JP2002167656A - Galvanized steel sheet having good appearance and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a galvanized steel sheet as a GI steel sheet having excellent appearance and usable under no coating which jointly has high workability and high corrosion resistance which have not been obtained in the conventional GA steel sheet, and good surface quality free from rugged defects which has not been obtained in the conventional GI steel sheet, and to provide its production method. SOLUTION: By mass, 0.20 to 1.0% Al and 0.5 to 6.0%, preferably, <2.5% Fe are incorporated into the plating film. In its production, a steel sheet stock is dipped into a galvanizing bath containing 0.15 to 0.3% Al, and the coating weight of the plating is controlled by gas wiping. After that, heating is performed thereto in the range of 420 to 600 deg.C as the temperature of the steel sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet which is a material of a steel sheet or a color steel sheet used for parts of electric appliances or automobiles, or a steel sheet used for building materials, and a method for producing the same. It is about.

[0002]

2. Description of the Related Art Conventionally, as a hot-dip galvanized steel sheet, Al
Is applied in a plating bath containing about 0.13% by mass or more, and then coated with a hot-dip galvanized steel sheet (hereinafter referred to as GI steel sheet) that is not heated after adjusting the amount of coating, and a plating bath containing about 0.15% by mass or less of Al. And then heating the steel sheet after adjusting the amount of adhesion so that the content of Fe in the plating film is 8% by mass or more.
There is an alloyed hot-dip galvanized steel sheet (hereinafter referred to as GA steel sheet) in which a plating film is alloyed so as to be 12% by mass.

[0003]

The GA steel sheet has a galvanized film alloyed with Fe in the above-mentioned range, so that it has better resistance weldability, chemical conversion property and coating adhesion than GI steel sheet, but has a coating layer. Since a brittle Fe-Zn-based alloy layer is deposited in the inside, workability, that is, plating adhesion is inferior. In addition, there are restrictions on the equipment restrictions of the alloying furnace, and in order to secure the minimum workability, the plating thickness cannot be increased,
High corrosion resistance cannot be obtained as compared with GI steel sheets.

Therefore, while securing the advantages of GA steel sheets,
Various proposals have conventionally been made to solve the above-mentioned problems. GA disclosed in JP-A-52-6340
The steel sheet has an η phase with an Al content of 0.25% or more on the alloyed layer,
By forming a surface layer composed of (η phase + ζ phase), the workability is to be improved as compared with the case of using only the alloyed layer.

[0005] Therefore, based on such a premise, in the method disclosed in the above publication, the Al concentration in the plating bath is reduced.
Since the plating bath is 0.3 to 5% by mass, which is higher than that of a normal GI steel plate plating bath, it is necessary to replace the plating bath and prepare two types of plating pots. Very expensive and poor productivity.
In addition, since the Al concentration in the plating bath is high, the Al concentration in the coating increases, and the weldability deteriorates.

A GA steel plate disclosed in Japanese Patent Application Laid-Open No. 62-4858 is also for improving workability, and an alloy layer is formed on 1/4 to 3/4 of the total plating layer thickness. A zinc layer is provided thereon, and the thickness of the alloy layer at the interface between the plating and the steel sheet is controlled to be small in order to improve workability.

However, the concentration of Al in the plating bath is 0.15% or less.
Despite the increase to 0.25%, the amount of Fe in the plating layer is rather increased from 2.5% to 4.3%. The effect cannot be inferred.
In fact, the brittle Fe-Zn
Since only the system alloy layer grows, the effect of improving the workability cannot be said to be sufficient, and this technique has not been put to practical use.

On the other hand, the GI steel sheet can be thicker in plating thickness than the GA steel sheet, and has high corrosion resistance.
Also, the workability is superior to that of the GA steel sheet. But G
I The steel sheet solidifies the hot-dip galvanized coating while the amount of coating is adjusted by gas wiping. Therefore, the unevenness of the plating caused by vibrations during gas wiping and the adhesion in the plating bath are caused on the surface of the plating. Dross (Fe −
Grains of a Zn-based alloy), and the surface appearance is inferior to GA steel sheets.

