the coating weight of Si is preferably smaller. If the surface roughness Ra of the steel sheet, i.e., mother sheet to be plated, is decreased, the surface area of the steel sheet can be decreased and the Sn content can be increased. Consequently, even if the Si content is smaller, the Sn surface can be coated, and satisfactory solderability and corrosion resistance are ensured. The present inventors have found that even if the coating weight of Si in the chemical conversion coating is less than 30 mg/m², by setting the surface roughness Ra of the steel sheet, i.e., mother sheet to be plated, at 1.5 µm or less, satisfactory solderability and corrosion resistance can be obtained. Additionally, the surface roughness of the steel sheet can be adjusted, for example, by controlling the surface roughness in temper rolling.
That is, as described above, when the coating weight of Si in the chemical conversion coating is small, the surface roughness of the mother sheet to be plated must be decreased. Even when the coating weight of Si in the chemical conversion coating is set at 3 to 30 mg/m², in order to satisfy the characteristics, such as satisfactory solderability and corrosion resistance, the surface roughness Ra of the steel sheet, i.e., mother sheet to be plated, must be set at 1.5 µm or less. Even when the coating weight of Si in the chemical conversion coating is set at 30 mg/m² or less, as described above, the coating weight of the tin plating layer is set at 5 g/cm² or more. In view of stability in corrosion resistance, the coating weight of the tin plating layer is set at preferably 7.5 g/m² or more and more preferably 10 g/m² or more.
If the coating weight of Si incorporated in the chemical conversion coating is less than 3 mg/m², even if the surface roughness Ra of the steel sheet, i.e., mother sheet to be soldered, is 1.5 µm or less, coverage of the chemical conversion coating becomes insufficient and tin oxides grow on the Sn surface with time, resulting in a degradation in solderability and corrosion resistance. Therefore, the coating weight of Si must be set at 3 mg/m² or more. If the surface roughness Ra of the steel sheet, i.e., mother sheet to be plated, is 1.5 µm or less, even if the coating weight of Si incorporated in the chemical conversion coating exceeds 30 mg/m², satisfactory solderability and corrosion resistance can be obtained. However, from the economical standpoint, the coating weight of Si incorporated in the chemical conversion coating is set at 30 mg/m² or less. Additionally, the coating weight of Si is measured by surface analysis using fluorescent X-rays.
In the present invention, Si is incorporated in the chemical conversion coating preferably by a silane coupling agent contained in the chemical conversion treatment solution. The general chemical formula of the silane coupling agent is X-Si-OR_2or3 (OR: alkoxy group).
The alkoxysilyl group (Si-OR) of the silane coupling agent is hydrolyzed by water to form a silanol group, which is brought into close contact with the OH-group on the surface of the metal and forms a strong film by dehydrocondensation.
Examples of the silane coupling agent which may be used include 3-methacryloxypropyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, N-2-(aminoethyl)3-aminopropyltrimethoxysilane, N-2-(aminoethyl)3-aminopropylmethyldimethoxysilane, 3-aminopropyltriethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, 3-mercaptopropylmethoxysilane, 3-chloropropyltrimethoxysilane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, and amino group-containing compounds, such as N-2-(aminoethyl)3-aminopropyltrimethoxysilane, N-2-(aminoethyl)3-aminopropylmethyldimethoxysilane, and 3-aminopropyltriethoxysilane. In particular, silane coupling agents with the general chemical formula X-Si-OR_2or3 wherein X contains an epoxy group, such as 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and 3-glycidoxypropyltrimethoxysilane, are preferably used.
In order to form the chemical conversion coating containing P and Si, for example, phosphoric acid-based conversion treatment is preferably used. In such a case, as the P source for the chemical conversion treatment solution, phosphoric acid, a metal salt, such as sodium phosphate, aluminum phosphate, or potassium phosphate, and/or a monohydrogen phosphate salt are more preferably used at a concentration of 1 to 80 g/l (in terms of phosphate ions). As the Si source, preferably, a chemical conversion treatment solution containing the silane coupling agent described above is used. In such a case, by adjusting the pH of the chemical conversion treatment solution in the range of 1.5 to 5.5, it is possible to uniformly dissolve the silane coupling agent in the chemical conversion treatment solution.
Additionally, a metal salt of Sn, Fe, or Ni, such as SnCl₂, FeCl₂, NiCl₂, SnSO₄, FeSO₄, or NiSO₄, may be added to the chemical conversion treatment solution as appropriate. In such a case, as accelerating agents, an oxidizing agent, such as sodium chlorate or a nitrite salt; and an etching agent, such as fluorine ions, may also be appropriately incorporated. In order to allow more uniform treatment, a surfactant, such as sodium lauryl sulfate or acetylene glycol, may also be appropriately added to the chemical conversion treatment solution.
In order to form the chemical conversion coating using the phosphoric-acid based conversion treatment, after the chemical conversion treatment solution is applied to the steel sheet or the steel sheet is immersed in the chemical conversion treatment solution, drying is performed.
As described above, according to the present invention, it has become possible to successfully satisfy all of solderability of Pb-free solder, corrosion resistance, and whisker resistance by forming a chemical conversion coating containing P and Si in the proper ranges described above on a tin-plating layer formed on the surface of a steel sheet, or by forming a chemical conversion coating containing P and Si in the proper ranges described above on a tin-plating layer formed on the surface of a steel sheet with a properly adjusted surface roughness.
Next, an embodiment of a specific production method according to the present invention will be described below.
After a cold-rolled steel sheet is tin-plated, melting (reflow) treatment is performed at a temperature not less than the melting point (231.9°C) of tin to form an intermediate layer composed of an Fe-Sn alloy and an upper metallic Sn layer, followed by conversion treatment by immersion. Additionally, in order to remove tin oxides generated on the surface after the reflow treatment, cathode treatment may be performed at 1 C/dm² in a 15 g/l sodium carbonate aqueous solution.
As a chemical conversion treatment solution, an aqueous solution containing 1 to 80 g/l (in terms of phosphate ions) phosphoric acid, 0.001 to 10 g/l (in terms of tin ions) stannous chloride, and 0.1 to 1.0 g/l sodium chlorate to which 0.5 to 20.0% by mass of a silane coupling agent is further added is used.
With respect to conversion the treatment conditions, preferably, the temperature is set at 40°C to 80°C, and the treatment (immersion) time is set at 1 to 5 seconds. After immersion in the chemical conversion treatment solution, the tin-plated steel sheet is dried at 80°C to 150°C and then washed with water, followed by drying by hot air.
It is to be understood that the present invention is not limited to the embodiment described above. The invention is intended to cover various modifications included within the scope of the appended claims.

