JPS62192597A - Plated steel sheet having superior powdering resistance - Google Patents

Abstract

PURPOSE:To form a thick Zn-Fe alloy layer causing no powdering during press working by forming a Zn-Fe alloy layer on the surface of a steel sheet by hot dip galvanizing and by electroplating the surface of the Zn-Fe alloy layer with a Zn-Fe alloy having a high Zn content. CONSTITUTION:A steel sheet is hot dip galvanized to a large thickness and a Zn-Fe alloy layer is formed by counter diffusion between Zn inn the resulting thick Zn layer and Fe in the steel sheet. The surface of the thick Zn-Fe alloy layer is then electroplated with a Zn-Fe alloy consisting of 80-95% Zn and the balance Fe or a Zn-Fe alloy consisting of 95-100% Zn and the balance Fe. The plated steel sheet causes no powdering during press working even in case of thick plating.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a toughened steel material having excellent groove laddering resistance, particularly to a corrosion-resistant steel material for automobiles.

(Prior Art) In recent years, the demand for enhanced rust prevention for automobiles has become increasingly high, and thick galvanized steel sheets are being required to guarantee 5 years of protection against rust on the exterior of cars and 10 years on perforated internal surfaces.

The conventionally used galvanized steel sheets are
Although it has excellent corrosion resistance and spot weldability, there is a problem of so-called powdering, in which the plated surface metal parts peel off into powder due to processing stress during press processing.

JP-A-60-39153 discloses improving workability by reducing the crystal grain size of plating to 3 μm or less, but this is somewhat difficult in actual operation.

A basis weight of less than 40t/- is a big problem in terms of quantity, but if it becomes a thick material of 4°Of/- or more, the laddering properties are significantly deteriorated, causing problems in press working. Therefore, in reality, melt-alloyed zinc-iron coated steel sheets of 40 f/- or more are rarely used;
Applications requiring the above requirements must be met with unalloyed steel sheets.

However, although unalloyed to semi-alloyed hot dip galvanized steel sheets do not cause nozzle laddering, their spot weldability and post-painting corrosion resistance are poorer than alloy materials, and when unalloyed to semi-alloyed hot dip galvanized steel sheets are used, Custom-made plating causes dross removal, and the dross removal part is hard, resulting in bumps during pressing, and similar to nog laddering, there are no press processing problems.As an automotive steel sheet, where surface quality after pressing is important. Due to these problems, electroplated materials are desired.

On the other hand, with the electroplating method, it is possible to create a zinc-iron alloy coated steel sheet with a desired thick thickness without occurrence of flaking loss and with almost no occurrence of laddering. The problem is that the amount of plating power is required in proportion to the amount of plating, and the production speed is considerably reduced, resulting in poor productivity and very high cost.

Therefore, there is a strong desire for a thicker hot-dip zinc-iron alloy plated steel sheet that has powdering resistance comparable to that of electroplating and is cheaper.

(Problems to be Solved by the Invention) The present invention provides an excellent zinc-iron alloy plated steel sheet that satisfies these requirements.

(Means for Solving the Problems) The present invention provides zinc 8
A galvanized steel sheet with excellent powdering resistance that is electrolytically plated with a zinc-iron alloy consisting of 0 to 95% residual iron, and a hot-fusion galvanized steel sheet with 95% zinc as the upper layer.
This is a toughened steel sheet with excellent powdering resistance, which has been electrolytically plated with a zinc-iron alloy consisting of ultra-100% residual iron.

The amount of electroplating film should be 5 to 302/-, and the amount of hot-melting galvanizing should be 40 to 100 t/fI?
It is desirable to do so.

(Function) The molten alloyed zinc-iron plating exhibits a thermally stable δl phase or P phase. Thermally stable δl phase or P
The plated coating of the phases is hard and has low ductility, and the separated r phase is particularly brittle. In addition, in the case of melting, since an alloy phase is thermally formed, there is a difference in the growth of the alloy phase between areas where iron is easy to diffuse and areas where it is difficult, resulting in the plating film exhibiting an uneven shape.

During press forming, the reason why hot-melted galvanized steel sheets cause flaking of the plating called powdering is because processing stress concentrates on the convex parts of the plating film, and because there are many r-phases that have poor workability. This is because it is formed. Therefore, as the plating becomes thicker, the thickness of the brittle P phase in the plating layer increases, and the unevenness of the plating film becomes even larger, so it is inevitable that the soiling property will deteriorate significantly.

On the other hand, electrolytic zinc-iron plating with 80 to 95% zinc and residual pig iron has a plating composition that is almost the same as that of hot-aluminum galvanized plating, but because it is electrically forced into plating, it is similar to hot-dip plating. Unlike a thermodynamically stable phase, it exhibits a metastable alloy phase, and the plating film is quite soft and ductile (with a plating composition of 80% zinc and the rest iron, electroplated materials have a Vickers' hardness compared to hot-dipped materials). Hardness Hv is approximately 25
0 is very low). In addition, since forced plating is applied electrically, the plating film does not exhibit uneven plating, unlike molten alloy galvanizing, and has the advantage that smooth plating can be obtained.

