JPH0441683A - Multi-ply zinc or zinc alloy plated steel sheet having superior spot weldability - Google Patents

Abstract

PURPOSE:To improve the spot weldability of a Zn or Zn alloy plated steel sheet while ensuring suitability to coating by electrodeposition by forming a coating layer of a metal mixed with a metal oxide on the plating film of the plated steel sheet. CONSTITUTION:A coating layer of a metal or alloy such as Fe, Al, Cr, Ni or Cu mixed with a metal oxide is formed on the surface of the plating film of a Zn or Zn alloy plated steel sheet. The metal improves suitability to coating by electrodeposition and the metal oxide improves weldability. The coating layer is formed so as to regulate the rate of exposure of the metal to 10-90% and the coating weight of the metal oxide to 10-500 mg/m<2>. The metal on the plating film ensures suitability to coating by electrodeposition for the plated steel sheet and the metal oxide reduces the damage of an electrode and ensures high density of generated heat to improve the spot weldability of the steel sheet. Superior corrosion resistance is further rendered.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is directed to galvanized multi-layered steel sheets, particularly those used as steel sheets for automobile bodies, home appliances, building materials, industrial equipment, etc. This invention relates to a galvanized multilayer steel sheet with excellent spot weldability.

(Prior Art) Since zinc-based plated steel sheets have excellent corrosion resistance, they have been used in large quantities in recent years, mainly for automobile bodies, and there are a wide variety of types. In recent years, alloy-plated steel sheets, organic resin-coated steel sheets, and the like have been developed and put into practical use.

However, on alloy-plated steel sheets, the coating weight is 20 to 45.
g/m'', the corrosion resistance is insufficient, and we are considering making the weld thicker, but the thicker the weld, the worse the spot weldability will be.

On the other hand, in the case of organic resin-coated steel sheets, the organic resin (approximately lp) is thermally decomposed during arc welding or spot welding, so the corrosion resistance of the welded parts deteriorates. Further, it is known that the spot weldability of ordinary galvanized steel sheets is insufficient even when the area weight is 30 g/m''.

Modern metal materials, such as steel plates for automobile bodies, are often assembled by spot welding, and therefore improvement of spot weldability is inevitable.

The following two points are considered to be the reason why the spot weldability of zinc-based plated steel sheets is poor.

First, during spot welding, the zinc layer on the surface of the steel sheet that comes into contact with the electrode melts due to the heat generated during welding, so the electrode tip also partially melts or forms an alloy layer, which can cause damage to the electrode due to continuous spot welding. This is more intense than in ordinary cold-rolled steel sheets, and as a result, the current path between the electrode and the plate changes, impairing the current conduction characteristics.

Second, the melting point of the galvanized layer is as low as 420°C, so it melts at the initial stage of energization.As a result, the energized area expands between the electrode and the plate or between the alternative plates, lowering the current density and nuggeting. It becomes difficult to generate. This phenomenon also applies to galvanized steel sheets or electrogalvanized steel sheets, but since the melting point of the plating layer of these steel sheets is about 850"C, the spot weldability is better than that of galvanized steel sheets.

However, the number of continuous dots is smaller than that of cold-rolled steel sheets, and as the coating layer becomes thicker from the viewpoint of improving corrosion resistance, the spot weldability of zinc-based plated steel sheets further deteriorates.

In the past, several methods have been proposed to improve the spot weldability of galvanized steel sheets.

For example, the Welding Society Resistance Welding Research Committee material, RW-1
73-80 "Spot Weldability of Surface-treated Steel Sheets", it is shown that the presence of oxides on the surface of the steel sheet increases the interelectrode voltage and improves the weldability. In other words, since oxides have extremely high resistance values, heat generation is promoted where the oxides are present, making it easier to form nuggets.

Furthermore, in JP-A-59-104463, an inert film is generated on the surface of a hot-dip galvanized steel sheet by heating to avoid direct contact between the electrode and the zinc plating, thereby preventing electrode melting and damage and extending the life of the galvanized steel sheet. A method for achieving this is disclosed.

On the other hand, JP-A-60-63394 and JP-A-63-1
In No. 86883, AQz03 is applied to the surface of a galvanized steel sheet.
By forming an oxide film such as or applying oil, we aim to improve weldability and extend the life of the electrode by avoiding contact between the electrode and the plating layer by utilizing the high electrical resistance and high melting point of the oxide. This has been disclosed.

