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This invention relates to a composite zinc-plated metal sheet which has good press formability and good corrosion resistance as well as a remarkably improved spot weldability. This invention also relates to a method for the production of such a plated metal sheet by electroplating with zinc metal.
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Surface-treated steel sheets are used in various applications including automobiles, electric appliances, and building materials. In particular, the use of rust-preventing steel sheets having a rust preventive surface coating formed thereon has been promoted in automobiles, particularly in automobile panels, to be sold in cold regions, in which they are exposed to a severe corrosive environment due to the use of rock salt on roads in order to prevent freezing. As rust-preventing steel sheets are used in increased quantities, requirements therefor with respect to rust preventing properties have become increasingly strict. For example, such steel sheets are required to have a long-term corrosion resistance intended to resist perforative corrosion for 10 years and surface rusting for 5 years.
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In addition to corrosion resistance, rust-preventing steel sheets are required to have press formability capable of withstanding severe press forming such as deep drawing, and spot weldability enabling bonding by resistance spot welding, particularly by continuous spot welding with an increased number of weld spots. They are also desired to have post-painting corrosion resistance, i.e., the capability of forming a painted film having good adhesion in a painting step subsequent to a press forming step such that they exhibit corrosion resistance in areas where the painted film is injured and in edge faces.
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The most popular rust-preventing steel sheets are zinc-plated steel sheets prepared by electroplating or hot dipping (galvanizing) with zinc metal.
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In order to further improve the corrosion resistance of zinc-plated steel sheets, a variety of zinc alloy-plated steel sheets have been proposed. These have a coating of a zinc alloy with one or more metals such as Fe, Co, Mn, Cr, or Al. Some of these sheets such as those having a Zn-Ni or Zn-Fe alloy coating, have already been used in practical applications.
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Zinc alloy-plated steel sheets generally have improved as-plated corrosion resistance to perforation and can protect the substrate steel sheet with a relatively thin coating. The use of a zinc alloy with one or both of Ni and Co as a plating material is particularly effective for this purpose. However, the press formability and spot weldability of these zinc alloy-plated steel sheets are still less than satisfactory, since zinc alloy plating does not contribute to improvement in these properties, and in some cases even results in deterioration of such properties. Furthermore, the use of expensive Ni and/or Co as an alloying element adds to the production costs of these steel sheets.
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In the following description, the term "zinc-base plating" is used to include both zinc plating and zinc alloy plating. Similarly, the term "zinc-base plated" steel sheet or metal sheet encompasses both a steel or metal sheet having a zinc plated coating or a zinc alloy plated coating.
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For further improvement of corrosion resistance of zinc-base plated steel sheets, multilayer zinc-base plated steel sheets which have a lower zinc-base plated layer and one or more upper coating layers have been proposed.
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Japanese Patent Application Laid-Open (Kokai) No. 60-215789(1985) describes a duplex plated steel sheet having a lower zinc plated coating with a weight of 10 - 300 g/m² and an upper zinc alloy plated coating with a weight of 1 - 20 g/m² which contains one or both of Ni and Co in a total amount of 15 - 30 wt%. This duplex plated steel sheet is costly since the upper coating contains a considerable proportion of expensive Ni or Co.
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Japanese Patent Publication (Kokoku) No. 58-15554(1983) describes a duplex plated steel sheet having good applicability to chemical conversion treatment and electrodeposition, thereby exhibiting good post-painting corrosion resistance. The duplex plated steel sheet comprises an alloyed galvanized steel sheet having a thin Fe-based coating formed thereon by flash plating.
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Even in these duplex plated steel sheets, the corrosion resistance basically relies on the lower zinc-base plated coating. Therefore, in order to achieve good corrosion resistance desired by users, the lower zinc-base plated layer must be a thick coating, thereby adversely affecting the press formability, post-painting corrosion resistance, and spot weldability. Furthermore, duplex plating makes the plating apparatus complicated.
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Accordingly, there remains a need for an inexpensive surface treated steel sheet having various properties required for automobile panels, i.e., press formability and spot weldability good enough for assembly while maintaining high corrosion resistance, as well as having good applicability to painting so as to give improved post-painting corrosion resistance.
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It is an object of this invention to provide a surface treated metal sheet having good corrosion resistance, press formability, and post-painting corrosion resistance, and also having improved spot weldability, particularly in continuous resistance spot welding.
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A more specific object of this invention is to provide an inexpensive zinc-plated metal sheet having improved press formability, post-painting corrosion resistance, and spot weldability while maintaining good bare (as-plated) corrosion resistance, even if the sheet has a single plated coating or a thick plated coating.
