JPS5848692A - Steel plate plated with alloyed zinc and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture the titled steel plate causing no cratering phenomenon by plating a steel plate with zinc in a zinc bath contg. Al and Mg at specified concns. and by alloying the plated layer by heating. CONSTITUTION:Al and Mg are added to a zinc bath in conventional continuous zinc plating equipment for steel so as to adjust the Al concn. to 0.05-0.25% and the Mg concn. to 0.2-1.5%. One side or both sides of a steel plate are plated with zinc in the zinc bath, and the plated layer is alloyed by heating to 500- 800 deg.C. By this method a steel plate plated with alloyed zinc can be manufactured easily. This steel plate causes no cratering even if subjected to cation electrodeposition coating, and it has high corrosion resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an alloyed galvanized steel sheet having an iron-zinc alloy plating layer suitable for use as a base for electrodeposition coating, and a method for manufacturing the same.

Conventionally, alloyed galvanized steel sheets obtained by heating hot-dip galvanized steel sheets have an alloy plating layer consisting of an iron-zinc alloy layer, which has problems with paint adhesion, corrosion resistance, workability, and weldability, which are disadvantages of galvanized steel sheets. It is known as an improved material. In addition, alloyed galvanized steel sheets do not have spangle patterns like galvanized iron sheets, but are coated with a fine crystalline alloy and have a precise surface shape, so you can use them without having to choose the paint or coating method. It also had an excellent feature in that it could easily produce a smooth and beautiful painted finish.

However, when applying cationic electrodeposition coating, which has recently been developed in the field of coating technology and has begun to spread as an excellent coating method in the automobile and electrical equipment fields, to alloyed galvanized steel sheets, small pinhole-like coatings may appear on the coated surface. A serious problem arises in that film defects (called cratering) occur, which not only impairs commercial value, but also reduces corrosion resistance if this occurs frequently. Moreover, this cratering phenomenon occurs only with alloyed galvanized steel sheets and does not occur with other steel materials such as cold-rolled steel sheets and galvanized steel sheets. Therefore, countermeasures should be taken at the electrodeposition equipment and work surface. There is a need for an improved galvanized alloyed steel sheet that does not have these problems.

In order to solve this problem, the present inventors first determined the correlation between the tendency of cratering to occur in an alloyed galvanized steel sheet and the alloy plating quality, and conducted a detailed property investigation of the plating layer. As a result, although the alloy plating layer of conventional alloyed galvanized steel sheets mainly consists of the δ1 phase, as can be seen by taking the X# diffraction spectrum, it has been found that
-The alloy plating layer has a complex composition due to the presence of Fe, etc., and in the X-ray diffraction spectrum, δ1
It was found that the peak height of the main diffraction line of the phase is smaller than that of other phases, and conversely, the more F phase and especially α-Fe are present, the more cratering occurs. Therefore, an essential condition for an improved alloyed galvanized steel sheet is to make the alloy plating layer a δl phase with a homogenized crystal structure and to eliminate the r phase and α-Fe by some means.

Thus, the present inventors have developed a method for producing an alloyed zinc-plated steel sheet that satisfies the quality of the alloy plating layer.
As a result of various studies, it was found that after zinc plating in a zinc bath containing sufficient amounts of KL and Mg as essential components, when an appropriate heat treatment is performed, the α
The present invention was completed based on the knowledge that an alloy plating layer containing no -Fe or F phase can be obtained.

That is, the present invention was made based on the above findings, and the gist thereof is (1) consisting of a phase in which H is 3.5 or more, δ, represented by the following formula, And α−
An alloyed galvanized steel sheet, characterized in that it has an iron-zinc alloy plating layer with a thickness of 3 to 20 μm that does not contain substantially Fe on at least one side, provided that II: interplanar spacing 't =
2.The peak height of the diffraction line at 20: #
d, = 2.15 I3: Peak height of diffraction line at interplanar spacing d, -2, 13 and (21kl: 0.05-0.25%, Mg: 0
．． After galvanizing at least one side of the zinc bath in a zinc bath containing 2 to 1.5% and the remainder consisting of unavoidable metals, the galvanized layer is heated at a temperature of 500 to 800°C to perform alloying treatment. A method for manufacturing an alloyed galvanized steel sheet according to claim 1, characterized in that:

The present invention will be explained in detail below.

First, in the alloyed galvanized steel sheet of the present invention, the alloy plating layer is made into a δ1 phase with a uniform crystal orientation to make the surface composition uniform in terms of crystal structure in order to suppress the occurrence of cratering. Specifically, when the alloy plating layer is subjected to X-ray diffraction, the spectrum is ds==2.1 of the δ1 phase.
3 shows a prominent peak and requires the absence of other strong diffraction lines.