However, such methods have not yet achieved satisfactory spot weldability on an industrial scale.

Today, zinc-plated steel sheets are seen as promising as rust-proof steel sheets, but with the trend towards thicker coatings,
There is a concern that the connection may deteriorate.

(Problems to be Solved by the Invention) The present inventors focused on the importance of spot weldability among the required performances of galvanized steel sheets, which are rust-proof steel sheets mainly used in automobiles, and mainly focused on improving the spot weldability. In addition, we conducted extensive research in order to develop a galvanized steel sheet that satisfies the performance requirements of corrosion resistance, electrocoatability, chemical conversion treatment, and workability.

As mentioned above, two methods can be considered to improve spot weldability. In other words, ■ Prevention of electrode damage, ■
This prevents current density from decreasing due to melting of the plating layer between electrodes and plates or between plates.

Therefore, the present inventors focused on a method of applying an oxide to the surface of a plated steel sheet as a method that satisfies these two points at the same time. According to this method, direct contact between the electrode and the plating layer is avoided to prevent damage to the electrode, and at the same time, the high electrical resistance and high melting point of the oxide are used to prevent the spread of the current-carrying area between the electrode and the plate and between the plates. can. However, according to the results of subsequent research, while it is true that spot weldability can be improved by simply applying an oxide, it creates a highly insulating film that causes problems during electrodeposition coating during the manufacture of automobile bodies. especially,
As electrodeposition coating is widely used today, there is a need for a galvanized steel sheet that simultaneously satisfies the contradictory properties of spot weldability and electrodeposition coating properties.

Thus, an object of the present invention is to provide a zinc-plated multilayer steel sheet that improves the spot weldability of conventional zinc-based plating fiIFi while ensuring electrodeposition coating properties.

(Means for solving the problem) Generally, when galvanized steel sheets are brought into continuous spot H16 contact,
A porous layer of Fe--Zn alloy is already formed at the 20th dot when the continuous dots start, and this layer expands to the periphery as the number of dots increases. Accordingly, the contact diameter between the electrode and the plate at the initial stage of energization increases, and the initial current density decreases, making it difficult to form nuggets. On the other hand, even after 2000 dots on the bare steel sheet, the diameter of contact between the electrode and the plate at the initial stage of energization is smaller than that of the zinc-plated steel sheet after 200 consecutive dots, and the weldability is quite good. In addition, although the Cu-Zn alloy layer is formed at the tip of the electrode,
Hard and brittle r-(C) with a Vickers hardness of 360 to 430
(u-Zn) layer is present, and this layer cracks due to impact during continuous impact, causing peeling of the alloy layer. This cracking and peeling of the tip of the electrode flattens the tip of the electrode and expands the current-carrying area between the electrode and the plate, making it difficult for nuggets to form.

One way to prevent Cu-Zn alloying is to use C
It is conceivable to coat the surface of the plating layer with an element that does not lower the melting point even when alloyed with u and Zn. That is, a double-layer plated steel sheet with a surface layer of Mn or Ni on the surface of the zinc-based plating layer improves continuous dot performance, but it is difficult to balance this with corrosion resistance or workability.

Further, a method of applying an oxide can also improve spot weldability, but there is a problem with electrodeposition coating properties.

As a result of intensive research, the inventors discovered that by coating the surface of a galvanized steel sheet with metal and coating it with an oxide, a zinc-based multi-layer coating with excellent spot weldability while ensuring electrodeposition coating properties was developed. After learning that steel plates can be obtained, the present invention was completed.

Therefore, the present invention has a coating layer in which a metal for improving electrodeposition coating properties and a metal oxide for improving weldability are mixed on the surface of the plating film of a zinc-based plated steel sheet, and Metal surface exposure rate is 10-90% for improving properties.
It is a zinc-based plated multilayer steel sheet with excellent electrodeposition coating properties and spot weldability, characterized by having a metal oxide content of 10 to 500 mg/m''.

(Function) The present invention ensures electrodepositability with the metal that covers the galvanized film (hereinafter also referred to as "coated metal" to distinguish it from the galvanized film), while reducing electrode damage and ensuring heat generation density through the oxide coating. This has achieved a dramatic improvement in spot weldability.