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Another object of this invention is to provide a method for producing the above-described zinc-plated metal sheet.
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These and other objects can be achieved by a composite zinc-plated metal sheet according to this invention which has good corrosion resistance, press-formability, and post-painting corrosion resistance as well as significantly improved spot weldability.
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The composite zinc-plated metal sheet comprises a metal sheet having, on at least one surface thereof, a carbon-containing composite zinc plated coating having a coating weight of 5 - 200 g/m² and a carbon content of 0.001 - 10 wt%, wherein the composite zinc plated coating has a carbon-rich surface layer with a thickness of 0.1 - 10 µm, the thickness constituting from 5% to 50% of the thickness of the plated coating.
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In another aspect, the composite zinc-plated metal sheet according to this invention comprises a metal sheet having, on at least one surface thereof, a carbon-containing composite zinc plated coating having a coating weight of 5 - 200 g/m² and a carbon content of 0.001 - 10 wt%, wherein the η-phase present in the composite zinc plated coating has an orientation index of not greater than 0.6 for the (00·2) plane and not less than 0.2 for the (10·1) plane.
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The above-described composite zinc-plated metal sheet can be produced by a method comprising subjecting a metal sheet to electroplating with zinc metal in an acidic plating solution which contains 0.001 - 10 wt% of lignin sulfonic acid or a salt thereof and 0.001 - 10 wt% in total of one or more organic compounds selected from the group consisting of higher alcohols, and ethers and esters both having a molecular weight of at least 100.
- Figure 1 shows a profile of the variation in concentrations of C, Zn, and Fe across the thickness of a composite zinc-plated steel sheet according to the present invention; and
- Figure 2 shows a similar profile across the thickness of a conventional zinc-plated steel sheet.
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As described above, a variety of zinc alloy-plated steel sheets have been proposed as rust-preventing steel sheets for use in automobiles. A zinc alloy plated coating is generally superior to a zinc metal plated coating with respect to bare (as-plated) corrosion resistance when both have the same coating weight. However, the bare corrosion resistance of a zinc metal plated coating can be enhanced to the same level as that of a zinc alloy plated coating by increasing the coating weight of the zinc metal coating to form a thick coating. Such a thick zinc metal coating exhibits post-painting corrosion resistance much more stable and superior to that of a zinc alloy coating in an edge corrosion test and in an injured corrosion test both performed after painting so as to simulate corrosion encountered in actual use. This is because the sacrificial anticorrosive effect of a zinc alloy coating is somewhat lost as compensation for improved bare corrosion resistance, while a zinc metal coating can achieve the maximum sacrificial anticorrosive effect of zinc.
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Accordingly, in general, zinc metal plating is not inferior to zinc alloy plating with respect to corrosion resistance but is more advantageous than zinc alloy plating in view of the lower cost of zinc metal plating.
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However, a zinc metal coating has problems during press forming and spot welding. Although a zinc metal coating is less susceptible to powdering during press forming due to the high ductility of zinc, it tends to result in seizure to a press forming die owing to the high coefficient of friction, thereby forming surface defects on the press-formed plated sheet. In resistance spot welding, the number of weldable spots between a pair of Cu-based electrodes in a spot welder is greatly decreased due to accelerated damage of the electrodes caused by diffusion of zinc into the electrodes. These phenomena which lead to deterioration in press formability and spot weldability become significant as the zinc coating has an increased weight.
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In this respect, the present invention can improve the press formability and spot weldability of a zinc metal coating, particularly a thick zinc metal coating, without sacrificing its good corrosion resistance, thereby making it possible to provide a zinc-plated steel sheet with corrosion resistance, press formability, and spot weldability sufficient for the sheet to be used as a rust-preventing steel sheet for automobiles.
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Such a zinc metal coating having improved press formability and spot weldability while maintaining corrosion resistance can be formed by electroplating with zinc metal in an acidic zinc plating solution to which a combination of lignin sulfonic acid or its salt and a particular organic compound has been added in a proportion within a specific range. The resulting zinc metal coating formed from such a plating solution is a composite zinc plated coating containing 0.001 - 10 wt% carbon (C) as a result of co-deposition of carbon derived from the organic substances added to the plating solution.
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Upon further investigations, it has been found that the carbon-containing composite zinc metal coating having improved press formability and spot weldability has the structural feature that it has a carbon-rich surface layer with a thickness which is in the range of 0.1 - 10 µm and which is up to a half the thickness of the coating (specifically constituting 5% to 50% of the thickness of the coating). Thus, the carbon concentration is not uniform across the thickness of the coating, but increases toward the surface whereby the carbon concentration in the surface area of the coating is much higher than that in the area adjacent to the substrate sheet.
