JPH04358051A - Manufacture of glavannealed steel sheet and its manufacturing device - Google Patents

Abstract

PURPOSE:To manufacture a galvanealed steel material excellent in an outer appearance quality without rudgedness flaw as well as dross flaw on a surface. CONSTITUTION:Plating treatment is beforehand applied to a fused galvanizing bath having a low Al concn. and successively, plating treatment is executed in a fused galvanizing bath having high Al concn. For example, the steel material 1 is dipped into an initial stage plating vessel 5 housing the galvanizing bath having <=0.10wt.% Al concn. obtd. by recovering the galvanizing bath incorporating bottom dross from the initial plating vessel 5 into a separating vessel 3 with a recovering pump 7 and supplying the clean fused Zn liquid separating the bottom dross in the separating vessel 3 with a supplying pump 6 to beforehand form Fe-Zn alloy layer on the surface and successively, the galvanizing treatment is applied in the fused Zn bath incorporating Al and thereafter, alloying treatment is applied.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] This invention relates to a method for manufacturing steel materials such as alloyed hot-dip Zn-plated steel sheets used for outer panels for automobiles, outer panels for home appliances, etc., and equipment for producing the same.
In particular, the present invention relates to a method for manufacturing alloyed Zn-plated steel material with extremely excellent appearance quality and equipment for manufacturing the same.

0002

[Prior Art] Alloyed hot-dip Zn-plated steel is a process in which steel is subjected to hot-dip Zn plating and then heated to cause interdiffusion between the Zn layer on the surface of the plating layer and the steel base material, thereby dissolving the entire plating layer. is made of Fe-Zn alloy. This alloyed hot-dip Zn-plated steel material has excellent paintability, corrosion resistance, weldability, etc., and is widely used for outer panels of automobiles, outer panels of home appliances, etc.

When a molten Zn bath is used in the hot-dip Zn plating process, Fe in the molten Zn bath exceeds its solubility limit and reacts with Zn to form an Fe--Zn alloy. Since this Fe-Zn alloy has a slightly higher specific gravity than the molten Zn bath, it precipitates in the form of sand grains at the bottom of the molten Zn bath. This is called bottom dross. This bottom dross has a slight difference in specific gravity from the molten Zn bath, and is rolled up by the accompanying flow from steel materials and sink rolls. The rolled up bottom dross adheres to the steel material and sink roll, and is pressed against the surface of the steel material by the sink roll,
It has an adverse effect on the appearance of the plating layer surface. Furthermore, if press forming is performed with bottom dross still attached to the surface of the steel material,
Not only does the appearance quality of the surface of the steel material on the side to which the bottom dross has adhered deteriorate, but the bottom dross is also transferred to the surface of the steel material on the opposite side to the side to which the bottom dross has adhered, significantly impairing the appearance quality. This deterioration in appearance quality due to the adhesion of bottom dross is called a dross defect.

[0004] In order to prevent dross defects from occurring on the surface of hot-dip Zn-plated steel materials, 0.10 to 0.2
Approximately 0% by weight (hereinafter, "%" means "weight%") of Al is added. When Al is added, the steel in the molten Zn bath reacts preferentially with Al before reacting with Zn, forming a thin Fe2Al5 or Fe-Al-Zn alloy layer on the surface of the steel, and removing Fe from the surface of the steel. Inhibits dissolution. At the same time, Fe dissolved in the molten Zn bath also preferentially reacts with Al to form Fe2Al5. This Fe2Al5
has a lower specific gravity than the molten Zn bath, so it floats on the surface of the molten Zn bath. This is called top loss. This top dross is regularly and easily removed during the hot-dip Zn plating process, and the top dross is prevented from adhering to the surface of the steel material.

[0005] In this way, Al is added to the molten Zn bath to actively create top dross, thereby reducing the formation of bottom dross and preventing the occurrence of dross defects.