Here, the above-mentioned zinc-based plated steel sheet is not particularly limited in the present invention, and may be a conventional one, such as a hot-dip galvanized steel plate, a hot-dip zinc alloy plated steel plate, an electrogalvanized conditioned plate, an electrolytic zinc alloy plated steel plate ( Example: Fe-Zn, Ni
-Zn), refers to steel sheets in general that are provided with a surface coating layer containing zinc, such as zinc sprayed steel sheets, zinc W-coated steel sheets, and alloyed hot-dip galvanized steel sheets. Therefore, other elements such as Cr, Mn, Ni, Sn, Pb
, AQ, Mo, Co, Ti etc. or 2 months
Even if more than one type of zinc is contained, as long as it contains at least zinc, it is included in the above-mentioned zinc-based plated steel sheet.

Incidentally, it is said that spot weldability of zinc-based plated steel sheets deteriorates when the coating is thickly coated, and therefore, the benefits of the present invention are particularly exhibited in the case of such thick coatings.

The purpose of the metal that further coats the surface of such galvanized steel sheets is to improve the electrodeposition coating properties.
Any metal may be used as long as it has a higher critical voltage for electrodeposited crater initiation than the underlying zinc-based plating film, and as long as it is not particularly limited, it is preferably Fe, AQ, Cr, Ni,
One or more metals selected from Co, which may be in the form of an alloy, a single metal, or a mixture; also Mn, Pb, Sn, Ti, Zn,
Mg, Cd, Ta, Nb, V, W, Sb
There is no problem even if the impurities such as , P, B, and S are contained up to about 10 wt%, and in particular, in the case of Mn and Zn, 40-
There is no problem even if it is less than t%.

The most common method for providing such a coating metal is plating, but it also includes electroplating from an aqueous solution or non-aqueous molten salt, electroless plating, PVD,
Although a CVD method is exemplified, an electroplating method is preferable for performing composite plating with an oxide, which will be described later.

For example, electroplating methods include pyrophosphoric acid bath, sulfuric acid bath,
Plating is based on Watt bath or Sargent bath, and for alloy plating systems, alloying elements are added to these baths with chloride, nitrate, sulfate, acetate, carbonate, molybdate, pyrophosphate, hypophosphite. , an organic metal salt, or a state in which the metal is previously dissolved in an acid, etc., is added to the plating bath to obtain the desired composition. It is generally sufficient that the ion concentration of the precipitated metal in the bath is 1 to 2 M/l. In either case, the plating current density is approximately 40-150A/da2
It is sufficient as long as it is of a certain extent.

The electroless plating method uses a generally commercially available bath containing a reducing agent or a bath for chemical displacement deposition, and the bath temperature is 30°C.
The immersion treatment may be carried out at a temperature of 80° C. to 80° C., and the amount of adhesion may be adjusted by adjusting the immersion time.

Other methods for providing the metal coating include thermal spraying, ion blasting, sputtering, and the like.

The reason why the surface exposure rate of the metal surface of the coated metal was set to 10% or more and 90% or less is because less than 10% has little effect on improving electrodepositability, and more than 90% has little effect on improving spot weldability. It is. Preferably it is 20-80%.

Here, the above-mentioned surface exposure ratio refers to the ratio of the coated metal region existing in the outermost surface layer that is not covered with oxide, as defined by the following formula.

Surface exposure rate (χ) = X100, where So: steel plate unit area Sl: exposed area of the outermost layer of coated metal The surface exposure rate is measured by an image analysis method of the surface analysis results. In addition, the surface exposure rate can be adjusted by appropriately changing the coating metal plating conditions including pretreatment and the oxide coating conditions. It can be divided into sections. Further, it can be changed depending on the surface morphology and activity of the lower layer Zn-based plating.

The metal oxide to be mixed with the coating metal is not particularly limited, but preferably 8Q, S
iS Cr, SbS TiS Fe, Zn,
Examples include oxides such as NiS Lf, Zr, and Mn.

Such a metal oxide layer may be provided by a coating method using an appropriate binder, PVD, CVD, static coating method, etc., and the operation methods of these methods are already well known to those skilled in the art. However, the present invention is not particularly limited in this respect.

For example, when applying a metal oxide onto a galvanized film using a coating method, the target metal oxide is applied in the form of a solid oxide, slurry, or It is mixed in a sol form with a solvent (e.g., water, alcohol, etc.) and applied onto the plating film by dipping it in a roll coater, squeezing it with a roll, using a bar coater, or brushing, and then drying it at, for example, 100 to 400°C. be.

