JP2630136B2 - Hot-dip galvanizing method for silicon-containing steel sheet - Google Patents

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 and a hot-dip galvanized steel sheet obtained by using a silicon-containing steel sheet as a base material and having an excellent surface quality without plating.

[0002]

2. Description of the Related Art In recent years, galvanized steel sheets have been used in large quantities in the industrial fields such as home appliances, building materials, and automobiles. Plated steel sheets are widely used.

[0003] A hot-dip galvanized steel sheet is usually subjected to an appropriate degreasing and washing step or without degreasing.
It is manufactured by preheating a steel sheet in a weakly oxidizing atmosphere or a reducing atmosphere, annealing in a reducing atmosphere of hydrogen and nitrogen, cooling to a temperature suitable for plating, and immersing it in molten zinc. You. It is considered desirable to form an oxide film with a thickness of about 80 nm on the steel sheet surface during preheating in the pre-annealing process from the viewpoint of wettability with molten zinc. Rather, it is considered to have an adverse effect on dross generation and adhesiveness of hot-dip plating.
As the zinc bath, a bath containing 0.08 to 0.18% by weight of Al is used, including the range of production of a galvannealed steel sheet described below.

An alloyed hot-dip galvanized steel sheet is prepared by heating a steel sheet that has been continuously hot-dip galvanized as described above in a heat treatment furnace at a material temperature of 500 to 600 ° C. for 3 to 30 seconds to obtain a zinc layer and a base steel sheet. It is manufactured by causing Zn and Fe to interdiffuse with each other and making the plating layer an Fe-Zn alloy. Accordingly, the plating layer of the alloyed hot-dip galvanized steel sheet is made of an intermetallic compound of Fe-Zn, and its average Fe concentration is generally adjusted to be 8 to 12% by weight.

[0005] coating weight of the galvannealed steel sheet is usually per side 25~70g / m ^2, well below the 25 g / m ² is difficult to manufacture in conventional means, 70 g / m ²
Those exceeding the above are not generally supplied because it is difficult to secure the powdering resistance of the plating layer.
In addition, the plating film usually contains about 0.12 to 0.2% by weight of Al.
Is often contained. This suppresses the formation of an alloy layer at the interface between the coating layer and the steel of a normal hot-dip galvanized steel sheet manufactured in the same equipment as the alloyed hot-dip galvanized steel sheet, and is applied to the plating bath to maintain the workability of the coating film. Although the added Al may inevitably mix into the plating film, the plating is usually performed from the viewpoint of securing the powdering resistance of the galvannealed film and suppressing the generation of dross during production. It is believed that it is rather appropriate to incorporate about 0.08 to 0.11% by weight of Al into the bath. Since Al in the plating bath tends to be enriched in the plating layer,
When plating is performed in a plating bath containing about 0.08 to 0.11% by weight of Al, the Al concentration in the plating film is in the range of 0.12 to 0.2% by weight.

Conventionally, low-carbon Al-killed steel sheets, ultra-low-carbon Ti-added steel sheets and the like have been mainly used as base materials for hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets. In some cases, higher strength is required, and a silicon-containing steel sheet containing 0.2% by weight or more of Si is being used. Si has the advantage of improving strength without impairing the ductility of steel, and in that sense, silicon-containing steel sheets are promising as high-strength materials for automobiles.

[0007]

However, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets based on silicon-containing steel sheets have problems in quality and production as described below.

[0008] When hot dip galvanizing a silicon-containing steel sheet by the above-described normal process, a trace amount of moisture in the atmosphere reacts with Si in the steel sheet in an annealing process before plating to form Si-Oxide on the steel sheet surface. Therefore, the wettability with molten zinc decreases rapidly with an increase in the Si concentration in the steel. In particular, in the case of steel sheets with Si added to the base of ultra-low carbon steel with Ti added to improve the formability of the steel sheet, the annealing temperature for recrystallization is as high as 800 ° C or more Therefore, precipitation of Si-Oxide on the surface of the steel sheet becomes more remarkable, and it becomes more difficult to secure wettability. As a result, non-plating frequently occurs.

In order to solve such a problem, it is known that the wettability is improved by heating a steel sheet in an oxidizing atmosphere in advance and forming iron oxide on the surface of the steel sheet. But,
In the case of a steel sheet having a Si content of 0.2% by weight or more, wettability is improved by preheating in an oxidizing atmosphere in a normal hot-dip galvanizing process, for example, an oxidizing atmosphere in which the air-fuel ratio of a non-oxidizing furnace is 1 to 1.35. In addition, not only is iron oxide not sufficiently formed, but also the alloying rate is extremely slow when alloying treatment is performed after plating, and the production efficiency is greatly impaired.

