JPS59162294A - Steel sheet having two-layered zn plating provided with superior workability and its manufacture - Google Patents

Abstract

PURPOSE:To improve the workability of a plated steel sheet by forming a Zn alloy layer contg. a specified amount of one or more among Fe, Ni, Co and Cu on each side of a steel sheet by electroplating by a specified extent of deposition as an under layer and by galvanizing the steel sheet or coating it with a Zn alloy by hot dipping to form a Zn or Zn alloy layer by a specified extent of sticking as an upper layer. CONSTITUTION:A steel sheet having two-layered Zn plating is obtd. by forming a Zn alloy layer contg. 3-30% in total of one or more among Fe, Ni, Co and Cu on each side of a steel sheet by electroplating by 5-40g/m<2> per one side as an under layer and by galvanizing the steel sheet or coating it with a Zn alloy by hot dipping to form a Zn or Zn alloy layer by 20-600g/m<2> per one side as an upper layer. Electrolysis is carried out using a steel sheet as a cathode in a Zn plating bath contg. ions of one or more among Fe, Ni, Co and Cu, the sheet is dipped in a molten Zn or Zn alloy bath at 430-550 deg.C, and the amount of the molten metal stuck is controlled by a conventional method. Thus, said steel sheet having two-layered Zn plating is manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses Zn that has poor processability, particularly peeling resistance and lubricity.
The present invention relates to a two-layer plated steel sheet and its manufacturing method.

Zn-plated steel sheets are widely used for automobiles, building materials, etc., but this is because Zn has a lower corrosion potential than Fe.
This is because it preferentially elutes in a corrosive environment and protects the underlying Fe from electrolytic corrosion. However, under severe corrosive environments, Zn
The elution rate of Zn is fast, and as soon as Zn disappears, corrosion of the underlying Fe begins. To deal with this, various Zn-based two-layer plated steel sheets have been developed in which a lower layer of metal having a corrosion potential intermediate between Zn and Fe is formed between the Zn layer and the underlying Fe.

The elution rate of Zn is influenced by the potential difference with the underlying Fe, but since the potential difference between the Zn layer and the underlying metal is smaller than the potential difference between Zn and Fe, Zn disappears slowly in a two-layer plated steel sheet. . Moreover, even after Zn disappears, the underlying metal having a corrosion potential lower than that of Fe electrolytically protects the underlying Fe. Due to both of these effects, the corrosion resistance of the Zn-based two-layer plated steel sheet is very good.

Zn is a double-layer plated steel sheet with the above properties.
-Ni/Zn plated steel sheets, Zn-Fe/ZH plated steel sheets, etc. are made by electroplating. However, on the other hand, these have the disadvantage that they have insufficient lubricity and are difficult to process. That is, the steel plate is deformed during press working, but the lubricity in that case is not sufficient. Poor lubricity causes great difficulties in its use, and improvements in processing methods and lubricants have been made to improve this problem, but these have not been sufficient countermeasures, and two-layer plating with excellent lubricity has been developed. The development of steel plates was desperately needed.

On the other hand, when a hot-dip Zn-plated steel plate is heated under appropriate temperature and time conditions, Zn F”e is deposited between the Zn and the underlying Fe
An alloy layer is formed, resulting in a Zn-Fe/Zn two-layer plated steel sheet. However, this two-layer plated steel sheet is easy to peel off after plating, that is, during press working, the plating peels off and adheres to the die. This becomes a difficult point in processing E (7). Therefore, there has been a strong desire to develop a double-layer plated steel sheet with excellent lubricity and peeling resistance.

As a result of various studies in view of the above circumstances, the inventors of the present invention found that the upper layer of a two-layer plated steel sheet made by electroplating has high hardness due to the residual stress and arrangement of crystals unique to electroplating, which causes poor lubricity. I found out that it was the cause. On the other hand, in a two-layer plated steel sheet made by heating a hot-dip Zn-plated steel sheet, the lower Fe'-Zn layer is formed by mutual diffusion of Fe and Zn, so F (capital gamma) δ1 (delta) Each phase such as ζ (theta) is arranged in order from the phase with the highest Fe content to the phase with the lowest Fe content,
It has also been found that this is the cause of matting and peeling.

Therefore, we created a two-layer plated steel sheet with a Zn alloy layer formed by electroplating as the lower layer and a Zn layer formed by bath melting as the upper layer, and found that it had good workability and achieved the corrosion resistance that was the original purpose of two-layer plating. The present invention has been made based on the above findings.