For this reason, GI steel sheets have not been able to be used without coating on visible panels of automobile exterior panel materials and home appliances, which are strict against unevenness defects on the surface. Here, an object of the present invention is to provide a hot-dip galvanized steel sheet having an excellent appearance which cannot be realized by a conventional GI steel sheet and which can be used without painting, and a method for manufacturing the same.

Another object of the present invention is to provide a high workability and a high corrosion resistance which cannot be obtained from a GA steel sheet and a conventional G steel sheet.
An object of the present invention is to provide a hot-dip galvanized steel sheet having a beautiful surface quality free from unevenness defects that cannot be obtained with an I steel sheet, and a method for producing the same.

[0011]

Means for Solving the Problems Here, the present inventors have
Hot-dip galvanizing is performed using a plating bath with an Al content of 0.3% by mass or less in the plating bath, and the Fe content of the plating film is reduced to 6.
When the heat treatment was carried out so as to be 0% by mass or less, specifically, less than 2.5% by mass, it was found that the problems of the prior art as described above were effectively solved, and the present invention was completed. did.

Therefore, the present invention is as follows. (1) Hot-dip galvanized steel sheet with a beautiful surface containing 0.20% by mass or more and 1.0% by mass or less of Fe and 0.5% by mass or more and 6.0% by mass or less of Fe in the plating film.

(2) 0.20% by mass or more of Al in the plating film 1.
A hot-dip galvanized steel sheet having a beautiful surface, characterized by containing 0% by mass or less, 0.5% by mass or more and 6% by mass or less of Fe, and further comprising 95 to 50 atm% of η phase in the galvanized film.

(3) 0.20% by mass or more of Al in the plating film 1.
0 wt% or less, and Fe is contained below 6% by mass to 0.5% by mass, beautiful galvanized steel sheet surface, characterized in that W _CM of the plated steel sheet surface is 10μm or less.

(4) A steel sheet is immersed in a hot-dip galvanizing bath containing 0.15% by mass or more and 0.3% by mass or less of Al, and then the coated steel sheet whose gas adhesion is adjusted by gas wiping is used.
By heating at a temperature of 420 ° C to 600 ° C at the temperature of the steel sheet, the galvanized film contains 0.2% to 1.0% by mass of Al and 0.5% to 6% by mass of Fe. A method for producing a hot-dip galvanized steel sheet having a beautiful plated surface in which an η phase is present at 95 to 50 atm%.

[0016]

As described above, according to the present invention, 0.20% to 1.0% by mass of Al and 0.1% by mass of Fe are contained in the plating film.
5 mass% to 6 mass%, or Fe is 0.5 mass% or more and 2.5 mass%
A galvanized steel sheet with a beautiful surface containing less than 20% by mass, and further containing 0.20% by mass to 1.0% by mass of Al and 0.5% by mass to 6% by mass of Fe in the galvanized film, η phase to be present 95~50atm%, and W _CM of / or plated steel sheet surface is a surface, wherein beautiful hot-dip galvanized steel sheet is provided that is 10μm or less in.

Here, the reason why the configuration of the plating film is defined as described above will be described as follows. If the Al concentration in the plating film is less than 0.20% by mass, Fe ₂ A generated at the interface between the plating and the steel sheet during plating
l A _five- phase alloy layer is unlikely to grow, and a brittle Fe-Zn-based alloy layer is likely to precipitate during the subsequent alloying treatment, and the adhesion of plating is impaired.

The Al concentration in the plating film is as high as more than 1.0% by mass.
If too much, Fe _TwoAl_FiveFeAl besides phase_Three
Phase precipitates and this FeAl_ThreePhase during subsequent alloying
Adhesion of plating to promote the growth of Fe-Zn alloy layer
Sex is impaired.

In a commonly used plating bath for producing GI steel sheets, the Al concentration in the plating film cannot exceed 1.0% by mass. If the content of Fe in the plating film is less than 0.5% by mass, when the coated steel sheet is heated after gas wiping, the temperature cannot be raised to a temperature at which the plating dissolves, and the unevenness defects of the plating cannot be eliminated.