As another method for improving the wettability with hot-dip zinc, Ni, Fe
Although a method of under-plating a metal such as is also known, this method causes an increase in cost, a sufficient effect cannot be obtained with a steel sheet having a high Si content, and further, when a steel sheet having a high Si content is used. Has a problem that uniform alloying treatment is very difficult.

For these reasons, silicon-containing steel sheets, which are attractive as high-strength materials for automobiles, lack a reasonable means for hot-dip galvanizing or alloying them.

The present invention has been made in view of the above circumstances, and has been developed to provide a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet using a silicon-containing steel sheet as a base material and having excellent non-plating surface quality. An object is to provide a hot-dip galvanizing method that can be manufactured efficiently.

[0013]

The present inventors have studied the wettability between a steel sheet and hot-dip zinc in hot-dip galvanizing of a silicon-containing steel sheet, and found that iron oxide is formed on the steel sheet surface in a preheating stage before annealing. This was confirmed to be effective in improving the wettability of the steel sheet after reduction annealing with molten zinc. However, the oxidation rate on the steel sheet surface is greatly reduced with an increase in the Si content in the steel. There is a limit to the formation of That is,
For silicon-containing steel sheets, the desired amount of iron oxide during the steel sheet preheating process
Despite being 0.5 g / m ² in terms of Fe, obtained only iron oxide content of about 0.05~0.25g / m ² in the prior art.

As a result of further study on this problem, it was found that by setting the effective Al concentration in the plating bath to a considerably low value, the wettability of the steel sheet with molten zinc was greatly improved, and furthermore, a small amount of Ni was contained in the plating bath. It has been found that the alloying is promoted by coexisting.

The present invention has been made based on such knowledge, and the gist of the present invention is that “Fe of 0.2 g / m ² or more and 5 g / m ² or more is added in advance to the surface of a steel sheet containing 0.2% by weight or more of Si. m ²
Less than iron oxide and then in a non-oxidizing atmosphere
After annealing in the temperature range of 500-900 ° C, the effective Al content is 0.03--0.
Hot-dip galvanizing method for silicon-containing steel sheet characterized by immersing and plating in a hot-dip zinc bath containing 08% by weight and 0.004 to 0.04% by weight of Ni.

[0016]

Hereinafter, the present invention will be described in more detail.

The hot-dip galvanizing method of the present invention comprises
For steel sheets containing more than 10% by weight. This is because, for a steel sheet having a Si content of less than 0.2% by weight, the normal hot-dip galvanizing method described above, that is, after pre-heating the steel sheet in a weakly oxidizing atmosphere annealing furnace or a reducing atmosphere annealing furnace, hydrogen + nitrogen Annealed in a reducing atmosphere, cooled to a temperature suitable for plating, and then immersed in a molten zinc bath to perform hot-dip galvanizing, a product without non-plating can be obtained.
This is because, in the case of a steel sheet of 0.2% by weight or more, non-plating occurs in this hot-dip galvanizing method, and a product of excellent quality cannot be obtained.

In the method of the present invention, the formation of oxides on the surface of the steel sheet is performed in a preheating stage of a continuous furnace (usually using a non-oxidizing furnace). Weight iron oxide, as Fe, i.e., poor wettability with molten zinc is less than 0.2 g / m ² expressed in an amount of iron contained in the iron oxide, since non-plating is likely to frequently, 0.2
g / m ² or more, and the preferable range is 0.5
g / m ² or more. The upper limit of the iron oxide amount, 5 g / m ² Not as Fe
Be full. This is because the effect is saturated when the amount is 5 g / m ² or more, and the iron oxide is liable to peel off during passing. Desirable is 2 g / m ² or less. On the surface of silicon-containing steel sheet,
Although it is difficult to form a large amount of iron oxide in a normal process using an oxidation-free furnace, for example, heating to 550 ° C or more in an oxidizing atmosphere where the partial pressure of oxygen is 0.05 to 0.4 with respect to the total pressure It is achieved by doing.