On the other hand, after hot-dip Zn plating, the thin Fe2Al5 or Fe-Al-Zn alloy layer formed on the surface of the steel material is , which not only suppresses the alloying reaction and causes unevenness in the alloying reaction rate and increases the alloying reaction time, but also deteriorates the appearance quality of the steel surface. Therefore, alloyed molten Zn with a beautiful appearance
When producing plated steel materials, it is necessary to reduce the Al concentration contained in the molten Zn bath as much as possible in the process of hot-dip Zn plating.

[0007] Usually, the concentration is set near the lower limit of the range of Al concentration required in the hot-dip Zn plating process.

[0008] As described above, Al, which is added to the molten Zn bath in order to prevent the occurrence of dross defects during the hot-dip Zn plating process, conversely deteriorates the appearance quality of the steel material surface during the subsequent alloying process. I am letting it go down.

[0009] In recent years, in proportion to the increase in demand for alloyed hot-dip Zn-plated steel materials, in addition to the original characteristics of alloyed hot-dip Zn-plated steel materials such as paintability, corrosion resistance, and weldability, there has been a strong demand for beautiful appearance. has been done. Tokuko Sho 63-58225
The publication states that the surface of a softened and annealed steel sheet is subjected to mechanical processing such as polishing, grinding, or shot blasting using a wire brush or an abrasive belt before or during immersion in a molten Zn bath. to give a new surface,
A method is disclosed in which the formation of an Al-enriched layer is suppressed in the hot-dip Zn plating process, thereby improving the alloying reaction rate in the alloying process. Also, JP-A-58-104
Publication No. 163 discloses that prior to reduction annealing before being immersed in a molten Zn bath, the surface of the steel plate is plated with iron to a thickness of 0.1 to 2.0 μm in a sulfuric acid bath, a chloride bath, etc. Zn
In the plating process, Al in the molten Zn bath is reacted with oxygen in the iron plating to reduce the Al concentration in the molten Zn bath near the steel plate surface and prevent the formation of a dense Fe-Al alloy layer.
A method for improving the alloying reaction rate in the alloying process is disclosed. Furthermore, in JP-A-2-11744, there is only a hot-dip Zn plating process and no alloying process;
．． A first plating process was performed in a molten Zn bath having an Al concentration of 11% or more to generate a large amount of top dross (i.e., to reduce the formation of bottom dross), and then a first plating process was performed to generate a large amount of top dross. Prevents top dross from entering,
Furthermore, a method and apparatus are disclosed in which the Al concentration is set to 0.11% or more but lower than that in the first plating process, and the second plating process is performed while suppressing the generation of top dross somewhat. Japanese Patent Publication No. 63-58225 and Japanese Patent Publication No. 58-
Although the method disclosed in Publication No. 104163 has the advantage of shortening the time for alloying treatment and improving the peeling resistance of the alloyed hot-dip Zn-plated layer, The appearance quality is still insufficient. In addition, although the method and apparatus disclosed in JP-A-2-11744 can be applied to the production of hot-dip Zn-plated steel sheets, the plating layer still contains a large amount of Al, so it cannot be applied to the production of alloyed hot-dip Zn-plated steel sheets. Can not.

0010

[Problems to be Solved by the Invention] In the process of hot-dip Zn plating, the Al concentration of the molten Zn bath is increased and alloyed molten Z
When dross defects are prevented from occurring on the surface of n-plated steel, thin Fe2Al5 or F
The e-Al-Zn alloy layer not only suppresses the alloying reaction in the next alloying process and increases the alloying reaction time, but also causes the alloying reaction to proceed unevenly. the result,
Areas where the alloying reaction has progressed become convex, areas where the alloying reaction has been delayed become concave, and once the alloying reaction has finished, the surface of the steel exhibits an uneven appearance (this is called an unevenness defect).
The product will have extremely poor appearance quality. Molten Zn
As an example of performing molten Zn plating treatment by increasing the Al concentration of the bath, the above-mentioned Japanese Patent Publication No. 63-58225 describes an example in which the Al concentration of the molten Zn bath was 0.2%. JP-A-104163 has an example in which the Al concentration of the molten Zn bath was 0.16%. However, in practice, alloyed hot-dip Zn-plated steel materials have not been manufactured by subjecting them to hot-dip Zn plating with a high Al concentration in a hot-dip Zn bath. Usually, the Al concentration in the molten Zn bath is set to about 0.10%, which is near the lower limit of the control range for Al concentration in the process of molten Zn plating.