EXAMPLES

The Examples of the present invention will be described in detail below.

Examples 1-1 to 1-7

Tin-plating layers were formed on both surfaces of cold-rolled steel sheets composed of a low carbon steel or ultra-low carbon steel with a thickness of 0.4 to 1.8 mm with Fe-Sn alloy layers therebetween, the coating weight of the tin-plating layer being 5.0 to 20.0 g/m² for each surface. Chemical conversion coatings were formed under the conversion treatment conditions shown in Table 1 on the tin-plated steel sheets. The compositions of the chemical conversion coatings formed are shown in Table 2.

Comparative Examples 1-1 to 1-6

For comparison, tin-plated steel sheets were produced. In each tin-plated steel sheet, at least one of the intermediate layer, tin-plating layer, and chemical conversion coating was out of the proper ranges of the present invention.

The surface roughness Ra of the cold-rolled steel sheet, i.e., mother sheet to be plated, used in each of Examples and Comparative Examples is a centerline average measured with a "Surfcom 500A" manufactured by Tokyo Seimitsu Co., Ltd. The coating weight of Sn of the tin-plating layer and the coating weights of P and Si contained in the chemical conversion coating were measured using fluorescent X-rays. The Sn coverage was measured by surface analysis using a scanning electron microscope (10 visual fields observed at a magnification of 5,000).