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The reason for the formation of a carbon-rich surface layer is not known, but it is thought to be because the rate of electrolytic reactions of the organic substances added to the plating solution to cause co-deposition of carbon is lower than that of electrodeposition of zinc, whereby carbon is deposited mainly in the late stage of electrolytic deposition and enriched in a surface layer of the resulting plated coating. Thus, although formed by a single plating step, the composite zinc plated coating has two layers, a carbon-rich surface layer and a carbon-poor lower layer, and it can function like a duplex (two layer) plated coating.
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The term "carbon-rich layer" used herein means an area of a plated coating where the carbon concentration is higher than the mean carbon concentration in the profile of carbon concentration taken across the thickness of the coating. The profile of carbon concentration across the thickness (in the depth direction) of a plated coating can be determined by glow discharge mass spectrometry, and the mean carbon concentration across the thickness can be then determined from the profile. The profile has an area adjacent to the outer surface in which the carbon concentrations are higher than the mean carbon concentration. This area corresponds to a carbon-rich surface layer, the thickness of which can be determined from the profile.
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Compared to a conventional carbon-free zinc coating, the carbon-rich surface layer formed in the carbon-containing composite zinc coating has an increased resistivity and generates a remarkably increased amount of heat in resistance spot welding, thereby facilitating the formation of a weld zone referred to as a nugget. The formation of such a carbon-rich layer solely in a surface region of the composite zinc coating results in improvement in spot weldability without deterioration in corrosion resistance, since the carbon concentration in the carbon-rich layer is merely on the order of 2 or 3 times the mean carbon concentration of the coating, which does not result in a significant deterioration in corrosion resistance. Moreover, the composite zinc coating has a lower carbon-poor zinc layer which is thicker than the upper carbon-rich surface layer or constitutes a half or more the thickness of the coating and which assures corrosion resistance. As a result, the formation of the carbon-rich surface layer does not adversely affect the corrosion resistance and sometimes even results in improved corrosion resistance.
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The composite zinc coating is also improved in press formability since the carbon-rich surface layer has a higher hardness and lower coefficient of friction than a pure zinc metal coating and is less susceptible to seizure with a press-forming die. Furthermore, the presence of the underlying ductile carbon-poor zinc layer serves to minimize or eliminate powdering of the composite zinc coating during press forming, a problem which is often encountered with a relatively hard coating.
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The thickness of the carbon-rich surface layer is in the range of 0.1 - 10 µm and constitutes 5% to 50% of the thickness of the composite zinc plated coating. When the thickness of the carbon-rich surface layer is less than 0.1 µm or constitutes less than 5% of the thickness of the plated coating, spot weldability is not improved sufficiently. When it is greater than 10 µm or constitutes greater than 50% of the thickness of the plated coating, the electrodes tend to be severely contaminated with carbon deposited at the center of the electrodes during continuous spot welding. As a result, an insulating area is formed on the electrodes, and this interferes with the formation of a nugget, thereby deteriorating spot weldability.
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Preferably the thickness of the carbon-rich surface layer is 0.1 - 5 µm and constitutes 5 - 40% of the plated coating and more preferably it is 0.5 - 3 µm and constitutes 10 - 30% of the thickness of the plated coating.
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The present inventors also made crystallographic investigations on the composite zinc metal coating containing co-deposited carbon. The composite zinc coating has a microstructure comprising a zinc metal phase called η-phase as a metallic matrix in which carbon is co-deposited. The η-phase can conveniently be identified by X-ray diffractometry since an X-ray diffraction pattern has characteristic peaks if an η-phase is present in the coating.
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The crystal structure of an η-phase is a close-packed hexagonal structure aligned in the direction of the c-axis, and it therefore has crystallographic orientation. As a result of a crystallographic investigation by X-ray diffractometry to determine the orientation of the η-phase in carbon-containing composite zinc coatings, it has been found that those coatings exhibiting improved press formability and spot weldability possess the crystallographic feature that they have an orientation index of not greater than 0.6 for the (00·2) plane of the η-phase and not less than 0.2 for the (10·1) plane thereof.
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The orientation index of a given plane of an η-phase is an indication of the orientation in the plane of a test sample relative to that of a standard zinc metal sample specified in ASTM. It is calculated from the intensities of diffraction of the planes of the η-phase by the following equation for the (00·2) plane, for example:
where
I
xx·x is the peak intensity of the (xx·x) plane in an X-ray diffraction pattern of a test sample; and
IR
xx·x is the peak intensity of the (xx·x) plane in an X-ray diffraction pattern of a standard sample.