Here, d3=2.13 of the δ1 phase has a plane index of 101 planes (h
, , and /ri, and the orientation scale can be expressed by a convenient index H value defined using the following formula (81).

However, 11: Interplanar spacing d Peak height of diffraction line at I-220 I2: ' δ2-2.15 I,: # δ3-2.13 Crater in alloyed galvanized steel sheet with H value less than 3.0 Rings do occur and do not occur when H is 3.0 or more, preferably 3.5 or more, but even if they do occur, they are very slight. By the way, in conventional alloyed galvanized steel sheets, H is 2.0 to 2.
．． 8, whereas in the alloyed galvanized steel sheet of the present invention, H is 3.5 or more. Although the upper limit is not particularly determined, the practical upper limit is about 8.0 in terms of production technology. The main diffraction lines of the r phase with a high correlation with cratering occurrence and α-Fe appear at peak = 2.11 and d2 = 2.02, respectively, so these are the d of the δ1 phase.

It is shown by the intensity ratio with the diffraction line of =2.13. For example α−
Regarding F and e, they are expressed by the following (bJ formula).

4 ・・・・・・・・・・・・・・・(bノ■3 However, I4: The peak height of the diffraction line that returns to the interplanar spacing d, -2,02) Of course, the smaller the H value, the better, R= 0 alloy plated steel sheet does not cause cratering.For F phase also α
-Shows the same tendency as Fe.

Note that the value of d of the diffraction line may deviate slightly from the specified value depending on the sample, but this must be determined with care as this often occurs when a solid solution is formed.

Regarding the thickness specification of the alloy plating layer, the range is 3 to 20
The value μ is chosen because it is the limit that can be obtained economically with the hot-dip plating method. That is, the lower limit thickness is controlled by the amount of zinc plating during hot-dip Zn plating and gas swiping, but a plating thickness of less than 3 μm cannot be uniformly controlled.

If the upper limit thickness exceeds 20 μm, quality control of the alloy layer becomes impossible.

Next, the production of the alloyed galvanized steel sheet with excellent electrodeposition coating properties according to the present invention can be carried out using ordinary continuous galvanizing equipment for steel (for example, the Zenzima one-type system).

Non-oxidizing furnace method, Tuck-Norman method, etc.) First.

A#=o, os~0.25%, Mg=0.2~1.5%
Galvanizing is carried out in a zinc bath containing the following:

Here, A# is added to the zinc bath for the purpose of improving the adhesion of the galvanized layer. Limiting AI to the above range is A.
If I is less than 0.05%, there is no addition effect, and if it exceeds 0.25%, a large amount of Fe-AZ compound will be generated at the interface between the plating layer and the base metal, and it will become alloyed with the galvanized layer in the subsequent process. This is because during treatment, diffusion of iron is suppressed, the growth of the alloy layer is hindered, and workability is significantly reduced. Mg is added for the purpose of refining the cast structure of zinc and generating a δ1 phase with high reactivity and a well-oriented crystal orientation through alloying treatment.

The reason why Mg is limited to the above range is that if it is less than 0.2%, there will be no effect, and if it exceeds 1.5%, the effect will not improve much, but rather it will cause the galvanized layer to become rough or have a poor appearance. This makes alloy processing in subsequent processes difficult. This is because Mg is easily oxidized in a zinc bath.

To prevent oxidation in zinc baths, a method of covering the bath surface with a non-oxidizing atmosphere (Iron Age, April 6, '8
1, 71) is well known, but it does not depart from the gist of the present invention to perform galvanizing using such equipment.

Metals other than Aj and Mg include pb of 0.2% or less,
Cd of 0.1% or less may be present as an impurity in the zinc bath. After the steel plate leaves the Zn bath, the zinc weight is immediately controlled by gas squeezing, etc.
Before cooling, heat treatment is performed to alloy the galvanized layer.

The heating method is direct gas heating.

Any method such as radiation heating or electric heating (resistance heating, induction heating, etc.) may be used, but the heating conditions are a temperature of 500 to 800 ℃.
°C (however, the temperature of the galvanized layer) and a heating time of 2 to 60 seconds is appropriate. If the heating temperature is less than 500'O, the phase and ζ phase tend to remain in the alloy layer, resulting in insufficient development of the δ1 phase. Moreover, the alloying speed becomes slow, which is economically disadvantageous. On the other hand, when the heating temperature exceeds 800° C., the development of the ζ phase becomes remarkable and α-Fe is likely to be generated. These changes in alloy phase change the amount of Fe diffusion depending on the amount of A# in the galvanizing bath, the thickness of galvanizing, etc., so it is necessary to control the heating conditions by paying attention to the heating time. As another heating method, a cold galvanized steel sheet may be heated in a box-type annealing furnace to form an alloy, but this requires a long processing time and is economically disadvantageous.

