JPH05320946A - Manufacture of alloyed galvanized steel sheet and galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To obtain a galvanized steel sheet having a plating film free from nonplating and having a good surface condition for a base metal steel sheet high in the content of Si and Mn. CONSTITUTION:The base metal steel sheet is annealed in a nonoxidizing atmosphere for iron, thereafter its surface is coated with Al or an Al alloy (such as an Al-Zn alloy, an Al-Mg alloy, etc.) having >=20Angstrom thickness. Thereafter, galvanizing is executed. At the time of executing alloying treatment, the thickness of the Al or Al alloy coating layer is regulated to 20 to 1000Angstrom so as to suitably execute the alloying.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a hot-dip galvanized steel sheet and alloy capable of obtaining a good galvanized surface without causing zinc non-adhesion (non-plating) even if the base steel sheet has a high Si or Mn content. The present invention relates to a method for producing a galvanized steel sheet.

[0002]

2. Description of the Related Art Hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets are widely used in industrial fields such as automobiles, home electric appliances, and building materials. In recent years, as the demand for weight reduction of automobile materials has increased, It has become necessary to increase the strength of the base steel sheet.

In order to improve the strength of steel without impairing the ductility, it is effective to increase the contents of Si and Mn. Therefore, conventionally, 0.15% by weight of Mn and 0.1% by weight of Si (as a base steel sheet) Hereinafter, "%" of alloying elements means "% by weight"), while a steel sheet containing about Mn was used.
Steel sheets containing up to 2% of Si and Si as needed have come to be used.

Hot-dip galvanized steel sheets are usually produced by annealing a cold-rolled steel sheet in a reducing atmosphere before plating, cooling it, and immersing it in molten zinc to deposit a zinc coating on the surface of the steel sheet. For example, in a continuous galvanizing line equipped with Zenzimer hot dip galvanizing equipment, cold-rolled steel sheet is annealed in a temperature range of 500 to 900 ° C in an atmosphere where iron is reduced and then cooled to about 500 ° C. Immerse in a hot dip zinc bath to obtain hot dip galvanized steel sheet. Alloyed hot dip galvanized steel sheet is the hot-dip galvanized steel sheet that has passed through the zinc bath immediately.
It is introduced into a heating furnace maintained at 450 to 700 ° C to cause mutual diffusion of Zn and Fe between the plating film and the steel substrate, and the plating layer is made of Fe.
-Manufactured by alloying with Zn.

However, when the content of alloy elements, particularly Si and Mn, of the cold-rolled steel sheet, which is the base material, increases, zinc does not adhere to the steel sheet surface and an unplated portion may occur on the surface of the steel sheet after plating. is there. This is due to the presence of oxides of Si and Mn on the surface of the steel sheet, which deteriorates the wettability between the steel sheet and molten zinc [eg, CAMP-ISI
J, Vol. 4 (1991) -1632].

On the surface of the cold-rolled steel sheet before being immersed in the molten zinc bath, Si or Mn produced in the annealing process before plating, or
There are oxides of elements that are easily oxidized, such as Al,
These oxides are usually contained in the molten zinc bath at 0.05 to 0.20% by weight.
Since it is reduced by the contained Al, an unplated portion does not occur on the surface of the steel sheet when the amount of oxide is small. However, in a steel sheet with an increased Si or Mn content, a large amount of Si or Mn oxide is present on the surface,
Since reduction with Al in the zinc bath is insufficient and these oxides remain and the wettability deteriorates, zinc does not adhere to the steel sheet surface and non-plating occurs.

[0007]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention As described above, the present invention solves the problem of zinc non-adhesion (non-plating) that tends to occur when the content of Si or Mn in the base steel sheet is increased. An object is to produce a hot-dip galvanized steel sheet and a galvannealed steel sheet having good properties.

[0008]

Means for Solving the Problems The inventors of the present invention have conducted various studies to solve the above-mentioned problems, and as a result, immediately after the annealing treatment of the steel sheet soaked in a molten zinc bath, a trace amount It was found that the presence of Al is very effective.