TABLE 1

Conversion treatment	Treatment solution		Treatment method
A	Phosphoric acid	1-80 g/L	Immersion
	Silane coupling agent (a)	0.5-20 mass%
	Stannous chloride	0.001-10 g/L
	Sodium chlorate	0.1-1.0 g/L
B	Phosphoric acid	1-80 g/L	Immersion
	Silane coupling agent (b)	0.5-20 mass%
	Ferrous chloride	0.001-10 g/L
	Sodium chlorate	0.1-1.0 g/L
C	Phosphoric acid	1-80 g/L	Roll coating
	Silane coupling agent (c)	0.5-20 mass%	Roll coating
	Nickel chloride	0.001-10 g/L
	Sodium chlorate	0.1-1.0 g/L

Silane coupling agent (a): γ-glycidoxypropyltrimethoxysilane (Epoxy type) Silane coupling agent (b): 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (Epoxy type) Silane coupling agent (c): N-2-(aminoethyl)3-aminopropyltrimethoxysilane (Amine type)

(Evaluation of properties)

With respect to the tin-plated steel sheet in each of Examples and Comparative Examples, solderability of Pb-free solder, corrosion resistance, and whisker resistance were evaluated.

(1) Evaluation of Solderability

As Pb-free solder, Sn-3.5%Ag-0.75%Cu solder manufactured by Senju Metal Industry Co., Ltd. was used. The solder temperature was set at 245°C, and using a "SAT-5100" system manufactured by Rhesca Co., Ltd. and by a wetting balance method, zero-crossing time at which the sheet became solderable was measured to evaluate the solderability. A sample with a sheet thickness of 0.6 mm was used for the evaluation after it was subjected to accelerated degradation by being exposed to a chamber at a temperature of 105°C, a relative humidity of 100%, and a pressure of 1.22 × 10⁵ Pa for 8 hours. The sample was dipped in a solder bath at a dipping rate of 3 mm/sec and a dipping depth of 3 mm. A zero-crossing time of 3 seconds or less was evaluated to be acceptable.

(2) Evaluation of corrosion resistance

A three-cycle corrosion test was carried out, in which each cycle consisted of 8 hours of salt spraying (according to JIS Z 2371) and 16 hours of spray halt, and corrosion resistance was evaluated based on the red rusting area rate (%).

(3) Whisker test

A sample was bent at a bend radius of 5 mm and was subjected to a 500-thermal cycle test in which -25°C and 120°C were repeated. The surface of the bent section was observed with a scanning electron microscope to check the generation of whiskers. Whisker resistance was evaluated based on the generation and length of whiskers.

As is evident from the evaluation results shown in Table 2, in each of Examples 1-1 to 1-7, all of the solderability, corrosion resistance, and whisker resistance are excellent. In contrast, in each of Comparative Examples 1-1 to 1-6, either one of the solderability, corrosion resistance, and whisker resistance is poor, and thus the plated steel sheet is not practical for use.

TABLE 2

	Surface roughness of steel sheet Ra (µm)	Sn plating layer			Chemical conversion coating			Evaluation of properties
	Surface roughness of steel sheet Ra (µm)	Tin melting treatment	Sn coating weight (g/m²)	Sn coverage (%)	Conversion treatment	P coating weight (mg/m²)	Si coating weight (mg/m²)	Solderability Zero-crossing time (sec)	Corrosion resistance Red rusting area rate (%)	Whisker resistance Length (µm)
Example 1-1	0.4	Performed	5.1	99.3	A	5.5	12.0	2.4	1	Not generated
Example 1-2	0.7	Performed	11.2	99.9	B	7.1	18.5	1.9	0	Not generated
Example 1-3	1.0	Performed	16.8	99.9	C	3.9	6.5	1.6	0	Not generated
Example 1-4	1.2	Performed	19.8	99.9	A	0.6	3.2	1.4	0	Not generated
Example 1-5	0.8	Performed	14.5	99.9	A	8.8	24.5	1.7	0	Not generated
Example 1-6	0.6	Performed	7.8	99.8	C	2.1	4.5	2.2	1	Not generated
Example 1-7	1.4	Performed	9.3	99.9	B	1.6	4.0	2.1	3	Not generated
Comparative Example 1-1	1.3	Not performed	9.0	98.0	B	1.8	3.9	2.6	3	200
Comparative Example 1-2	0.9	Performed	4.7	99.1	A	4.2	5.5	4.1	10	Not generated
Comparative Example 1-3	1.1	Performed	11.8	99.9	B	0.4	2.7	7.8	10	20
Comparative Example 1-4	0.6	Performed	5.1	98.0	C	0.5	2.9	8.1	25	20
Comparative Example 1-5	0.8	Performed	9.3	99.9	A	12.0	27.0	6.8	0	Not generated
Comparative Example 1-6	1.6	Performed	7.8	99.9	C	2.0	4.3	3.3	15	Not generated