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The angles of diffraction (Co 2ϑ) of the planes of the η-phase are 42.4° for the (00·2) plane, 45.6° for the (10·0) plane, 50.7° for the (10·1) plane, 64.0° for the (10·2) plane, 83.6° for the (10·3) plane, and 84.4° for the (11·0) plane.
-
Thus, an orientation index of 1 (one) for a given plane indicates that the sample has the same degree of orientation for the plane as that of the standard sample. As the orientation index decreases from 1, the degree of orientation decreases from that of the standard sample.
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In accordance with this invention, the η-phase has an orientation index of not greater than 0.6 for the (00·2) plane, which means that the degree of orientation for that plane is significantly lower than that of the standard sample. The η-phase has an orientation index of at least 0.2 for the (10·1) plane. It is thought that such orientation of the η-phase increases the electrical anisotropy of the plated coating, thereby increasing the contact resistance with the electrodes of a spot welder and suppressing diffusion of Zn into the electrodes, both of which lead to improvement in spot weldability.
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The spot weldability of the composite zinc-plated metal sheet is significantly deteriorated when the orientation index of the η-phase is greater than 0.6 for the (00·2) plane or less than 0.2 for the (10·1) plane.
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The orientation index for the (00·2) plane is preferably not greater than 0.5, more preferably not greater than 0.4, and most preferably not greater than 0.3, while that for the (10·1) plane is preferably not less than 0.3, more preferably not less than 0.5, and most preferably not less than 1.0.
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The carbon-containing composite zinc plated coating possesses both the above-described features in chemical structure and orientation of the η-phase. However, it is not necessary to examine whether a given composite zinc coating has both features. Accordingly, in order to obtain the above-described improved properties, it is sufficient to examine either the chemical structure of a composite zinc coating to confirm that it has the above-described carbon-rich surface layer or the crystallographic orientation of the η-phase to confirm that it has the above-described orientation indices. For example, when the substrate metal sheet on which the composite zinc coating is formed is also a zinc- or zinc alloy-plated steel sheet having an η-phase, the X-ray diffraction pattern of a composite zinc alloy plated coating formed on the substrate sheet includes those diffractions from the η-phase present in the substrate sheet, too. Therefore, it is impossible to examine the orientation of the η-phase present in the composite plated coating on the substrate sheet from the X-ray diffraction pattern. In such cases, only the chemical structure of the plated coating can be examined.
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The composite zinc plated coating has a coating weight of 5 - 200 g/m². Since the corrosion resistance basically depends on the coating weight, a coating weight of less than 5 g/m² will not provide a sufficient rust-preventing effect by the zinc coating and also makes it difficult to form a carbon-rich surface layer with a thickness of at least 0.1 µm. On the other hand, an extremely thick coating with a weight of more than 200 g/m² is costly and adversely affects the press formability and spot weldability of the plated metal sheet.
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The composite zinc-plated metal sheet according to this invention has the advantage that good press formability and spot weldability can be maintained even if it has a thick coating weight, e.g., on the order of 40 g/m² or higher. In order to fully gain such advantage, it is preferred that the coating weight be in the range of 40 - 100 g/m² and more preferably in the range of 40 - 80 g/m².
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If the substrate metal sheet is a plated metal sheet, as described below, the composite zinc plated coating formed on the plated sheet may be thin enough to provide the plated metal sheet with improved spot weldability and press formability. Therefore, it is preferable that the composite zinc plated coating be a thin coating having a weight in the range of 5 - 50 g/m² and preferably in the range of 10 - 40 g/m². A thicker coating weight may result in a deterioration in press formability or spot weldability.
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The composite zinc plated coating has a carbon content of 0.001 - 10 wt%. A carbon content of less than 0.001 wt% is too low to form a composite zinc plated coating having the above-described carbon-rich surface layer or the orientation of the η-phase. The presence of more than 10 wt% carbon causes the plated coating to have a decreased ductility, thereby rendering it more susceptible to powdering in press forming, and also deteriorates the surface appearance of the plated coating. The carbon content of the composite zinc coating is preferably 0.05 - 5 wt% and more preferably 0.5 - 3 wt%.
Substrate Metal Sheet
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The metal sheet serving as a substrate to be plated according to this invention is not critical. The substrate metal sheet is usually a steel sheet, particularly a cold-rolled steel sheet, but it may be selected from other metal sheets depending on the end use and environment of use of the plated metal sheet. For instance, it may be an aluminum sheet for lighter weight.
