JP3387415B2 - Manufacturing method of galvannealed steel sheet with excellent surface appearance - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a galvannealed steel sheet having excellent surface appearance. [0002] Galvannealed steel sheets are widely used due to their excellent properties such as press formability, corrosion resistance and weldability. Recently, galvannealed steel sheets have come to be used for applications requiring beautiful surface appearance, such as automotive body outer panels, instead of cold-rolled steel sheets and electroplated steel sheets. [0003] However, it is more difficult for an alloyed hot-dip galvanized steel sheet to obtain a beautiful surface appearance than a cold-rolled steel sheet or an electroplated steel sheet. One reason for this is that an intermetallic compound called dross, which is inevitably generated in the production line of hot-dip galvanized steel sheets, sometimes adheres. Normally, in the case of producing an alloyed hot-dip galvanized steel sheet, the steel sheet to be plated is immersed in a hot-dip zinc bath in a plating bath, turned around by a sink roll in the bath, lifted up from the plating bath, and wiped by a wiping device. After adjusting to the plating adhesion amount, an alloying treatment is performed. In such a manufacturing facility, Fe eluted from a steel plate to be plated or equipment in the bath, and Zn or Al in the molten zinc bath by a chemical reaction,
It is inevitable that a -Zn-based and / or Fe-Al-based intermetallic compound is generated. Usually, the intermetallic compound formed in these baths is called dross. If the molten zinc bath is in a quiet state, the generated dross settles and deposits on the bottom of the plating tank or floats on the bath surface depending on the specific gravity difference from the molten zinc. However, the molten zinc bath is constantly stirred by the passage of a steel plate to be plated and the rotation of a roll in the bath such as a sink roll. Fe-Zn dross has a higher specific gravity than molten Zn, but the difference in specific gravity is relatively small. Therefore, when the stirring of the bath is increased, the dross easily floats and adheres to the steel sheet in the bath. On the other hand, Fe-Al dross has a large specific gravity difference with molten Zn, so it easily floats on the bath surface,
The steel sheet easily adheres when passing through the bath surface. The attached dross is crushed at the time of temper rolling or press working, which causes a point-like defect called a dross defect. [0006] In order to prevent such dross defects, as a means for removing dross from the molten zinc bath without stopping the operation, a molten zinc bath containing dross is taken out of the plating tank and sent to a sub-pot. Many techniques have been proposed for cleaning the molten zinc bath inside and then returning it to the plating tank. Among these prior arts, for example, Japanese Patent Application Laid-Open
Japanese Patent No. 98405 discloses that as a method for more efficiently removing dross, dross having a sphere equivalent diameter of 50 μm or more in a molten zinc bath is settled and separated in a subpot, then returned to a plating tank, and a sphere equivalent diameter that is difficult to remove. A method has been proposed in which dross having a diameter of 50 μm or less is adhered to a steel sheet and removed, and is eliminated and rendered harmless by alloying treatment or temper rolling. In Japanese Patent Application Laid-Open No. 3-211262, a small diameter formed by reacting Zn and Al with a steel plate to be plated is described.
A method for producing a hot-dip galvanized steel sheet that draws out dross of an Fe-Zn alloy and / or Fe-Al alloy of 50 μm or less while adhering the dross, and then eliminates and renders harmless dross adhered by alloying treatment or temper rolling, and Proposes an apparatus for manufacturing a hot-dip galvanized steel sheet having a cover around a steel sheet to be plated in a plating tank. [0009] However, in the prior art represented by the above-mentioned Japanese Patent Application Laid-Open No. 5-98405, in order to remove dross from the molten zinc bath, a pump for extracting the molten zinc bath, Since large-scale remodeling of facilities is required, such as the construction of a new sub-pot, a large amount of capital investment is required. In addition, when installing on an existing production line, there is a problem of installation space. Alternatively, labor is required for maintenance, inspection, and the like of the apparatus, which does not necessarily lead to improvement in productivity. In Japanese Patent Application Laid-Open No. 3-212126, Japanese Patent Laid-Open Publication No. Hei 3-211262 discloses that dross having a diameter of 50 μm or less
Since it is extinguished by the diffusion of Al and Zn, the conditions for alloying treatment are restricted, and there is a concern that quality other than surface quality such as powdering property may be degraded. In the case of detoxification by temper rolling, the conditions for temper rolling are similarly restricted. Further, in order to limit the dross to be adhered to a size of 50 μm or less in diameter, it is necessary to clean a zinc bath such as a cover in the plating tank or a sub-pot as proposed in Japanese Patent Application Laid-Open No. 5-98405. Equipment is required, which requires capital investment and labor for maintenance work. [0012] The present invention provides a method for reducing
Another object of the present invention is to provide a method for producing an alloyed hot-dip galvanized steel sheet having excellent surface appearance without requiring large-scale modification of existing production equipment. [0013] means for solving the above-mentioned object, according to an aspect of, after performing the hot dip coating a steel sheet by continuously Tsuban-immersed in the molten zinc bath in the plating bath, the steel sheet In the method for producing an alloyed hot-dip galvanized steel sheet in which the plating layer is alloyed by heating the steel, the Al concentration in the bath is maintained within a range of 0.15 ± 0.01 wt% for at least one day, and the surface appearance is excellent. This is a method for producing a galvannealed steel sheet . The present invention will be described below.