The object of the present invention is to perform hot-dip Zn plating in a molten Zn bath with a high Al concentration so that dross defects do not occur on the surface, and to maintain excellent appearance quality without unevenness defects even after alloying treatment is performed. An object of the present invention is to provide a method for manufacturing alloyed hot-dip Zn-plated steel material and equipment thereof.

0012

[Means for solving the problems] The gist of the present invention is as follows:
The manufacturing method for alloyed hot-dip Zn-plated steel material and the manufacturing equipment thereof in (1).

■ Molten Zn in a molten Zn bath containing Al
A method for manufacturing an alloyed hot-dip Zn-plated steel material, in which the steel material is immersed in a molten Zn bath having an Al concentration of 0.10% or less before being subjected to plating treatment.

[0014] ■ An initial plating tank containing a molten Zn bath with an Al concentration of 0.10% by weight or less, a recovery pump that recovers the molten Zn bath containing bottom dross from the initial plating tank, and a recovery pump that collects the bottom dross from the recovered molten Zn bath. A production facility for alloyed hot-dip Zn-plated steel material, which has an initial plating device consisting of a separation tank that separates the bottom dross and a supply pump that supplies a clean molten Zn bath from which bottom dross has been separated to the initial plating tank.

In the method of the present invention, molten Zn is heated in a molten Zn bath containing a considerable amount of Al in order to prevent the generation of dross defects.
It is assumed that plating treatment will be applied. In the present invention, if an Fe-Zn alloy layer is formed on the surface of the steel material in advance, the subsequent alloying treatment will proceed quickly, and the alloyed hot-dip Zn-plated steel material will have an excellent appearance quality with no unevenness defects on the surface. This was done based on the knowledge that it can be obtained.

[0016] Here, the term steel mainly refers to cold rolled steel plates, cold rolled steel strips, hot rolled steel plates and hot rolled steel strips.

[0017]

[Operation] FIG. 1 is a sectional view showing an example of the manufacturing equipment of the present invention.

Steel material (herein, steel strip) 1 is led from a snout 4 to a main plating tank 2 through a non-oxidizing furnace, a reducing furnace, and a cooling zone, which are not shown in FIG. 1, and is subjected to hot-dip Zn plating treatment. Ru. Next, the direction of movement of the steel material 1 is changed by a sink roll 8 in the molten Zn bath, and the steel material 1 is pulled upward. The steel material 1 pulled upward from the molten Zn bath is passed through a coating amount control device (hereinafter referred to as a gas wiping nozzle).
High-pressure gas is blown by 10, and excess Zn is squeezed out by a gas jet. Thereafter, the steel material 1 is heated in a heating furnace 11 for alloying treatment, and is subjected to an alloying treatment in which mutual diffusion progresses between the steel base and the attached Zn layer, so that the entire plating layer becomes an Fe-Zn alloy. , resulting in an alloyed hot-dip Zn-plated steel material. In addition, 9 in Fig. 1 is a snap roll, and 1
2 is the top roll.

The Al concentration of the molten Zn bath in the main plating tank 2 is
0.14 to 3.0% is desirable. The reason is that melting Z
This is because if the Al concentration of the n-bath is less than 0.14%, dross defects are likely to occur on the surface of the steel material 1 during the hot-dip Zn plating process. In addition, the Al concentration of the molten Zn bath is 3.0%.
If it exceeds 3.0%, the upper limit of Al dissolved in the plating layer of the steel material 1 is 3.0%, so when the steel material 1 is subjected to alloying treatment in the heating furnace 11 for alloying treatment, Fe This is because there is a possibility that a Fe--Al alloy or an Al--Zn alloy is formed in addition to the -Zn alloy. F in the Fe-Zn alloy layer
If an e-Al alloy or Al-Zn alloy is present, cracks will occur between the alloys during processing, which will not only become a starting point for corrosion, but in the worst case, the plating film itself will peel off. . Melting Z of main plating tank 2
When the Al concentration of the n-bath is set to a high value of 0.14 to 3.0% and hot-dip Zn plating is applied to the steel material 1, Al reacts preferentially with Fe dissolved in the molten Zn bath. It is possible to generate top dross and reduce the generation of bottom dross, which is the cause of dross defects.