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In order to further improve corrosion resistance, the substrate metal sheet may be a plated metal sheet having a plated coating formed on one or both sides in a conventional manner. In such cases, it is preferred that the plated metal sheet serving as a substrate be a steel sheet plated with zinc or a zinc alloy or aluminum or an aluminum alloy. The plating method is not limited and may be electroplating in a solution or in a molten salt, hot dipping including alloyed galvanizing, or vapor plating. The weight of the plated coating in the substrate plated metal sheet is not critical, but the coating is preferably a thin coating having a weight of not greater than 50 g/m² for each side.
Plating Method
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The above-described composite zinc plated coating containing co-deposited carbon is formed on one or both surfaces of a substrate metal sheet by electroplating, i.e., cathodic electrodeposition in an acidic plating solution to which two classes of the following organic compounds (1) and (2) have been added:
- (1) 0.001 - 10 wt% of lignin sulfonic acid or a salt thereof, and
- (2) 0.001 - 10 wt% in total of one or more organic compounds selected from the group consisting of higher alcohols, and ethers and esters both having a molecular weight of at least 100.
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Lignin sulfonic acid or its salt (1) is a product formed by treating wood or other lignin-containing material with a sulfurous acid or its derivative. It is a by-product produced in large quantities from the waste solution of pulp production in the paper making industry and therefore is available inexpensively. Either one or both of free lignin sulfonic acid and its salt may be used. The lignin sulfonate salt include alkali metal salts such as sodium and potassium salt, alkaline earth metal salts such as calcium and magnesium salts, and various heavy metal salts.
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Among the organic compounds (2), the higher alcohol is a straight or branched chain aliphatic alcohol containing 6 or more carbon atoms. The higher alcohol may be either monohydric or polyhydric and it includes an unsaturated alcohols having one or more double or triple bonds in the chain. Examples of useful higher alcohols include 1-hexanol, 1-octanol, 2-ethyl-1-hexanol, nonyl alcohol, decyl alcohol, 1-octen-3-ol, 1-hexyn-3-ol, 1,6-hexane diol, and the like.
-
The ether used in this invention has a molecular weight of at least 100. It is either a monoether having a single ether bond or a polyether having two or more ether bonds. Examples of polyethers are poly(lower alkylene glycols) such as a poly(ethylene glycol) and poly(propylene glycol). The chain of the ether molecule may have unsaturation and it may be branched. Useful ethers include monoethers such as di-n-butyl ether, di-n-amyl ether, and d-n-hexyl ether, as well as polyethers such as a poly(ethylene glycol) and poly(propylene glycols) each having a molecular weight of approximately 1,000.
-
The esters used in this invention also have a molecular weight of at least 100 and may be either a monoester having a single ester bond or a polyester having two or more ester bonds. Useful esters include monoesters such as acetylsalicylates, and polyesters such as a methyl lactate polyester and ethyl tartrate polyester each having a molecular weight of approximately 10,000.
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If the alcohol is a lower alcohol or the ether or ester has a molecular weight of less than 100, the desired carbon-rich surface layer will not be formed in the resulting zinc plated coating or the orientation of the η-phase will not be controlled in the above-described manner. The maximum molecular weight for the organic compound (2) is not limited. When the organic compound is an alcohol, monoether, or monoester, however, the molecular weight should not be so high as to make it difficult to dissolve the compound in the aqueous acidic plating solution in a sufficient concentration.
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The amounts of these organic compounds present in the plating solution are 0.001 - 10 wt% for each of lignin sulfonic acid or its salt (1) and alcohol, ether, or ester (2). Lignin sulfonic acid or its salt (1) is preferably present in an amount of 0.05 - 5 wt% and more preferably 0.1 - 3 wt%. Organic compound (2) is preferably present in an amount of 0.02 - 2 wt% and more preferably 0.05 - 1 wt%. It is also preferable that one of organic compounds (1) and (2) be present in an amount of at least 0.5%. One or more compounds may be used each for organic compounds (1) and (2).
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The acidic plating solution used for electroplating with zinc metal may be a sulfate solution or a chloride solution. The electroplating may be performed by cathodic electrolysis in a conventional manner except that the above-described organic compounds (1) and (2) are added to the plating solution in the bath.