Method for producing tin-plated steel sheet

The present inventors have conducted thorough research on a method for stably forming a chemical conversion coating containing P and Si on a tin-based plating layer, and in particular, a production method in which an Si coating weight of 3 mg/m² or more can be obtained stably. As a result, it has been newly found that a chemical conversion coating can be stably formed in a short period of time by a method including the steps of immersing a tin-based plated steel sheet in a chemical conversion treatment solution containing phosphate ions and a silane coupling agent or applying the chemical conversion treatment solution to the steel sheet, heat-drying the steel sheet at 80°C to 200°C with the chemical conversion treatment solution being present on the plating layer, washing the dried steel sheet with water, and drying the water-washed steel sheet.
By incorporating a surfactant in the chemical conversion treatment solution, the film of the chemical conversion treatment solution on the steel sheet becomes more uniform during heat drying, and thus a stable chemical conversion coating can be obtained, which is preferable.
The construction of the present invention will be described in detail below.
In the present invention, a "tin-based plated steel sheet" means a steel sheet provided with a plating layer containing tin on one or both surfaces. Examples of the plating layer containing tin include, but are not limited to, an alloy layer containing Sn and at least one metal selected from the group consisting of Ni, Fe, Zn, Bi, and Cu; and a two-layered plating film including a metallic tin layer and an intermediate layer formed between the metallic tin layer and the steel sheet, the intermediate layer being composed of a tin alloy containing at least one metal selected from the group consisting of Fe and Ni.
In the present invention, the intermediate layer may be a two-layered film including an Fe-Ni alloy layer and an Fe-Sn-Ni alloy layer formed on the upper surface of the Fe-Ni alloy layer. In such a case, in the Fe-Ni alloy layer, the ratio Ni/(Fe + Ni) (by mass) is preferably 0.02 to 0.50. If the ratio Ni/(Fe + Ni) (by mass) is less than 0.02, an alloy layer mainly composed of an Fe-Sn alloy with tetragonal crystals is formed, and thereby the amount of interstices increases, resulting in a decrease in corrosion resistance. It also becomes difficult to continuously form the silane film, resulting in a small improvement in paint adhesion. On the other hand, if the ratio Ni/(Fe + Ni) (by mass) exceeds 0.50, the crystal state of the Fe-Sn-Ni alloy becomes sparse, resulting in a decrease in the corrosion resistance of the steel sheet itself. It also becomes impossible to form a dense silane film, resulting in a small improvement in paint adhesion. Additionally, in the present invention, an undercoat may be applied by nickel plating or the like between the steel sheet and the plating layer containing tin.
The chemical conversion coating containing P and Si is preferably formed, for example, by phosphoric acid-based conversion treatment. In such a case, as the P source for the chemical conversion treatment solution, phosphoric acid, a metal salt, such as sodium phosphate, aluminum phosphate, or potassium phosphate, and/or a monohydrogen phosphate salt are more preferably used at a concentration of 1 to 80 g/l (in terms of phosphate ions).
The reason for setting the preferred concentration in terms of phosphate ions in the chemical conversion treatment solution to be in the range of 1 to 80 g/l is as follows. If the concentration is less than 1 g/l, paint adhesion and corrosion resistance are degraded. On the other hand, if the concentration exceeds 80 g/l, defects easily occur in the chemical conversion coating, resulting in a degradation in paint adhesion and corrosion resistance. In addition, there may be a case in which unreacted phosphoric acid remains, resulting in a degradation in paint adhesion.
As the Si source for the chemical conversion treatment solution, a silane coupling agent is used. The general chemical formula of the silane coupling agent is X-Si-OR_2or3 (OR: alkoxy group). The alkoxysilyl group (Si-OR) is hydrolyzed by water to form a silanol group, which is brought into close contact with the OH-group on the surface of the metal by dehydrocondensation. The pH of the chemical conversion treatment solution is preferably in the range of 1.5 to 5.5. That is, by adjusting the pH of the chemical conversion treatment solution in the range of 1.5 to 5.5, it is possible to uniformly dissolve the silane coupling agent in the chemical conversion treatment solution.
Examples of the silane coupling agent which may be used include 3-methacryloxypropyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, N-2-(aminoethyl)3-aminopropyltrimethoxysilane, N-2-(aminoethyl)3-aminopropylmethyldimethoxysilane, 3-aminopropyltriethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, 3-mercaptopropylmethoxysilane, 3-chloropropyltrimethoxysilane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, and amino group-containing compounds, such as N-2-(aminoethyl)3-aminopropyltrimethoxysilane, N-2-(aminoethyl)3-aminopropylmethyldimethoxysilane, and 3-aminopropyltriethoxysilane. In particular, silane coupling agents with the general chemical formula X-Si-OR_2or3 wherein X contains an epoxy group, such as 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and 3-glycidoxypropyltrimethoxysilane, are preferably used.