In addition, these metal oxides are provided in a mixed state with the above-mentioned coating metal, and various forms can be considered for the mixed form, but in short, in the case of the present invention, the surface exposure rate is 10 to 90%.
In addition, there is no particular restriction on any form as long as the amount of oxide adhesion can be limited to 10 to 50011 g/m2. It can be roughly divided into the following two types.

■When both are sequentially provided on the galvanized film and exist in a mixed state.

■When both are provided on a zinc film using composite plating.

It is advantageous to set the oxide particle size to 10AIII or less because stable production can be achieved.

In FIGS. 1(a) to 10), some forms of coexistence of the coating metal layer and the metal oxide according to the present invention in the cases (1) and (2) described above will be briefly explained in more detail.

Figure 1 (a) to C), (c), (1) and (0
) is a case where one type of coating metal and one or more oxides are attached to the galvanized steel sheet. Metal oxides may adhere directly to the galvanized film as shown in FIGS. 1(a) and (c), or may adhere to the coated metal as shown in FIG. 1(c). In FIG. 1(c), the galvanized film is completely covered with the first and second coating metals, and the metal oxide is adhered thereon.

FIG. 1(d) shows a case where two types of coating metals are deposited on a galvanized film, and a metal oxide is deposited thereon. In FIG. 1fe), the first coating metal forms a layer and covers the galvanized film, and the second coating metal and metal oxide are deposited thereon.

FIG. 1(f) shows a case where the first coating metal is provided directly on the galvanized film, and a portion of the second coating metal and metal oxide are also directly attached onto the galvanized film.

Figure 1 (8) to (n) show examples in which oxides are composited with the coating metal, and in particular, in Figure 1 (i) and 1), oxides are also composited with the A-based plating film. . Figure 1 (0) shows the case where the oxide is mechanically pushed into the surface of the coated metal layer. If there are many layers, a layered coating layer will be seen.

In order to balance the amount of deposited metal and the coverage rate, it is also possible to deposit the coated metal in a convex shape.

Further, there is no problem even if the coating metal is made of two or more layers.

According to the preferred US of the present invention, the coating metal preferably has a weight of 0.1 g/+w2 or more per side. 0.1g
This is because if it is less than / m 2 , it is difficult to make the outermost layer exposure rate 10% or more.

Further, if the amount of adhesion exceeds 20 g/m2, the performance will be saturated and it will be disadvantageous in terms of cost, so a range of 0.1 to 20 g/m'' is preferable.

The reason for setting the metal oxide content to 10mg/m' or more per side is because
This is because if it is less than that, the effect of improving spot weldability will not be observed much. The upper limit is not particularly limited in terms of performance, but peeling tends to occur at 500a+g/m"il, and preferably from 10 to 500 g/m"il.

In a more preferred embodiment of the present invention, the coating metal is 0.1 gem" or more per side and the coverage is 10% or more. If the coating is less than 0.1 g/lIz and less than 0x,
This is because the effect of improving electrodeposition coating properties is small.

Here, the coverage rate is defined by the following formula.

AI Coverage rate (χ) = -X100 A.

However, AI: coverage area angle by coating metal 〇: unit area of galvanized film Note that since the above coverage rate is proportional to the amount of coating metal,
The basis weight of the coated metal can be adjusted by changing the amount.

Next, the present invention will be explained in more detail with reference to examples thereof.

Example The present invention will be specifically described below with reference to Examples.

A cold-rolled steel sheet with a thickness of 0.8 m111 was used, pre-treated using a conventional method, and zinc-based plating was performed using common methods such as hot-dip plating, electroplating, dry process, and post-plating heat treatment. .

After that, the coating metal was plated, and in that case, the plating method was basically a birophosphate bath, a sulfuric acid bath, a Watt bath, or a Sargent bath.
For alloy plating systems, alloying elements are added to these baths with chloride,
Nitrates, sulfates, acetates, carbonates, molybdates, pyrophosphates, hypophosphites, organic metal salts, or metals dissolved in acid in advance in a plating bath to achieve the desired composition. added to. The ion concentration of the precipitated metal in the bath is 1~
2 gates/r.

However, electroplating from a molten salt bath was carried out using a chloride molten salt at a temperature of 200±10°C.