If the content of Fe in the plating film exceeds 6% by mass, it means that the alloying treatment was performed at a high temperature for a long time.
Therefore, many brittle Fe-Zn-based alloy layers are precipitated, and the adhesion of plating is impaired. The preferred Fe content is less than 2.5% by weight.

The Al concentration in the plating film is 0.20 to 1.0% by mass
Under the condition that the Fe ₂ Al ₅ phase with good adhesion exists in the plating film, the Zn in the plating film and Fe of the steel sheet are alloyed, and the ζ phase of the Fe-Zn alloy phase is preferably within a certain range. (ex.5 ~
By precipitating at 50 atm%), a hot-dip galvanized steel sheet having a beautiful surface with no unevenness on the plating surface can be obtained while maintaining the adhesion of the plating.

The composition analysis of such a plating film is performed by the ICP method.
(Inductively-coupled high-frequency plasma spectroscopy).
This is an emission spectroscopy method in which a sample in the form of a solution is mainly introduced into non-polar discharge plasma generated when a high-frequency current is passed from a high-frequency oscillator with an output of 2 to 10 kW in the MHz range. The medium stay time is long and high-sensitivity simultaneous multi-element quantification is possible. Used in steel mills to analyze hundreds of samples per day.

As described above, the present invention is characterized in that the degree of the alloying treatment is limited to a certain range.
One expression is to define the Al and Fe concentrations in the plating film, and another expression is to further define the amount of η phase in the plating film.

According to the embodiment of the present invention, the unevenness of the plating surface is basically caused by the magnitude of the amount of coating in the same plane, but by alloying Zn in the plating film with Fe of the steel sheet, Heat input to the interface between the plating and the steel sheet increases in the part with a small amount of plating compared to the part with a large amount of coating, and the alloying proceeds rapidly, so that the ζ phase precipitates and the volume increases due to the precipitation of the ζ phase In addition, unevenness can be reduced as the plating thickness.

Therefore, in a preferred embodiment of the present invention, when the η phase in the plating film exceeds 95 atm%, alloying of the plating film does not proceed, and the effect of reducing unevenness on the plating surface is poor, and the plating surface is beautiful. Can not. Ζ in plating film
If the Fe—Zn alloy layer such as a phase grows thickly and the η phase becomes less than 50 atm%, the plating film becomes too brittle and the adhesion of the plating decreases.

The thickness of the plating film in the hot-dip galvanized steel sheet according to the present invention is not particularly limited, but is preferably about 20 to 200 g / m ² (plating film thickness: 2.8 to 28 μm). Plating reference W _CM defined by JIS B0610 respect uneven surface of the steel sheet length 30 mm, and evaluated by the value measured at a cut-off 0.8 mm, which is equal to 10μm or less, the surface unevenness defects such as automobile outer panels Even in strict applications, it is good because it does not appear in appearance.

The hot-dip galvanized steel sheet according to the present invention may be manufactured basically according to the method of manufacturing a GI steel sheet. In particular, as a production technique for suppressing the unevenness of the plating surface and ensuring the adhesion of the plating, the Al content in the zinc plating bath is such that the Al in the plating film is 0.20% by mass or less.
The range is 0.15% by mass to 0.3% by mass so as to be 1.0% by mass.

This is because Al has a higher reactivity with Fe than Zn, and a Fe ₂ Al ₅ phase is precipitated at the interface between the plating and the steel sheet, so that the Al is concentrated in the coating to a concentration higher than the concentration of the plating bath. . In addition, the Al concentration in the plating bath is a normal GI plating bath. In practicing the present invention, the bath used for the production of a hot-dip galvanized steel sheet that does not undergo alloying may be used as it is. Since it is not necessary, the product can be provided at low cost.

The purpose of heating the plated steel sheet of which the amount of plating has been adjusted by gas wiping is to redissolve the plating surface. In a conventional GI steel sheet, the zinc plating solidifies in the state of gas wiping, so that the plating surface has irregularities (wavy irregularities) due to vibration during gas wiping and dross adhering in the plating bath. Exists. All of these impair the appearance.
In the present invention, by re-melting the surface of the plating and solidifying the plating in a state where it is released from vibration during wiping, irregularities on the plating surface disappear, and at the same time, the plating is reheated and alloyed. In addition, dross adhering in the plating bath is moved to the interface between the plating and the steel sheet, and the surface can be made beautiful.