[0020] Usually, the Al concentration of the molten Zn bath is 0.14~
When hot-dip Zn plating is performed in the main plating bath 2 with a high Zn content of 3.0% and then alloyed in the heating furnace 11 for alloying, uneven defects occur on the surface of the steel material 1. The reason is that the Fe2Al5 or Fe-Al-Zn alloy layer formed on the surface of the steel material 1 during the hot-dip Zn plating process becomes thicker than when the Al concentration in the hot-dip Zn bath is less than 0.14%. Therefore, the thick alloy layer suppresses the alloying reaction during the alloying process, making the alloying reaction rate non-uniform.

Therefore, in order to prevent irregularities from occurring on the surface of the steel material 1, an Fe-Zn alloy layer is formed on the surface of the steel material 1 immediately before hot-dip Zn plating in the main plating tank 2. be. Fe-Z is applied to the surface of steel material 1 in advance.
The method for forming the n-alloy layer is as follows. Steel material 1
Before being led to the main plating tank 2, the steel material 1 is led to an initial plating tank 5 and subjected to hot-dip Zn plating treatment in advance. Initial plating tank 5
The Al concentration of the molten Zn bath shall be 0.10% or less. The reason is that the Al concentration of the molten Zn bath in the initial plating tank 5 is 0.
．． This is because if it exceeds 10%, even if hot-dip Zn plating is performed in advance in the initial plating tank 5, a uniform Fe-Zn alloy layer cannot be formed.

When molten Zn plating is performed in the initial plating tank 5 containing a molten Zn bath with an Al concentration of 0.10% or less, bottom dross is likely to be generated in the initial plating tank 5. This bottom dross is rolled up by the accompanying flow accompanying the movement of the steel material 1 and adheres to the steel material 1. If the steel material 1 is guided to the next main plating tank 2 and subjected to hot-dip Zn plating with bottom dross still attached, the bottom dross will be pressed against it by the sink roll 8 and it will have dross defects. Therefore, the molten Z containing bottom dross is removed from the initial plating tank 5.
The n bath is recovered to the separation tank 3 by the recovery pump 7. In the separation tank 3, bottom dross is first precipitated in the sedimentation tank 3-1. Next, the molten Zn bath from which the bottom dross has been separated is supplied from the settling tank 3-1 to the cleaning tank 3-3 by the pump 3-2. In addition, the separation tank 3 has a structure in which the bottom dross precipitated in the sedimentation tank 3-1 is collected and removed as necessary.
The structure is such that bottom dross is removed by a piping system that supplies a molten Zn bath from -1 to cleaning tank 3-3. The clean molten Zn bath from which the bottom dross has been separated and removed is returned to the initial plating tank 5 by the supply pump 6. In this way, the molten Zn bath in the initial plating bath 5 is always free from bottom dross.

The initial plating tank 5 may be built into the snout 4 as shown in FIG. 1, or may be built into the main plating tank 2. Further, it is desirable that the bath temperature of the initial plating tank 5 is higher than the bath temperature of the main plating tank 2. The reason for this is that if the bath temperature of the initial plating tank 5 is lower than the bath temperature of the main plating tank 2, an Fe-Zn alloy layer cannot be uniformly formed even if hot-dip Zn plating is performed in the initial plating tank 5 in advance. be. Furthermore, the thickness of the formed Fe-Zn alloy layer should be uniform and without any breaks, and may be 0.5 to 2.0.
It is desirable to set it to μm. The composition of the Fe-Zn alloy layer is
It is desirable that the Fe concentration be 1 to 20%, the Zn concentration be 99 to 77%, and the rest be impurities. Al as an impurity
Concentrations up to about 3% are allowed.