FIG. 1 shows a cold rolled steel sheet containing 0.67% Si and 1.47% Mn at 850 ° C. in a reducing atmosphere (26% by volume H ₂ + N ₂ ).
When an annealing treatment of × 90 seconds is performed (A in the figure), and the steel sheet is subjected to Al vapor deposition treatment (room temperature), the surface with a thickness of 20Å
It is a figure which shows the result of having investigated the reduction state of Mn oxide by the photoelectron spectroscopy analysis about the case where the film of Al is formed (B in a figure). In this figure, only the peak of the Mn oxide is observed in A just annealed, whereas the height of the peak of Mn oxide decreases in B which is Al vapor deposition after the annealing. At the same time, a peak showing the presence of metal Mn (Met.Mn) is seen. That is, it is shown that it is possible to reduce the Mn oxide by coating a steel sheet having Mn oxide on the surface with a thin layer of Al. When the steel sheet in such a state is immersed in the molten zinc bath, non-adhesion of zinc does not occur and a plating film having a good surface quality can be obtained.

The present invention has been made on the basis of such findings, and the gist thereof lies in the following method for producing a galvanized steel sheet and the following method for producing a galvannealed steel sheet.

The base material steel sheet is annealed in a non-oxidizing atmosphere with respect to iron, the surface thereof is coated with Al or Al alloy having a thickness of 20 Å or more, and then hot dip galvanizing is performed. Manufacturing method of hot-dip galvanized steel sheet.

After the base material steel sheet is annealed in a non-oxidizing atmosphere with respect to iron, its surface is coated with Al or Al alloy having a thickness of 20 to 1000 Å, and then hot-dip galvanized, and further alloyed. A method for producing an alloyed hot-dip galvanized steel sheet, which comprises performing an alloying treatment.

The base steel sheet is usually a steel sheet used as a base material for hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets, and contains Si and Mn which are apt to cause non-plating by ordinary methods. High quantity steel plate, ie
It is possible to use a steel sheet containing Si and Mn in an amount of 3% in excess of the usual content.

As a method for coating the surface of the steel sheet with Al or an Al alloy, a vapor deposition method is suitable, but a sputtering method or any method capable of depositing atomic Al on the surface of the steel sheet can be applied. The Al alloy is Al
And an alloy of a metal that does not adversely affect plating, such as an Al-Fe alloy and an Al-Zn alloy.

[0015]

[Operation] By applying Al vapor deposition treatment to a steel sheet on the surface of which Mn or Si oxide exists, a thin layer of Mn or Si is formed.
The reason why it is possible to reduce the oxide and to prevent the occurrence of non-plating during hot dip galvanization to obtain a good plating film is considered as follows.

FIG. 2 shows a case where an annealed steel sheet having an oxide of Mn or Si present on the surface is subjected to Al vapor deposition treatment for hot dip galvanizing, and a case where plating is performed without Al vapor deposition treatment. It is a figure which shows typically the situation in which a plating film is formed.

When the Al vapor deposition process is not carried out, the oxides of Mn and Si existing on the surface of the steel sheet after the annealing process (Mn in the figure
-Si-O) impedes the wettability between the molten zinc and the steel sheet, resulting in non-plating, so a uniform plating film is not formed. On the other hand, when Al is vapor-deposited on the steel plate surface, Mn and
Oxide of Si is reduced by Al to become metal Mn or metal Si (Met.Mn or Met.Si in the figure), while part of evaporated Al becomes oxide (Al-O in the figure). , The balance is metallic Al (Met.
Coating the surface as Al). Oxide of Al does not affect the wettability because it exists only in the lower layer of metallic Al, that is, at the interface between the steel plate and the evaporated film of Al, and when this steel plate is dipped in a molten zinc bath, it becomes uniform. Plating film is formed. At this time, the Al oxide becomes Al ₂ O ₃ and floats up to become dross.

At the interface between the plating film (Zn) and the steel plate (Fe)
An Fe-Al-Zn alloy layer is formed, and this alloy layer is the same as the alloy layer formed when hot dip galvanizing is performed in a plating bath containing 0.05 to 0.20% by weight of Al. It has excellent adhesion to the plating film (Zn). In the present invention, an Al vapor deposition process is performed before plating to form an Al layer on the surface of a steel sheet, and thereby an Fe-Al-Zn alloy layer is generated during plating.