Additionally, a metal salt of Sn, Fe, or Ni, such as SnCl₂, FeCl₂, NiCl₂, SnSO₄, FeSO₄, or NiSO₄, may be added to the chemical conversion treatment solution as appropriate. In such a case, as accelerating agents, an oxidizing agent, such as sodium chlorate or a nitrite salt; and an etching agent, such as fluorine ions, may also be appropriately incorporated.
In order to allow more uniform treatment, more preferably, a surfactant, such as sodium lauryl sulfate or acetylene glycol, is appropriately added to the chemical conversion treatment solution.
The tin-based plated steel sheet is immersed in the chemical conversion treatment solution at 40°C to 80°C for 1 to 5 seconds and is then drawn with a roller or the like so that the chemical conversion treatment solution forms a film with a proper thickness. The steel sheet provided with the chemical conversion treatment solution is dried by heating at 80°C to 200°C. In the heat-drying step, dehydrocondensation is accelerated between the silanol group resulting from the hydrolysis of the alkoxysilyl group (Si-OR) and the OH group on the surface of the metal, and thereby the chemical conversion coating is formed stably. It is difficult to obtain a Si coating weight of 5 mg/m² or more in the film only by immersion treatment because of slow dehydrocondensation reaction.
Heating must be performed with the chemical conversion treatment solution being present on the steel sheet. Therefore, a hot air blowing method which is usually industrially used is not suitable. Preferably, infrared heating, induction heating, or radiation heating is performed.
With respect to the heating temperature, the steel sheet temperature must be 80°C to 200°C. If the temperature is less than 80°C, the dehydrocondensation rate is decreased and the formation of the chemical conversion coating becomes unstable. Consequently, it is not possible to obtain a sufficient Si amount. If the temperature exceeds 200°C, although the dehydrocondensation proceeds quickly enough, tin is oxidized on the surface of the tin-based plating layer, and also heating energy is consumed excessively, which is not desirable.
Immediately after the chemical conversion treatment solution is dried by heating, water washing is performed to remove unreacted phosphate ions and silane coupling agent. If the unreacted phosphate ions and silane coupling agent remain on the surface, paint adhesion and corrosion resistance are degraded. Therefore, it is essential to remove the unreacted phosphate ions and silane coupling agent by water washing. After water washing, usual hot air drying is performed.
In order to form the film of the chemical conversion treatment solution on the plating layer, instead of the immersion method described above, a method may be employed in which the chemical conversion treatment solution is applied to the steel sheet using a roll coater that makes it easy to control the thickness of the film of the solution on the steel sheet.
As described above, according to the present invention, it has become possible to stably form a chemical conversion coating containing P and Si on the surface of a tin-based plating layer.
Next, an embodiment of a specific production method according to the present invention will be described below.
After a cold-rolled steel sheet is tin-plated, melting (reflow) treatment is performed at a temperature not less than the melting point (231.9°C) of tin to form a tin-based plating layer including an Fe-Sn alloy layer (intermediate layer) and a metallic Sn layer (upper layer), followed by conversion treatment by immersion. Additionally, in order to remove tin oxides generated on the surface after the reflow treatment, cathode treatment may be performed at 1 C/dm² in a 15 g/l sodium carbonate aqueous solution.
As a chemical conversion treatment solution, an aqueous solution containing 1 to 80 g/l (in terms of phosphate ions) phosphoric acid, 0.001 to 10 g/l (in terms of tin ions) stannous chloride, and 0.1 to 1.0 g/l sodium chlorate to which 0.5 to 20.0% by mass of a silane coupling agent is further added is used.
With respect to the conversion treatment conditions, preferably, the temperature is set at 40°C to 80°C, and the treatment (immersion) time is set at 1 to 5 seconds. The tin-plated steel sheet which has been subjected to conversion treatment is drawn with a ringer roll so that the film of the chemical conversion treatment solution has a predetermined thickness. The steel sheet is then heat dried at 110°C with an infrared heater. Immediately after drying, the steel sheet is water-washed, followed by drying with hot air at 35°C to 90°C.
It is to be understood that the present invention is not limited to the embodiment described above. The invention is intended to cover various modifications included within the scope of the appended claims.
The Examples of the present invention will be described in detail below.