The steel sheet having the iron oxide formed on its surface is annealed at 500 to 900 ° C. in a non-oxidizing atmosphere containing hydrogen, which may be performed according to a conventional method. If the temperature is lower than 500 ° C, the reduction is insufficient.If the temperature is higher than 900 ° C, the ferrite crystal grains grow abnormally, and the mechanical properties decrease.In addition, the concentration of Si on the steel sheet surface during the heating process becomes significant, It has a significant adverse effect on wettability. In this step, the iron oxide on the steel sheet surface is reduced to metallic Fe.

After annealing, the steel sheet is cooled to a temperature suitable for plating and then immersed in a plating bath to perform hot-dip galvanizing. If the effective Al concentration of the plating bath is less than 0.03% by weight, dross generation increases. Hindered operation, 0.08% by weight
If it exceeds the value, non-plating is likely to occur, so the effective Al concentration is set to 0.03 to 0.08% by weight. In addition, the effective Al
The concentration is the concentration obtained by subtracting the total concentration of Fe determined by chemical analysis from the total concentration of Al determined by chemical analysis,
Of plating bath from which Al existing as dross in bath is subtracted
It is a value usually used for convenience as an index of free Al concentration.

Ni in the plating bath has a large effect of suppressing the occurrence of non-plating and promoting the alloying reaction. However, if the concentration is less than 0.004% by weight, the effect is insufficient,
If it exceeds weight%, it will exceed the solubility limit and become dross,
Ni concentration shall be 0.004 to 0.04% by weight.

In the case of performing the alloying treatment, after the above steps, a heat treatment may be performed according to a conventional method. In the case of the present invention, since the alloying reaction is promoted, the alloying treatment can be performed at an alloying temperature of 520 ° C. or less, and therefore, a product having relatively excellent powdering resistance can be manufactured.

[0023]

[Examples] Five types of ultra-low carbon steel cold rolled steel sheets (unannealed material, thickness 0.8 mm) shown in Table 1 were cut into 250 mm x 100 mm to obtain test materials, which were mixed in a mixed gas of oxygen and nitrogen. After oxidation with
Hot-dip plating was performed using a vertical hot-dip plating apparatus capable of performing a heat treatment in a predetermined atmosphere and directly charging a test material from a reducing atmosphere into a hot-dip plating bath. That is, after the specimen is degreased in advance with a 10% NaOH solution, oxygen is reduced to 2%.
Preheating in a nitrogen atmosphere (total pressure 1 atm) containing various ratios of ppm to 15 vol%, and then N ₂ + with an oxygen concentration of 2 ppm
Perform annealing at 850 ° C for 60 seconds in a 40% H ₂ mixed gas (dew point: -30 ° C), cool to 520 ° C, and immerse in a molten zinc bath with various Al and Ni concentrations to perform plating Processing was performed. The plating time was 1 second, and the amount of Zn deposited was adjusted to about 50 g / m ² (per one side) by a gas wiper.

After plating, the occurrence of non-plating was examined.
Furthermore, the test piece after the hot-dip plating was alloyed in a salt bath at 500 ° C., and the time until the completion of the alloying was measured.

The results are shown in Table 2. As is clear from this table, in Nos. 3 to 9 (Examples of the present invention), no non-plating occurred, and good plating properties could be ensured for steel containing 0.2% or more of Si. Was. Further, the time required for alloying has been reduced. On the other hand, the plating bath does not contain Ni,
The effective Al is also out of the range defined by the method of the present invention.
In No. 11 (Comparative Example), non-plating occurred frequently, and in No. 2 and No. 10 (Comparative Example) in which the bath composition was out of the range specified by the method of the present invention even when Ni was contained, the effect of suppressing the occurrence of non-plating was observed. However, there was a problem that the time required for alloying was extremely long.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

As described above, when a silicon-containing steel sheet is plated by the hot-dip galvanizing method of the present invention, a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet having excellent non-plating surface quality can be obtained. Can be manufactured well.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-134147 (JP, A) JP-A-4-276057 (JP, A) JP-A-5-51714 (JP, A) JP-A-5-51714 65612 (JP, A) JP-A-5-70920 (JP, A)

Claims (1)

(57) [Claims] To 1. A surface of the steel sheet containing not less than 0.2 wt% of Si, previously, to form iron oxide of less than 5 g / m ² at 0.2 g / m ² or more as Fe, then in a non-oxidizing atmosphere 500 ~ 900 ℃
After annealing in the temperature range of 0.03 to 0.08% by weight of effective Al, Ni
Hot-dip galvanizing method for a silicon-containing steel sheet, characterized by immersing and plating in a hot-dip zinc bath containing 0.004 to 0.04% by weight.