In the manufacturing method of the present invention, the exact reason why the alloying reaction is promoted at a uniform rate by forming an Fe-Zn alloy layer on the surface of the steel material in advance is unknown, but the reason is explained as follows. It can be estimated as follows. That is, considering that the Fe-Al alloy layer formed on the surface of the steel material during hot-dip Zn plating treatment in a hot-dip Zn bath containing Al suppresses the alloying reaction, it is assumed that before hot-dip Zn plating treatment is performed in the main plating tank, Formation of an Fe-Al alloy layer that suppresses alloying reaction during hot-dip Zn plating in the main plating tank by performing hot-dip Zn plating in the initial plating tank and pre-existing an Fe-Zn alloy layer on the surface of the steel material. It is thought that this prevents

[0025]

Example 1 An alloyed hot-dip Zn-plated steel sheet was manufactured using the manufacturing equipment shown in FIG.

A steel strip of low carbon aluminium-killed steel with a plate thickness of 0.8 mm and a plate width of 1000 mm was placed in an initial plating bath 5 containing a molten Zn bath with an Al concentration and bath temperature shown in Tables 1-1 and 1-2.
A Fe-Zn alloy layer was formed on the surface of the steel strip. The thickness of the Fe-Zn alloy layer was measured using a scanning electron microscope,
It was confirmed that it was formed uniformly. Next, Table 1-1
A molten Zn plating process was performed in the main plating tank 2 containing a molten Zn bath having an Al concentration and bath temperature shown in Table 1-2. Thereafter, the amount of Fe--Zn alloy deposited was adjusted to 70 g/m2 using a gas wiping nozzle 10, and alloying was performed in a heating furnace 11 for alloying at an average ambient temperature of 520 DEG C. and for a processing time of 10 seconds.

The surface of the obtained alloyed hot-dip Zn-plated steel sheet was observed with an optical microscope at a magnification of 10 times, and the appearance quality was judged from the occurrence of dross defects and unevenness defects on the surface. Dross defects are determined by the number of dross present on the steel plate surface per 1 m2, and it is determined that there are no dross defects (Table 1).
-1, symbol "○" in Table 1-2)" and "Less than 10 dross defects/m2 (symbol "○" in Table 1-1, Table 1-2)
If the product had 10 or more dross defects/m2 and there was a problem with its commercial value (symbol '×' in Tables 1-1 and 1-2), it was considered to be defective. In addition, unevenness defects are determined by the presence or absence of an uneven pattern on the surface of the steel plate. Almost none (Table 1-1,
In Table 1-2, it is considered good if the symbol is ``△''), and if it is ``there is a problem with the product value due to the presence of uneven patterns (symbol ``×'' in Tables 1-1 and 1-2), it is considered defective. . The results are shown in Table 1-
1. Also listed in Table 1-2.

Test No. 1 to 21 are examples of the present invention. In both cases, molten Zn has a high Al concentration of 0.10 to 0.16%.
Since the hot-dip Zn plating treatment was performed in a bath, there are few dross defects. In particular, when molten Zn plating is performed in a molten Zn bath with an Al concentration of 0.14% or more, there are no dross defects at all. Moreover, since a Fe-Zn alloy layer was formed uniformly on the surface of the steel sheet by immersing it in a molten Zn bath with an Al concentration of 0.10% or less, the molten Zn was immersed in a molten Zn bath with a high Al concentration.
Despite the plating treatment, the subsequent alloying treatment proceeded satisfactorily in a short time of 10 seconds, and almost no unevenness defects were generated on the surface of the steel sheet. In particular, if the temperature of the molten Zn bath used to form an Fe-Zn alloy layer on the surface of the steel sheet is higher than the bath temperature of the molten Zn bath used to perform the hot-dip Zn plating process, no unevenness defects will occur on the surface of the steel sheet. There wasn't. That is, it was possible to obtain an alloyed hot-dip Zn-plated steel sheet having excellent appearance quality.