Examples 2-1 to 2-8

Tin-based plating layers having the compositions shown in Table 3 were formed on both surfaces of cold-rolled steel sheets composed of a low carbon steel or ultra-low carbon steel with a thickness of 0.1 to 2.0 mm, the coating weight of the tin-based plating layer being 10 g/m² for each surface. Each steel sheet was immersed in a chemical conversion treatment solution selected from the three chemical conversion treatment solutions A to C shown in Table 1 or subjected to roll coating using the chemical conversion treatment solution. Immediately after heat drying, water washing was performed, followed by hot air drying. Chemical conversion coatings were thereby formed. The heating method and heating temperature in the heat-drying step for forming the chemical conversion coatings are also shown in Table 3.

Comparative Examples 2-1 to 2-4

For comparison, tin-based plated steel sheets were produced by methods in which chemical conversion coatings were formed under the conditions that were out of proper ranges of the present invention.

TABLE 3

	Sn-based plating layer	Chemical conversion treatment solution	Surfactant (g/L)	Heating method	Sheet temperature (°C)	Post water washing	Chemical conversion coating		Red rusting area rate (%)
	Sn-based plating layer	Chemical conversion treatment solution	Surfactant (g/L)	Heating method	Sheet temperature (°C)	Post water washing	P coating weight (mg/m²)	Si coating weight (mg/m²)	Red rusting area rate (%)
Example 2.1	Fe-Sn/Sn	A	(a): 0.1 g/L	Infrared heating	85	Performed	6.0	17	1
Example 2-2	Sn-Bi	B	(a): 0.5 g/L	Induction heating	190	Performed	9.0	102	0
Example 2-3	Ni/Sn	C	(a): 0.2 g/L	Radiation heating	105	Performed	4.0	60	0
Example 2-4	Sn-Zn	A	(b): 0:1 g/L	Infrared heating	95	Performed	2.0	32	0
Example 2-5	Fe-Ni/Fe-Ni-Sn/Sn	B	(c): 0.1 g/L	Radiation heating	150	Performed	3.0	80	0
Example 2-6	FeSn	C	(b):0.1 g/L	Infrared heating	100	Performed	2.0	8	2
Example 2.7	FeSn	B	(c): 0.2 g/L	Radiation heating	135	Performed	1.7	4	5
Example 2-0	Fe-Sn/Sn	C	None	Radiation heating	125	Performed	2.2	15	6
Comparative Example 2-1	Fe-Sn/Sn	B	None	Hot air heating	65	Performed	2.0	1.5	25
Comparative Example 2.2	Sn-Zn	A	None	Hot air heating	70	Performed	1.5	2.5	15
Comparative Example 2-3	Sn-Bi	C	None	None	40	Performed	1.8	0.5	30
Comparative Example 2.4	FeSn	B	None	Infrared heating	100	Not performed	18.5	215	40