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Typical plating conditions for zinc electroplating in a sulfate plating solution are as follows. However, they are illustrative and not restrictive.
| ZnSO₄·7H₂O | 10 - 60 wt% |
| Na₂SO₄ and/or (NH₄)₂SO₄ | 5 - 20 wt% |
| pH | 1 - 4 |
| Bath temperature | 40 - 65 °C |
| Current density | 40 - 150 A/dm² |
| Solution flow rate | 0.01 - 3 m/sec |
-
The cathodic electrolysis for electroplating can be performed by passing a direct current as in conventional electroplating.
-
In a preferred embodiment, the cathodic electrolysis is performed by using either (a) a pulse current having an off-time (separation time) of 1 msec to 1 sec and a duty factor of at least 0.5, or (b) a direct or pulse current on which an alternating current (AC) having a frequency of 1 - 100 Hz and a current variation peak of ±1% - ±50% is superimposed (hereinafter referred to as "superimposed AC on direct or pulse current"). Electroplating with such a pulse current or a superimposed AC on direct or pulse current in a plating solution containing the organic compounds results in the formation of a composite zinc coating having a significantly increased carbon content compared to that formed by direct current electroplating in the same plating solution having the same content of the organic compounds. This is thought to be because the organic compounds are relatively readily accessible to the substrate metal sheet and are effectively adsorbed thereby while the pulse current is in the off state or the superimposed AC is in a low-current state, where interference with the polarity of the substrate metal sheet to which a voltage is applied is minimized. As a result, the resulting composite zinc coating has a carbon-rich surface layer with an increased thickness or the η-phase in the coating has a decreased orientation index for the (00·2) plane and an increased one for the (10·1) plane.
-
Furthermore, a composite zinc-base plated coating formed by electroplating with the above-described pulse current or superimposed AC on direct or pulse current is advantageous in that it has a decreased level of internal stress and a reduced number of cracks and pinholes. Therefore, the resulting plated coating has improved adhesion to the substrate sheet, adhesion to painting, and bare corrosion resistance, in addition to improved spot weldability and press formability.
-
Electroplating with zinc metal in an acidic plating solution which contains 0.001 - 10 wt% each of the above-described organic compounds (1) and (2) results in the formation of a zinc coating containing 0.001 - 10 wt% of co-deposited carbon in which a carbon-rich surface layer is formed to the above-described thickness or the η-phase is oriented in the above-described manner.
-
The composite zinc-plated metal sheet according to this invention, which has the above-described composite zinc coating on at least one surface of a substrate metal sheet, has improved press formability and spot weldability, even if the coating is thick, in addition to good as-plated (bare) corrosion resistance. Therefore, it is suitable for use, as is, as a rust-preventing metal sheet in assembly of automobile panels.
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Automobile panels are normally assembled by press forming followed by painting. The press-formed sheet is subjected to chemical conversion treatment, usually phosphating treatment before painting. The composite zinc coating has good adhesion to the chemical conversion treatment, resulting in improved post-painting corrosion resistance, which is also desired for automobile panels.
-
The composite zinc-plated metal sheet according to this invention is also useful in other applications including building materials and electric appliances.
-
The corrosion resistance can be further improved by coating the composite zinc-plated metal sheet with a chromate film, which has an anticorrosive effect, and optionally further with a thin organic resin coating, e.g., an epoxy resin-based coating with a thickness of 2 µm or less, which acts as a barrier from a corrosive environment. Therefore, such further coating may be applied, if desired, onto one or both surfaces of the composite zinc-plated metal sheet.
-
The following examples are presented to further illustrate the present invention. These examples are to be considered in all respects as illustrative and not restrictive. In the examples, all percents and parts are by weight unless otherwise indicated, and coating weights are for each side of a sheet.
EXAMPLE 1
[Preparation of Composite Zinc Plated Coating]
-
Either a 0.8 mm thick cold-rolled steel sheet or a zinc-plated steel sheet having a zinc electroplated coating weighing 30 g/m² on both surfaces of a 0.8 mm thick cold rolled steel sheet was used as a substrate metal sheet. The substrate metal sheet was subjected to electroplating with zinc metal by cathodic electrolysis in an acidic sulfate plating solution to which the above-described two classes of organic compounds (1) and (2) had been added as shown in Table 2. The basic plating conditions were as follows.
| Composition of Plating Solution | Electroplating Conditions |
| ZnSO₄·7H₂O: | 80-160 g/l | Bath temp.: | 50±10°C |
| Na₂SO₄: | 65 g/l | Flow rate: | 0.06-1.40 m/sec |
| pH: | 1.6 - 2.6 | Anode: | Pb-1% Ag |
-
The electrolysis was performed by passing a direct current, a pulse current, or an alternating current superimposed on a direct or pulse current as shown in Table 1. In Table 1, Current No. 0 was a direct current, Currents Nos. 1 to 3 were pulse currents, Currents Nos. 4 and 5 were superimposed AC's on DC's, and Current No. 6 was a superimposed AC on pulse current. For example, Current No. 4 was a direct current having a current density of 60 A/dm² on which an alternating current having a current variation peak of ± 0.6 A (corresponding to ± 1.0% of 60 A) was superimposed. Similarly, Current No. 5 was a direct current having a current density of 90 A/dm² on which an alternating current having a current variation peak of ± 45 A (corresponding to ± 50% of 90 A) was superimposed.