The reason why the base steel sheet is annealed in a non-oxidizing atmosphere with respect to iron before coating with Al is as follows. When the base steel sheet is annealed in such an atmosphere, the surface of the steel sheet is coated with an iron oxide film ranging from several hundred Å to several μm, and Al in the plating bath or the Al coating according to the method of the present invention also causes Not all of the oxide film can be reduced. Therefore, when hot dip galvanizing is performed, adhesion of the plating occurs, but the plating film is easily peeled off. Therefore, the annealing atmosphere of the base steel sheet should be an atmosphere in which iron is not oxidized.

When coating the surface of the steel sheet with Al or an Al alloy by vapor deposition treatment, the atmosphere during vapor deposition should be 10 ^-5 to 10 ^-4.
A high vacuum with a pressure not higher than Torr may be used, or an atmosphere reducing iron may be used. The temperature of the steel plate during vapor deposition is 800
If the temperature exceeds ° C, the oxidation rate of Al will increase, so the temperature should be 800 ° C or less. If vapor deposition is performed in such an atmosphere,
Oxides of Si and Mn are reduced during vapor deposition.

It is considered that the thickness of the vapor-deposited film of Al is basically effective as long as the surface of the steel sheet is coated with Al, even if it is a coating of one atomic layer. However, in practice, it is difficult to uniformly deposit Al with a thickness of several Å, and as shown in the examples, in order to coat the entire surface of the steel sheet with Al, the thickness of the deposited film is 20 Å It is necessary to do the above. Further, even when the thickness exceeds 1000 Å, no galvanization occurs, and a galvanized steel sheet having sufficient performance as a plating film can be obtained (the above invention). However, when producing alloyed hot-dip galvanized steel sheet, the thickness of the deposited film is 10
If it exceeds 00Å, the alloying rate will be delayed, which is not preferable in practice. Therefore, when performing the alloying treatment, it is preferable that the thickness of the vapor deposition film of Al be 20 to 1000 Å.

The coating of the surface of the steel sheet with Al is performed by using pure Al, but is not limited to Al-Fe alloy, Al which does not adversely affect plating.
It may be performed using a —Zn alloy or the like. Even Al that constitutes such an alloy has an action of reducing oxides of Si and Mn. Further, Fe and Zn in the alloy are both elements present in the base steel plate and the plating film, and there is no problem even if they are present in the coating layer that covers the steel plate surface.

[0023]

Example 1 A cold-rolled steel sheet full-hard material (sheet thickness 0.8 mm) having the chemical composition shown in Table 1 was used as a base steel sheet, and after degreasing the surface with acetone, a reducing property of 26% by volume H ₂ + N ₂ was obtained. Annealing treatment at 850 ° C for 90 seconds in the atmosphere, then Al vapor deposition treatment at the vapor deposition temperature shown in Table 2 in the same reducing atmosphere at the time of cooling, cooling to 500 ° C, and then immersing in the plating bath Hot dip galvanizing was performed. Figure 3 shows the heat pattern during this time.
Shown in (a). Plating bath temperature is 460 ℃, Al in plating bath
The concentration was 0.1% by weight and the immersion time was 1 second. In addition, for some steel sheets, as shown in Fig. 3 (b), after annealing treatment in the same reducing atmosphere as above, the steel sheet was cooled to 25 ° C and then in vacuum.
Al vapor deposition treatment was performed, heating was performed again to 500 ° C. in the same reducing atmosphere, and then immersion in a plating bath. For comparison, the same base material steel sheet was used, and hot dip galvanizing treatment was performed without performing Al vapor deposition treatment.

With respect to the plating film thus obtained, it was investigated whether or not the zinc adhesion was good, that is, the presence or absence of non-plating. Moreover, when a good plating film without non-plating was obtained, the alloying treatment was performed at 500 ° C. for 60 seconds in the air atmosphere.