Surfactant
(a): Acetylene glycol
(b): Sodium lauryl sulfate
(c): Polyoxyethylene sorbitan monooleate

(Evaluation of chemical conversion coating)

With respect to the tin-based plated steel sheet in each of Examples and Comparative Examples, the coating weights of P and Si in the chemical conversion coating were measured by surface analysis using fluorescent X-rays. The evaluation results are shown in Table 3.

(Evaluation of corrosion resistance)

With respect to the tin-based plated steel sheet in each of Examples and Comparative Examples, a salt spray test (according to JIS Z 2371) was carried out for 24 hours, and corrosion resistance was evaluated based on the red rusting area rate (%). The evaluation results are shown in Table 3.
As is evident from the evaluation results shown in Table 3, in each of Examples 2-1 to 2-8, the Si coating weight is stable at 3 mg/m² or more in the chemical conversion coating and satisfactory corrosion resistance is exhibited. In contrast, in each of Comparative Examples 2-1 to 2-4, the Si coating weight is less than 3 mg/m² in the chemical conversion coating, and thus the plated steel sheet is not practical for use.

Claims

A tin-plated steel sheet comprising:
a steel;

an Fe-Sn alloy layer formed on a surface of the steel sheet;

a tin-plating layer formed on the Fe-Sn alloy layer, the tin-plating layer having a coverage of more than 99%; and

a chemical conversion coating containing P and Si provided on the tin-plating layer,

the chemical conversion coating having a coating weight of P: 0.5 to 10 mg/m² and a coating weight of Si: 3 to 30 mg/m²

wherein

the tin-plating layer having a coating weight of 7.5 g/m² to 20 g/m², and the steel sheet having a surface roughness Ra of 1,5 µm or less.
A tin-plated steel sheet according to claim 1, wherein the Fe-Sn alloy layer has a coating weight of 1 g/m² or less.
A tin-plated steel sheet according to claim 1, wherein the chemical conversion coating has a ration Si/P (by mass) of 5 to 30.
A method for producing a tin-plated steel sheet with a chemical conversion coating having a coating weight of P: 0.5 to 10 mg/m² and a coating weight of Si: 3 to 30 mg/m² comprising the steps of:
forming a tin-containing plating layer having a coating weight of 7.5 g/m² to 20 g/m² on at least one surface of a steel sheet by performing melting treatment at a temperature not less than the melting point of tin;

immersing the steel sheet provided with the plating layer in a chemical conversion treatment solution containing phosphate ions and a silane coupling agent, or applying a chemical conversion treatment solution containing phosphate ions and a silane coupling agent to the steel sheet;

heating the steel sheet with the chemical conversion treatment solution being present on the plating layer to a temperature of 80°C to 200°C to dry the steel sheet;

washing the dried steel sheet with water; and

drying the water-washed steel sheet.
A method for producing a tin-plated steel sheet according to claim 4, wherein the chemical conversion treatment solution contains a surfactant.
A method for producing a tin-plated steel sheet according to claim 4, wherein the silane coupling agent contains an epoxy group.