In either case, the plating current density is 40-150A/da
I went with +z.

In addition, in the case of electroless plating, a commercially available bath containing a reducing agent or a bath for chemical substitution deposition is used, and the bath temperature is 30
The coating was immersed at a temperature of ~80°C, and the amount of adhesion was adjusted by adjusting the immersion time.

As a dry process method, Zn, 8Q or T1,
Furthermore, regarding alloy plating containing these as main components, P
It was carried out under known conditions using the method of VD, CVI).

Next, the metal oxide was deposited as follows.

A chromate solution, silane coupling agent, or chromium coupling agent is used as a binder, and a metal oxide is mixed in these solutions and in a dispersed state, coated with a bar coater to the desired amount of adhesion, and heated at 100 to 400°C. was heated and baked.

As an evaluation of the obtained sample, spot weldability was determined by using a double R type Cu-Cr alloy as an electrode, rolling it down at 2450 N, and performing continuous dots at 1 dot/second with a welding current of 11 kA. 8 of the welding current for every 100 dots
Welding was carried out at a current value of 5%, and the welding was performed using a number of welds that could secure the nugget diameter of 3.5 liters, and the following five-level evaluation was performed.

◎...8,000 dots or more O...4,000' △...2I000〃 ×...1,000 〃 ××...Less than 1.000 dots Also, as an evaluation of electrodeposition coating properties, Electrodeposition voltage 300V, 2
At 8°C, the electrodeposited film thickness was 20±1p, and 175
The number of craters per 100 cm after baking for 25 minutes was evaluated on a 5-grade scale as follows.

◎... 0 pieces O... 3 pieces or less △... 10 pieces or less It was used for one year.

When evaluating corrosion resistance, the electrodeposition coating was applied under relatively mild conditions of 200ν (other conditions were the same as above),
Cross-cuts were made and subjected to a compound corrosion test, and the number of cycles until perforation was investigated. Compared to a material without an upper coating layer, 01 has improved corrosion resistance, Δ is equivalent to that of material with improved corrosion resistance,
Items that have deteriorated are evaluated as ×.

Note that the corrosion cycle is as follows.

(1 cycle) The test results are summarized in Tables 1 and 2 together with the processing conditions. Table 1 shows the invention examples and Table 2 shows the comparative examples.

In the case of single-sided plating in the plating sample in the example, the combination of membrane surfaces before welding was made to be in the same direction. In addition, although both sides of the double-sided plated material are produced under the same conditions, the same effect can be obtained even if the film has a different form. However, all lower layer galvanizing is double-sided plating.

In addition, the numbers written before the elements in the table are alloy compositions in weight percent.
It is shown by . Furthermore, the power/saw after the type shows the plating method, EL ('! air plating), HD (hot dip plating), OP (dry process), NE (electroless plating), and post-plating heat treatment. Those that have been marked are marked with an A after the symbol. (e.g. ELA, HDA, e
tc,) As the coating oxide, what is shown as Crux is Cr
The ratio between `` and Cr'° is not clear, and it also contains hydroxides.

Comparative Examples Nos. 1 to 30 correspond to Invention Examples No. 1 to 30, and are made of only the lower layer Zn-based plating material, which is the standard for corrosion resistance evaluation of Invention Examples, so the corrosion resistance evaluation column is blank.

It can be seen that Comparative Examples No. 31 and after have upper layer coating conditions outside the scope of the present invention, and the results are unsatisfactory in either weldability or electrodepositability.

(Effects of the invention) By using the galvanized multilayer steel sheet of the present invention,
It can significantly improve spot weldability, and has excellent effects on all properties such as corrosion resistance and electrocoatability, and satisfies all of the properties currently required in industry for zinc-based coated steel sheets. Therefore, the present invention has great practical value.

[Brief explanation of drawings]

FIGS. 1(a) to 1(0) are schematic enlarged explanatory views of the surface state of a plated steel sheet according to the present invention.

Claims (1)

[Claims] On the surface of the plating film of the zinc-based plated steel sheet, there is a coating layer containing a mixture of a metal for improving electrodeposition coating properties and a metal oxide for improving weldability, and A galvanized multilayer steel sheet with excellent electrodeposition coating properties and spot weldability, characterized in that the surface exposure rate of the metal surface is 10 to 90% and the metal oxide content is 10 to 500 mg/m^2. .