The quality of the plating layer of the thus obtained alloyed galvanized steel sheet can be easily controlled by the H value and the H value as described above by X-ray diffraction.

Thus, according to the present invention, alloyed galvanized plating with good corrosion resistance can be achieved without impairing productivity or workability using continuous galvanizing equipment for steel, without causing cratering even when cationic electrodeposition is applied. The steel sheet can be easily manufactured and provides significantly superior properties compared to conventional alloyed galvanized steel sheets.

Next, the effects of the present invention will be explained in more detail with reference to Examples.

EXAMPLE Alloyed galvanized steel sheets were manufactured using hot-dip galvanizing equipment using a non-oxidizing furnace method by changing the composition of the galvanizing bath and the heat treatment conditions. i.e. A#, M to galvanized bath
The addition of g was carried out through the master alloy with zinc, and the concentration in the zinc bath was varied. When the Mg concentration was 0.7% or more, N and a seal box were used to prevent oxidation of the zinc bath.

The heat treatment after galvanizing was carried out using a combination of radiant heating and direct gas heating. The plating properties of the obtained alloyed galvanized steel sheet were analyzed by X-ray diffraction.

An evaluation test for the amount of cratering generated by cationic electrodeposition coating was conducted as follows. That is, two types of typical commercially available electrodeposition paints were prepared in an electrodeposition bath under standard conditions. Next, the alloyed galvanized steel sheet was degreased and washed, then electrodeposited using it as a cathode under predetermined conditions, washed with water, baked in an electric hot air drying oven, and cooled. Next, the painted surface was observed with the naked eye or with a microscope, and the amount of cratering generated per unit area was determined. The detailed electrodeposition conditions are as follows.

'Table 1 shows the test results.

As can be seen from Table 1, the alloyed galvanized steel sheet according to the present invention does not generate cratering, or even if cratering occurs, it is very minimal.
Much superior to conventional alloyed galvanized steel sheets. In addition, the alloyed galvanized steel sheet of the present invention has a large H value, δ, well-developed phase, and good orientation, and conversely, a small R value, with no α-Fe mixed in, and is effective in suppressing cratering. It has a great effect.

Procedural amendment F@October 19856 Z7 Japan Patent Office Commissioner Shima 1) Indication of Haruki Tono l case 11 fl Japanese 56 year f Permit am 144623 No. 2 Name of invention Alloyed galvanized steel sheet and method of fabricating the same 3 Make amendments Relationship with the patent case Patent applicant address 2-6-3 Otemachi, Chiyoda-ku, Higashihara-to Name
(665) Nippon Steel Corporation Representative Takeshi 1) Yutaka 4th generation Masato The number of inventions will increase due to the 105 Den (503) 48776 amendment.

7. Subject of amendment In the field of inventors other than the above in the application, in the description of invention q in the specification 8 Contents of amendment (1) In the inventors other than the above in the application, the furigana for "Yoshitaka Miura" is "Yoshitaka". ” is incorrect, so we will submit an amended application on a separate sheet.

(2) On page 7, line 11 of the specification.

The phrase ``The results are as thick as possible...'' is corrected to ``The results are as thick as possible...''

(3) On page 7, lines 12-13.

It says "Gas Quiping", Correct it with "gas wiping."

(4) On page 9, lines 5-6, It's called "gas quiping". Correct it with "gas wiping."

4

Claims (1)

[Claims] (1) It consists of a δ1 phase with H expressed by the following formula of 3.5 or more, and has an iron-zinc alloy plating layer on at least one side with a thickness of 3 to 20 μm that does not substantially contain α-Fe. Alloyed galvanized steel sheet featuring: However, the peak height I2 of the diffraction line at the interplanar spacing d, -2,20: # d, -2,15
Is:' d3-2.13(2)
At: 0.05-0.25%, Mg: 0.2-15%
Alloyed zinc is characterized in that at least one side of the zinc bath is galvanized in a zinc bath, the remainder of which is made of an unavoidable metal, and then the galvanized layer is heated at a temperature of 500 to 800°C to undergo an alloying treatment. Method of manufacturing plated steel sheets.