The survey results are shown in Table 2. The alloying (%) in the table is the content of Fe in the plating film after the alloying treatment. The quality of zinc adhesion is visually observed. When there is no unplating (when the ratio of the area of the unplated part to the total surface area is 0%), the ratio of the area of the unplated part is less than 20%. Is indicated by Δ, and when 20% or more is indicated by x. In addition, the thickness of the Al vapor deposition film was obtained by measuring the thickness of the vapor deposition film formed on the pure iron surface by performing annealing and vapor deposition treatment using the pure iron test material under the same conditions as the above test material. It was The film thickness is measured by 150
Photoelectron spectroscopy was performed at a thickness of about Å, and SIMS (secondary ion mass spectrometer) was used when the thickness was exceeded.

As is clear from the results shown in Table 2, when a vapor deposition film of Al having a thickness of 20 Å or more is formed before being immersed in a plating bath, a hot-dip galvanized steel sheet without unplating can be manufactured. Also, if the thickness of the deposited film is 1000 Å or less,
Fe content in the plating film is 8 to 13% under specified conditions
Can be appropriately alloyed, and a galvannealed steel sheet having a good surface condition can be produced. When the Fe content in the plating film is 8 to 13%, the plating layer does not peel during processing.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

Example 2 A steel sheet having the chemical composition shown in Table 1 was reduction-annealed under the conditions shown in Example 1 and cooled to room temperature.
Al-Zn alloy (55% Al-45% Zn) was sputtered on the surface, and Al-Mg alloy (97% Al-3% Mg) and
Zn was deposited by the vacuum deposition method.

After vapor deposition, under the same conditions as in Example 1, 5
After heating to 00 ° C, plating treatment was performed.

With respect to the plating film thus obtained, it was investigated whether or not the zinc adhesion was good, that is, whether or not there was non-plating. Moreover, when a good plating film without non-plating was obtained, the alloying treatment was performed at 500 ° C. for 60 seconds in the air atmosphere.

The survey results are shown in Table 3. The determination of the quality of the zinc adhesion and the measurement of the thickness of the vapor-deposited film of Al alloy are the same as in the first embodiment.

As shown in Table 3, even if an Al alloy is used as a coating material for the base steel sheet after the annealing treatment, the thickness of the aluminum alloy is 20 Å or more.
By forming a vapor-deposited film of Al alloy, it is possible to manufacture a galvanized steel sheet without unplating. Further, alloying could be performed without any trouble.

In the Al-Zn alloy vapor deposition, Zn is also vapor deposited in a large amount. For the purpose of investigating the effect of this Zn, a test was also conducted when only Zn was vapor-deposited, but as shown in Table 3, the plating adhesion was poor. As described above, when the Al-Zn alloy is vapor-deposited, the base steel plate surface
Since the oxides of Mn and Si are reduced by Al, the adhesion of the plating is improved, but when only Zn is vapor-deposited,
It shows that the adhesion of the plating is not improved because the oxides of Mn and Si are not reduced.

[0035]

[Table 3]

[0036]

EFFECTS OF THE INVENTION By applying the method of the present invention, even when a steel sheet having a high Si or Mn content and easily unplated by the ordinary method is used as a base steel sheet, there is no unplating and a good plating surface. It is possible to manufacture a hot-dip galvanized steel sheet having the above and an alloyed hot-dip galvanized steel sheet.

[Brief description of drawings]

FIG. 1 is a diagram showing a result of investigating a reduction state of Mn oxide on a surface of a steel sheet with Al by photoelectron spectroscopy.

FIG. 2 is a diagram schematically showing a plating film formation situation when a vapor deposition process is performed on a steel sheet having an oxide of Mn or Si on the surface and when it is not performed.

FIG. 3 is a diagram showing a heat pattern from the annealing treatment performed in the example to the hot dip galvanizing.

Claims (2)

[Claims] 1. A base steel sheet is annealed in a non-oxidizing atmosphere with respect to iron, and then the surface thereof is made of Al or Al having a thickness of 20 Å or more.
A method for producing a hot-dip galvanized steel sheet, which comprises coating with an alloy and then hot-dip galvanizing. 2. A base material steel sheet is annealed in a non-oxidizing atmosphere with respect to iron, and then the surface thereof is made of Al having a thickness of 20 to 1000Å or
A method for producing an alloyed hot-dip galvanized steel sheet, which comprises coating with an Al alloy, hot-dip galvanizing, and then an alloying treatment.