That is, the gist of the present invention is that Fe, Ni, Co, Cu
The lower layer is an electrolytic Zn alloy plating layer containing one or more of 8 to 30% in total, with a coating weight of 5 to 4 p7rrt per side, and a coating weight of 20 to 600 PA per side.
Zn-based two-layer plated steel sheets and steel sheets with excellent workability characterized by having a molten Zn or molten Zn alloy plated layer as an upper layer of d are coated with Fe ions, N1 ions, Co, Z +i containing one or more types
After cathodic electrolysis in a plating solution, it was immersed in a molten Zn or molten Zn alloy plating bath at 430 to 550°C, and then
．． ) A method for producing a Zn-based double-layer plated steel sheet σ) with excellent workability, which is characterized by controlling the coating amount using a conventional method.

The present invention will be explained in detail below.

First, the scope of the present invention is defined as described above. +bl
Well, it is as follows. 1 of Fe, Ni, Co, Cu
The Zn layer containing one or more species has a corrosion potential or Zn and F.
It is between 14 and
It has anti-corrosion effect, but the total alloy metal concentration is less than 8%.
;The potential difference between the upper layer and Zn is small, so the upper layer is Zn? Since the elution suppressing effect on this is weak, the lower limit of the concentration was set at 8%. Furthermore, if the total concentration exceeds 30%, the potential difference with Fe becomes small and the anticorrosion effect on the underlying Fe becomes weak, so the upper limit was set at 30%.

When a Zn alloy layer is formed by electroplating,
The alloy layer often does not have a uniform structure, for example f'f
In Zn-Fe alloy with 15% Fe, it depends on the electrolytic conditions, but
η phase, δ1 phase, p phase, etc. are mixed. However, the corrosion potential is mostly determined by the alloying element concentration and has little dependence on the structure of the alloy layer, so corrosion resistance can be maintained by controlling only the average concentration of the alloying element. As mentioned above, the Fe-Zn alloy layer formed by heating the Zn-plated steel sheet is formed by mutual diffusion, so the phases such as F, δ1. As a result, it is easy to cause plating peeling, but Zn made by electroplating
The alloy layer does not have a structure due to interdiffusion,
Therefore, the peeling resistance is good.

The lower layer referred to in the present invention may be a single layer or two or more layers as long as the average concentration of one or more of Pe, Ni, Co, and Cu is in the range of 8 to 30%. It's okay. However, for a lower layer having a structure of two or more layers, if an electroplated layer with a low melting point of pure Zn or close to pure Zn is formed as the outermost layer in the manufacturing process, the low melting point layer will be replaced with molten Zn in the next step. It melts and disappears in the plating bath.

The lower limit of the amount of Zn alloy layer deposited on one side! M/z was determined as 5 tan? This is because if the adhesion is less than 1, there will be many pinholes, and therefore the anticorrosion effect on the underlying Fe will be insufficient. The upper limit is 40 P/rr? It is inefficient to apply plating exceeding 401"/,
,? And so.

The lower limit of the adhesion amount of the bath-dipped Zn plating layer which is the upper layer is 20//
The reason I gave it a 1 was 20! /n? This is because if the basis weight is less than that, the upper layer will disappear in a short time in a corrosive environment, resulting in insufficient corrosion resistance. As for the upper limit, it is difficult to uniformly deposit Zn in an amount exceeding 600 f/I on the surface of a steel plate using the normal basis weight control method, so 600 J'/+yl per side.
”.

The hot-dip Zn or bath-dip Zn alloy plating layer forming the upper layer is made of Zn, A, or G 10% or less, Fe 3% or less, Mg 3% or less, Sn 5% or less, and S that are used in normal hot-dip plating.
Zn containing 1% or less b, 1% or less Pb
It may be formed in a bath. Hereinafter, these will be collectively referred to as a hot-dip Zn plating layer. The hot-dip Zn plating layer has lower hardness than the electrolytic Zn plating layer, and therefore has excellent lubricity.

The method for forming such a steel plate includes degreasing and pickling the steel plate in the usual way, and then applying a sulfuric acid bath, which is a normal Zn plating bath.
Or Fe ions, Ni ions in a chloride bath.

A lower alloy layer can be obtained by cathodic electrolysis in a bath containing at least one of Co ions and Cu ions.

The composition and deposition amount of the lower layer can be controlled by controlling the coexisting ion concentration and electrolytic conditions.

The steel plate on which the lower layer was formed was heated to 430-550°C with molten Zn.
Upon immersion in the bath, an upper molten Zn layer is obtained.