TABLE 1 | No. | Current Density (A/dm²) | Shape of Pulse Current | Wave Form of Superimposed Alternating Current |
| | | Off time (msec) | Duty factor | Frequency (Hz) | Current variation peak |
| 0 | 80 | -- | -- | -- | -- |
| 1 | 80 | 1000 | 0.5 | -- | -- |
| 2 | 80 | 1 | 0.99 | -- | -- |
| 3 | 60 | 500 | 0.8 | -- | -- |
| 4 | 60 | -- | -- | 100 | ±1.0% |
| 5 | 90 | -- | -- | 1 | ±50% |
| 6 | 90 | 400 | 0.9 | 50 | ±25% |
[Chemical and Crystallographic Structure of Zinc Coating]
-
The carbon content of the resulting composite zinc plated coating was determined by combustion of a mechanically removed sample of the plated coating and gas analysis of the generated gas.
-
The profile of the carbon concentration of the plated steel sheet across the thickness was measured by a glow discharge mass spectrometer (Shimadzu GDLS-5017). Figure 1 shows an example of such a profile of carbon concentration along with the profiles of Fe and Zn in a composite zinc-plated steel sheet according to this invention. For comparison, Figure 2 shows an example of such profiles for a comparative zinc-plated steel sheet formed by electroplating in a plating solution to which no organic compounds were added.
-
From the profile of carbon concentration, the thickness of a carbon-rich surface layer was determined as an absolute value and as a percentage of the thickness of the zinc alloy plated coating.
-
The orientation indices of the (00·2) plane and the (10·1) plane of the η-phase present in the composite zinc coating were determined by X-ray diffractometry in the above-described manner using a high-voltage X-ray diffraction apparatus equipped with a cobalt target. In Runs Nos. 21 - 24 in which the substrate was a zinc-plated steel sheet, the diffractometric measurements were not performed since the orientation indices could not be determined accurately for the reason described previously.
[Testing Procedures for Plated Steel Sheet]
-
The resulting composite zinc-plated steel sheets were evaluated with respect to post-painting corrosion resistance in an injured area and edge faces, press formability, and spot weldability according to the following test methods.
[Post-Painting Corrosion Resistance]
Corrosion resistance in injured areas
-
A test specimen measuring 70 mm X 150 mm was cut from each plated steel sheet and painted by a process comprising treatment with a degreasing agent FC 4336, then with a conditioner PZT, and finally with a phosphating solution PB-L3080 (all manufactured by Nippon Parkerizing), then coating with a cationic electrodeposition coating to a thickness of 20±1 µm using a coating composition U-80 (Nippon Paint) followed by baking for 25 minutes at 175°C, intercoating with an alkyd-based coating composition for automobiles to a thickness of 40 µm followed by baking, and topcoating with a melamine-polyester coating composition to a thickness of 40 µm followed by baking.
-
The resulting painted test specimen was injured by scribing crossed lines with a knife to a depth sufficient to reach the substrate steel sheet and was subjected to an accelerated cyclic corrosion test with a 24 hour-cycle consisting of salt spraying for 7 hours at 35°C using a 5% NaCl solution, drying for 2 hours at 50°C, and humidifying for 15 hours at 50°C and a relative humidity of 85%.
-
After 30 cycles, the width of blisters (W) observed along the scribed lines was measured, and post-painting corrosion resistance in the injured area was evaluated as follows.
- Ⓞ
- : W < 0.5 mm
- O
- : 0.5 mm ≦ W < 1 0 mm
- △
- : 1.0 mm ≦ W < 2.0 mm
- X
- : 2.0 mm ≦ W < 3.0 mm
- XX
- : W ≧ 3.0 mm
Corrosion resistance in edge
-
A test specimen was blanked out by a press in which the die clearance was adjusted such that a burr was formed to a height corresponding to 10% of the sheet thickness. The blanked test specimen was then painted by the above-described process, and the painted test specimen was subjected to the above-described accelerated cyclic corrosion test.