Therefore, during press processing, the processing stress is not concentrated in one area but is applied uniformly, and although cracks may occur due to processing, powdering, in which the plating layer peels off into powder, hardly occurs. However, as the plating becomes thicker, the plating power increases and the productivity decreases, resulting in a major manufacturing disadvantage that significantly increases the manufacturing cost.

Therefore, as a result of various studies as a method to reliably produce thick plating that is less likely to cause dusting at a lower cost, we have developed a method for producing molten alloyed galvanized plating (5 to 15% iron remaining) as in the present invention. Zinc) desirably 40-1
It has been found that a plated steel sheet with almost no powdering can be obtained by applying electrolytic zinc alloy plating at a rate of 00 t/ffI' and then applying electrolytic zinc alloy plating preferably at 5 to 30 f/ff.

As in the present invention, the lower layer is subjected to molten alloy galvanizing,
Powdering can be significantly improved even with thick coatings by applying electroalloy galvanizing to the upper layer.■ By applying electroplating to the upper layer, the smoothing effect of electroplating reduces the surface of the lower layer plating. 1) The ductility of the upper electroplated film is lower than that of the lower layer. It is thought that this is because it exerts the underlayer effect of the coating and prevents the powdery plating from peeling off in the lower layer plating.

The reason why the upper layer electroplating film composition was specified to be 80-95% zinc and the rest iron is because if the composition is less than 80% zinc, the plating film will become hard and porous even if it is electroplated, and the powdering resistance will improve. This is because it does not contribute. Also, zinc is 95
If the plating composition exceeds %, the majority of the plating alloy phase is a galvanic phase, so the plating film is soft and ductile, which is effective in improving powdering resistance, but it may cause a reaction with the copper electrode chip during spot welding. This is because it has high properties and has the disadvantage of significantly reducing weldability.

The amount of electroplating film on the upper layer is preferably 5 to 30 f/d because if it is less than 5 f/-, the above-mentioned smoothing effect, processing stress absorption and relaxation effect, and no-inder effect are insufficient.
Electroplating over 30 f/m'' is undesirable in terms of manufacturing costs.

For this reason, if you don't mind sacrificing spot weldability to some extent, electrolytic zinc-iron plating of over 95% to 100% zinc and remaining iron may be applied to the upper layer of the hot-fusion alloyed galvanized steel sheet. , kt is effective as a method for improving powdering properties. At that time, the plating amount of electrolytic zinc-iron plating with more than 95% to 100% zinc and remaining iron is 5 to 30%.
t/- is desirable.

If it is less than 52/-, the above-mentioned smoothing effect, processing stress absorption/relaxation effect, and nonoinder effect are insufficient;
This is because it is undesirable to perform electroplating with a thickness exceeding /d because it is undesirable in terms of manufacturing cost.

On the other hand, the basis weight of the lower layer molten alloyed zinc-iron plating is 4.
O-LOOP/- is desirable. If it is less than 40 y/, /, the powdering resistance is good even with hot dipping alone, so there is no need to apply a powdering prevention coating on the upper layer, and if a thick coating is required, the powdering resistance of the lower layer is good. From a cost perspective, it is desirable to apply as much plating film as possible and minimize the amount of the upper layer film. In addition, in order to prevent laddering of the lower layer plated film with a thickness of more than 100 f/-, K is set to 30 f/-.
- Requires a super electroplated coating and is expensive;
And 130? /-Since there is almost no demand for super-plated coatings, there is no practical need for them.

(Examples) Next, Examples of the present invention are shown in Table 1 along with comparative examples.

Note 1: The lower layer plating in Examples and Comparative Examples was hot-dip galvanized and then heat treated, and the entire metal layer was treated with an alloy base (steel plate thickness: 0.8 m).Note 2: The upper layer was electrolytically zinc-plated. Iron alloy plated.

Electroplating conditions: Fe” = 40-90 t/t
Zn2+ = 40~702/1 p) (=l~2 Organic additive = 1 ?/ Bath temperature 55℃ Plating current density 80~200A/dm' Note 3, resistance) J? f)/ Ringability evaluation was performed by bending a plated steel plate into a V shape (V angle 60 degrees), returning it to a flat plate shape, pasting cellophane tape on the inside of the bent part, peeling it off, and checking the inside of the bent part. The peeling width of plated metal was measured (
There is no practical problem if the peeling width is less than 3.0 mm).

Note 4: Spot welding conditions are: electrode diameter 5.1φ, pressing force 2
Chilled at 40kf, welding time 8Hz, welding current 10KA.

The greater the number of consecutive hits, the better the workability.

(Effects of the Invention) According to the present invention, it is possible to obtain a thick zinc-iron alloy plating at a low cost, which does not cause blanking during press working. Furthermore, the weldability is not deteriorated and the function as a rust-preventing steel plate is fully exhibited, and it has a great industrial effect.

Agent Patent Attorney Masaaki Akizawa 2 others

Claims (2)

[Claims] (1) Zinc 80~
A laminated steel sheet with excellent powdering resistance that has been electroplated with a zinc-iron alloy consisting of 95% residual iron. (2) 95% zinc on the upper layer of molten alloyed galvanized steel sheet
A laminated steel sheet with excellent powdering resistance that has been electrolytically plated with a zinc-iron alloy made of ultra-100% residual iron.