In this case, the molten plating bath temperature was set at 430 to 550°C because, although the melting point of Zn is 419°C, the viscosity of molten Zn is high at temperatures below 430°C, making plating difficult. The upper limit was set at 55°C, taking into account that at temperatures exceeding 550°C, a large amount of dross is generated, which inhibits the plating work, and the evaporation of Zn increases, reducing workability.
The temperature was 0°C. The immersion time is not particularly determined, but it is 3 to 60 minutes.
About seconds is appropriate.

The amount of adhesion is controlled by any means such as the usual gas wiping method or roll squeezing method.

As described above, a sulfuric acid bath or a chloride bath is used as the electric Zn alloy plating bath, and the metal ion concentration is desirably about 01 to 20 mol/j of Zn'''r (on the contrary,
Fe ions (Fe + Fe), Ni ions.

C02+ ion and 0112 blind on are each 0.01 to 20
Mol/! The degree is appropriate. Other than that, Na ions, ni ions, NH ions, etc. are also added.Although the electrolytic conditions are not particularly defined, the desirable range is that the cathode current density is approximately 5 to 200 A/dd, and the electrolysis time is approximately 1 to 200 A/dd. 400
About seconds is appropriate.

The Zn-based two-layer plated steel sheet of the present invention obtained in this way does not have residual stress or crystal alignment peculiar to electroplating because the upper layer is made by hot dipping, and therefore does not have high hardness. On the other hand, since the lower layer is made by electroplating, Zn
Each phase of the alloy is arranged at the top of the rungs and does not have an ordered structure.As a result, it has excellent lubricity and extremely good peeling resistance. Further, as a lower layer, Zn-Fe system, Zn-N, which has an intermediate potential between Zn and Fe.
Corrosion resistance can also be improved by selecting a Zn-Co alloy layer, a Zn-Co alloy layer, or a ZnCu alloy layer.

The effects of the present invention will be illustrated in more detail with reference to Examples below.

Example After the surface of the steel plate was degreased and pickled in the usual manner, cathodic electrolysis was carried out under various conditions in various Zn alloy plating baths, and then molten Zn was
Various Zn-based two-layer plated steel sheets were obtained by immersion in a plating bath. Details are shown in Table 1.

Next, the Zn-based two-layer plated steel sheets prepared by the method shown in Table 1 were examined for corrosion resistance by a salt spray test, lubricity by a metal wear test, and peeling resistance by a die galling test. The results are shown in Table 2.

The numbers in Table 1 and the numbers in Table 2 correspond to each other.

2.3, 4, 6, 8, 10, 12.14 in the table are comparative examples,
The others are steel plates of the present invention. It was difficult to rank 2 and 3 cases in which the hot-dip Zn plating bath temperature was outside the range of the present invention. Steel plate No. 4, which has a lower layer made by heating after hot-dip Zn plating, has a plated layer that easily peels off, and steel plate No. 6, which has an upper layer made by electric Zn plating, has poor lubricity. None of the 8°10 steel plates whose lower layer composition is outside the scope of the present invention, the 12 steel plates whose lower layer basis weight is outside the scope of the present invention, and the 14 steel plates whose upper layer basis weight is outside the scope of the present invention. It also has poor corrosion resistance.

On the other hand, all of the Zn-based two-layer plated steel sheets of the present invention have excellent corrosion resistance, lubricity, and peeling resistance. In addition, the method of the present invention can satisfactorily produce a Zn-based two-layer plated steel sheet.

As detailed above, the present invention makes it possible to obtain a Zn-based 21@-plated steel sheet with excellent workability, particularly lubricity and peeling resistance, and its industrial value is truly great.

Claims (2)

[Claims] (1) The lower layer is an electrolytic Zn alloy plating layer containing one or more of Fe, Ni, Co, and Cu in a total of 8 to 30% and with a coating amount of 5 to 4071/nl per side. Adhesion amount 20~6001! A Zn-based two-layer plated steel sheet with excellent workability, characterized by having an upper layer of hot-dip Zn or hot-dip Zn alloy plating of /d. (2) Fe ion, Ni ion, Co ion on the steel plate. After cathodic electrolysis in a Zn plating solution containing one or more CIJ ions, molten Zn at 430 to 550 °C
Alternatively, a method for producing a Zn-based double-layer plated steel sheet with excellent workability, which comprises immersing it in a hot-dip Zn alloy plating bath, and then controlling the amount of adhesion using a conventional method.