-
After 60 cycles, the percent of the area of the edge face covered with red rust (S) was measured, and post-painting corrosion resistance of the edge face was evaluated as follows.
- Ⓞ
- : No red rust
- O
- : S ≦ 5%
- △
- : 5% < S ≦ 10%
- X
- : 10% < S ≦ 30%
- XX
- : S > 30%
[Press Formability]
Workability
-
A round blank having a diameter of 90 mm was cut and subjected to deep drawing into a cylindrical cup measuring 50 mm in diameter and 28 mm in depth. The plated coating on the outer side wall of the press-formed cup was subjected to a Scotch tape test to measure the degree of plated coating peeled off by adhesion to the tape. The results were evaluated in terms of the percent of the area of the tape on which peeled coating was adhered (T) as follows.
- 5 : No adhesion of peeled coating
- 4 : T < 10%
- 3 : 10% ≦ T < 30%
- 2 : 30% ≦ T < 50%
- 1 : All-over adhesion of peeled coating
Formability
-
The formability was evaluated in the following manner by visually observing the blank during the above-described deep drawing with respect to fracture of the blank.
- Ⓞ
- : No fracture
- X
- : Fractured
[Spot Weldability]
-
Two test specimens of the plated steel sheet obtained in each run were welded by continuous spot welding using a single-phase AC spot welder under the following conditions:
| Current passed: | 10,000 A |
| Welding force: | 200 kgf |
| Weld time: | 12 cycles (at 60 Hz) |
| Shape of electrodes: | Dome shape |
-
Continuous spot welding was performed with a welding cycle consisting of welding of consecutive 20 spots at intervals of 2 seconds followed by a rest of 40 seconds. Three welded spots were taken at random from each 100 spots to determine the nugget diameters after the welded spots were pulled apart. The spot weldability was evaluated by the number of welded spots (N) before the nugget diameter decreased to 3.6 mm or smaller.
-
The test results are shown in Table 2 along with the type of substrate, weight and carbon content of the composite zinc coating, thickness of the carbon-rich surface layer and its percentage in the coating, and the orientation indices of the η-phase.
-
As can be seen from Table 2, each composite zinc plated coating formed by electroplating in an acidic plating solution which contains 0.001 - 10 wt% of a lignin sulfonic acid salt and 0.001 - 10 wt % of a specific alcohols, ether, or ester in accordance with this invention had a carbon-rich surface layer with a thickness of 0.1 - 10 µm, which constituted 5 - 50% of the thickness of the plated coating. It also had an η-phase in the microstructure in which the orientation index was not greater than 0.6 for the (00·2) plane and not less than 0.2 for the (10·1) plane.
-
As a result, the composite zinc-plated steel sheets exhibited good post-painting corrosion resistance both in edge and injured areas. Furthermore, even if they had a thick plated coating of 60 g/m², they had good press formability and significantly improved spot weldability that 8,000 spots or more were weldable in a continuous spot welding test.
-
By comparison of the results of Runs Nos. 12 - 15 to Runs Nos. 17 - 20, it can be seen that the passage of a pulse current or a superimposed AC on direct or pulse current made it possible to form a composite zinc plated coating having an increased carbon content with the same or a smaller amounts of organic compounds added. The increased carbon content also resulted in the formation of a carbon-rich surface layer with an increased thickness and the η-phase with a decreased orientation index for the (00·2) plane and an increased one for the (10·1) plane in the composite zinc coating.
-
It is noted that the number of weldable spots exceeded 10,000 when the orientation index is not greater than 0.4 for the (00·2) plane and not less than 0.5 for the (10·1) plane, and it reached 11,000 - 13,000 when the orientation index is not greater than 0.3 for the (00·2) plane and not less than 1.0 for the (10·1) plane.
-
In contrast, one of the two classes of organic compounds was not added to the plating solution, the carbon-rich surface layer formed in the composite zinc coating had a thickness which was less than 5% of the thickness of the coating, resulting in a deterioration in spot weldability. When none of the organic compounds were added or the amounts thereof added were both insufficient, a carbon-rich surface layer was not formed in the resulting zinc coating, resulting in a significant deterioration in spot weldability and press formability.
-
Addition of one of the organic compounds in excess of 10 wt% resulted in a deterioration in press formability due to powdering.
-
The use of toluenesulfonic acid or methanol in place of lignin sulfonic acid or a higher alcohol, respectively, failed to provide the resulting coating with all the desired properties.
-
It will be appreciated by those skilled in the art that numerous variations and modifications may be made to the invention as described above with respect to specific embodiments without departing from the spirit or scope of the invention as broadly described.