Test No. 22 to 30 are comparative examples. Since these were also subjected to molten Zn plating treatment in a molten Zn bath with a high Al concentration of 0.14 to 0.16%, there were no dross defects at all. However, A of the pre-immersed molten Zn bath
Since the l concentration exceeded 0.10%, Fe-Z was added to the surface of the steel plate.
The n-alloy layer was not formed uniformly. Therefore, even if the steel plate was then subjected to hot-dip Zn plating treatment and then alloying treatment, unevenness defects were generated on the surface of the steel sheet.

[0030]

[Table 1-1]

[0031]

[Table 1-2]

[0032]

[Example 2] Test No. 2 is an example of the present invention in Example 1. 4~
A steel strip having the same dimensions and material as in No. 21 was used, and under exactly the same conditions, it was immersed in the initial plating bath 5 to uniformly form an Fe-Zn alloy layer on the surface of the steel strip, and then the main plating was carried out. Hot-dip Zn plating was performed in tank 2, and then the amount of Fe--Zn alloy deposited was adjusted using gas wiping nozzle 10. Only the average ambient temperature of the heating furnace 11 for alloying treatment was changed compared to Example 1, and two conditions were set: 520°C and 450°C. The alloying processing time required to complete alloying at each ambient temperature was investigated. The results are shown in FIGS. 2 and 3.

FIG. 2 shows the case where the average ambient temperature of the heating furnace 11 for alloying treatment was 520° C., and the alloying treatment completion time was within 10 seconds in all cases.

FIG. 3 shows a case where the average ambient temperature of the heating furnace 11 for alloying treatment was 450° C., and the time to complete the alloying treatment was 15 to 20 seconds. Test No. of Example 1
In the alloying treatments corresponding to 7 to 12 and 16 to 21, the bath temperature of the molten Zn bath in which the Fe-Zn alloy layer is previously formed on the surface of the steel strip is lower than the bath temperature of the molten Zn bath in which the molten Zn plating process is performed. It can be seen that the alloying process completion time tends to be shorter because the alloying temperature is also higher.

[0035]

[Effects of the Invention] According to the manufacturing method and manufacturing equipment of the present invention, alloyed hot-dip Zn-plated steel material with a beautiful appearance and without not only dross defects but also unevenness defects caused by non-uniform alloying reaction on the surface can be produced. can be manufactured.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an example of manufacturing equipment of the present invention.

FIG. 2 is a diagram showing the alloying treatment time when the alloying treatment temperature is 520°C.

FIG. 3 is a diagram showing the alloying treatment time when the alloying treatment temperature is 450°C.

[Explanation of symbols]

1 is steel, 2 is the main plating tank, 3 is the separation tank, 3-1 is the settling tank, 3-2 is the pump, 3-3 is the cleaning tank, 4 is the snout,
5 is an initial plating tank, 6 is a supply pump, 7 is a recovery pump,
8 is a sink roll, 9 is a snap roll, 10 is a coating amount control device (gas wiping nozzle), 11 is a heating furnace for alloying treatment, and 12 is a top roll.

Claims (2)

[Claims] [Claim 1] Prior to hot-dip Zn plating treatment in a hot-dip Zn bath containing Al, the steel material is coated with 0.10% by weight or less of Al.
1. A method for producing alloyed hot-dip Zn-plated steel material, which comprises immersing the steel material in a concentrated hot-dip Zn bath. 2. An initial plating tank containing a molten Zn bath with an Al concentration of 0.10% by weight or less, and a recovery pump that recovers the molten Zn bath containing bottom dross from the initial plating tank.
Alloying melting characterized by having an initial plating device consisting of a separation tank that separates bottom dross from the recovered molten Zn bath and a supply pump that supplies the clean molten Zn bath from which the bottom dross has been separated to the initial plating tank. Manufacturing equipment for Zn-plated steel.