In order to re-dissolve the surface of zinc plating,
The temperature must be equal to or higher than the melting point of zinc. However, when heated to a high temperature exceeding 600 ° C., when the Al concentration in the plating bath is 0.3% by mass or less, the Fe concentration in the plating film exceeds 6% by mass, and a brittle Fe-Zn alloy layer is formed at the interface between the plating and the steel sheet. Precipitation causes the adhesion of the plating to be impaired, and uneven color tone such as alloying unevenness occurs on the surface of the plating. Preferred heating temperature has a lower limit of 450
℃, the upper limit is 550 ℃.

[0031]

EXAMPLES Table 1 shows examples of the present invention and Table 2 shows comparative examples. In the examples of the present invention, a hot-rolled steel sheet obtained by heating and hot-rolling a continuously cast slab, and a cold-rolled steel sheet obtained by further pickling and cold-rolling the slab were used for the material of the galvanized steel sheet (the material of the present invention). May be a hot-rolled steel sheet).

As the material of the steel plate, a low carbon steel, a P-added steel and an ultra low carbon steel having the following steel compositions were used. Low carbon steel: [C] = 0.02 to 0.18% by mass, [Si] ≦ 0.20% by mass, [Mn] = 0.05 to 1.00% by mass, [P] ≦ 0.05% by mass,
[S] ≦ 0.05% by mass, balance unavoidable impurity element and iron. P-added steel: [C] = 0.001 to 0.01% by mass, [Si] ≦ 0.50% by mass, [Mn] = 0.05 to 1.00% by mass, [P] ≧ 0.05% by mass,
[S] ≦ 0.05% by mass, balance unavoidable impurity element and iron. Ultra low carbon steel: [C] ≤ 0.02 mass%, [Si] ≤ 0.20 mass%, [M
n] ≦ 0.10% by mass [P] ≦ 0.05% by mass, [S] ≦ 0.05% by mass, [Ti] = 0.003 to 0.05% by mass, [Nb] = 0.001 to 0.05% by mass, balance inevitable impurity element and iron.

The above-mentioned material was immersed in a continuous hot-dip galvanizing line in a bath having an Al concentration of 0.15 to 0.30% by mass in a galvanizing bath, followed by gas wiping to obtain a coating weight of 30 g / m ^{2 on} one side. the plated steel sheet after the plating in the range of to 150 g / m ² was re-heated at a temperature in the range of 420 to 600 ° C. at a steel sheet temperature.

The contents of Fe and Al in the plating film were determined by quantitatively analyzing a solution in which the plating film was dissolved with hydrochloric acid by an ICP method. The volume ratio of η phase in the plating film was determined by using an electrolytic potential method.

That is, ZrSO ₄ .7H ₂ O: 100 g / l, NaCl:
In a 200 g / l electrolytic solution, low current electrolysis is performed using the sample as an anode at a current density of 20 mA / cm ² , and the potential is measured using Ag as a reference electrode.

At this time, electrolysis is started from the outermost layer as a starting point, and the range showing a potential of less than -750 mV with respect to the Ag electrode is η.
Phase. The electroplating was started with the outermost layer as a starting point, and a range showing a potential of -290 mV with respect to the Ag electrode was defined as a plating layer constituting a plating film.

Here, after starting electrolysis of the outermost layer, Ag
The time required to show a potential of -750 mV with respect to the electrode is A second,
After starting the outermost layer electrolysis, -290 mV with respect to the Ag electrode
Is assumed to be T seconds, the ratio of the η phase in the plating film can be obtained by η phase atm% = A × 100 / T.

With respect to the corrosion resistance, a salt spray test specified in JIS Z2371 was performed in the as-plated state without any post-treatment, and evaluated by the time until red rust was generated. Regarding the workability, the plated steel sheet was closely bent, and it was observed whether cracks or peeling occurred in the plating film. 1: No cracking and peeling occurred, 2: Cracks occurred but no peeling occurred, 3 : Evaluated using a three-step index of occurrence of plating exfoliation.

As an index of surface beautification, surface irregularities are measured by JI
Reference W _CM defined by S B0610 length 30 mm, cut-off
Evaluated by the value measured at 0.8 mm, the plating surface shown in FIG.
W _CM and W _CM from the evaluation results after the painting was considered good 10μm or less.

The results are summarized in Tables 1 and 2. As can be seen from the results in Tables 1 and 2, by performing plating within the range of the Al concentration in the zinc plating bath specified in the present invention, the Al content in the plating film is reduced to 0.20 to 1.0% by mass. The reheating temperature after plating is 420 ~
By raising the temperature to 600 ° C, the Fe content in the plating film can be reduced.
0.5-6% by mass, and the η phase in the plating film is 95-
Was 50 atm%, W _CM further plated surface is at 10μm or less, it was possible to obtain a beautiful steel sheet surface irregularities and excellent less workability.

On the other hand, as shown in the comparative example of Table 2, if the reheating temperature after plating is lower than 420 ° C. and too low, the Fe% in the plating film is less than 0.5%, and the volume ratio of the η phase is less than 0.5%. 95at
%, and irregularities remain on the plating surface.

If the reheating temperature is higher than 600 ° C., the growth of the Fe—Zn alloy layer in the plating film proceeds, and the Fe% in the plating film exceeds 6% by mass. The volume ratio of the phase is less than 50 atm%, the adhesion of the plating is deteriorated, and the workability is poor.

When the concentration of Al in the zinc plating bath is less than 0.15% by mass, the content of Fe in the plating film rapidly increases upon reheating after the plating, so that the present invention needs to be contained in the plating film. In this case, the Fe content in the range described above cannot be obtained, and the adhesion of the plating is inferior.

If the Al concentration in the zinc plating bath exceeds 0.30% by mass, the Fe% in the plating film does not increase at all, and the Fe content in the range required by the present invention cannot be obtained in the plating film, and No effect is observed in reducing surface irregularities.

[0045]

[Table 1]

[0046]

[Table 2]

[0047]

As described above, according to the present invention, a GI steel sheet having the advantages of a GA steel sheet can be obtained.
Despite the low production cost, a product superior in performance to any conventional galvanized steel sheet can be obtained, and the present invention will greatly contribute to the development of the art.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of Examples.

Continued on the front page (72) Inventor Yoshikazu Ando 1850 Minato, Wakayama-shi, Wakayama Sumitomo Metal Industries, Ltd. Wakayama Works F-term (reference) 4K027 AA02 AA22 AB15 AB28 AB35 AB42 AB44 AC73 AE03 AE12 AE25 AE27

Claims (4)

[Claims] Claims: 1. A plating film containing 0.20% by mass or more of Al
A hot-dip galvanized steel sheet with a beautiful surface content of 0.5% by mass or less and Fe of 0.5% by mass or more and 6.0% by mass or less. 2. The method according to claim 1, wherein the plating film contains 0.20% by mass or more of Al.
A hot-dip galvanized steel sheet having a beautiful surface, characterized by containing not more than 0.5% by mass, not less than 0.5% by mass and not more than 6% by mass of Fe, and having 95 to 50 atm% of η phase in a galvanized film. 3. The method according to claim 1, wherein the plating film contains 0.20% by mass or more of Al.
Mass% or less, and Fe is contained below 6% by mass to 0.5% by mass, beautiful galvanized steel sheet surface, characterized in that W _CM of the plated steel sheet surface is 10μm or less. 4. A steel sheet immersed in a hot-dip galvanizing bath containing 0.15% by mass or more and 0.3% by mass or less of Al, and then a coated steel sheet whose gas adhesion is adjusted by gas wiping is heated to a temperature of 420 ° C. or more. By heating in the range of 600 ° C or less, Al in the plating film is 0.20% by mass or more and 1.0% by mass
And Fe is contained in an amount of 0.5% by mass or more and 6% by mass or less,
A method for producing a hot-dip galvanized steel sheet having a beautiful galvanized surface in which 95 to 50 atm% of the η phase is present in the galvanized film.