After the dross in the bath is completely removed, when the operation is started in a clean bath and the production of the hot-dip coated steel sheet is continued, Fe is gradually supplied from the steel sheet and equipment in the bath, and dross is generated. The dross grows at first even if it is fine, and if dross having a diameter greater than a certain value adheres to the steel sheet, point-like surface defects are generated. For example, Japanese Patent Application Laid-Open No. 3-2121262 states that a hot-dip coated steel sheet to which dross having a diameter of 50 μm or more has adhered will have surface defects after alloying treatment. It is known that the composition of dross has a close relationship with the Al concentration in the bath. For example, in a zinc bath maintained at 465 ° C., dross generated when the Al concentration in the bath is about 0.15% or more is of the Fe-Al type, while dross generated at a lower Al concentration in the bath is Dross of the δ1 phase of the Fe-Zn system is generated. If the Al concentration in the bath is further reduced, Fe-Zn based d phase dross is generated. That is, the dross phase equilibrated in the bath is determined by the holding temperature of the bath and the Al concentration in the bath. No matter which dross phase is stable, the dross increases and coarsens over time, even if the operation is carried out while maintaining the temperature and the Al concentration in the bath, causing dross defects. Further, even when the Al concentration in the bath in which dross already exists is greatly changed, the above change occurs, and the dross in the bath changes to a stable phase according to the bath composition and the temperature. The present inventors have conducted intensive studies on the relationship between bath composition and dross, and have obtained the following findings. When the Al concentration in the zinc bath in which dross of the δ1 phase is present is increased to change the Fe-Al phase to a stable composition, the dross in the bath changes to the Fe-Al phase. At this time,
The dross of the Fe-Al phase nucleates on the surface of the dross of the δ ₁ phase,
grow up. Further, the plurality of dross one [delta] _1-phase Fe-A
l-phase dross occurs. That is, when the bath conditions are changed and a stable dross phase change occurs, one dross is divided into a plurality of particles. A similar phenomenon occurs again when the bath composition is changed in the opposite direction. When the operation is performed while maintaining the Al concentration in the bath at a composition exceeding 0.15 wt%, the Fe-Al phase becomes a stable region, and dross of the Fe-Al phase grows with time. On the other hand, the bath Al concentration performed operations below the 0.15 wt%, and a [delta] ₁ phase stability region, dross δ1 phase with the lapse of time to grow. For example, when the composition of a zinc bath containing dross in the Fe-Al phase when the bath composition is 0.15 wt% or more is changed to 0.15 wt% or less, the dross present in the bath becomes a stable phase determined by the bath composition, that miniaturized in the process of changing to the [delta] ₁ phase. However, greatly changing the bath composition for the dross refining treatment has a serious problem in terms of stable production of galvannealed steel sheets. For example, it is difficult to change the bath composition rapidly, and it is particularly difficult to lower the Al concentration. In addition, when the Al concentration is greatly increased, the alloying hot-dip galvanized steel sheet manufactured during that time becomes difficult to alloy. Therefore, as a result of further study, the Al concentration in the bath was adjusted to a value near the concentration at which phase transformation occurs, that is, 0.15 ± 0.01 wt%.
It has been found that by keeping the content within the range, growth of dross can be suppressed, and the effect of suppressing dross is higher. In actual operation, even if an effort is made to keep the bath conditions constant, in practice, local temperature and nonuniform bath composition cannot be avoided. For example, the supply of a zinc bath is performed by dissolving a Zn ingot or a Zn alloy ingot in a plating tank, so that the bath composition is different around the ingot. In addition, the zinc bath is naturally cooled near the bath surface and around the ingot, and the bath temperature decreases. On the other hand, there is a region where the bath temperature is partially high due to the heating device provided for maintaining the bath temperature. Further, the zinc bath around the steel plate to be plated is heated or cooled by the temperature difference between the steel plate to be plated and the zinc bath. As described above, most of the contents in the plating tank are temperature,
Although the composition is uniform, there are some parts where the composition and temperature of the zinc bath differ locally. Therefore, when the zinc bath containing dross whose Al concentration in the bath is close to 0.15 wt% reaches the above-described region where the bath composition is different, the bath composition changes by more than 0.15 wt%, whereby a stable dross phase is formed. It changes and refinement accompanying the phase transformation of dross occurs. Also, when this bath recovers to the same composition as most of the plating tank, dross refinement occurs again due to the dross phase transformation, and dross refinement is continuously performed. This is because growth of dross is suppressed. In order to suppress the growth of dross by using the inevitable nonuniform temperature of the bath temperature in the plating tank,
The bath composition of a uniform part occupying most of the plating tank
It has been found that the control is performed within a range of 15% ± 0.01 wt%. That is, it was found that the bath composition measured by the normal operation management was controlled within the range of 0.15 ± 0.01 wt% determined by the similarly measured bath temperature. According to this method, most of the bath composition in the plating tank does not fluctuate, so that it is highly stable in producing an alloyed hot-dip galvanized steel sheet. Further, even if dross adheres to the plated steel sheet, it is fine, so that the surface appearance of the alloyed hot-dip galvanized steel sheet manufactured by further performing an alloying treatment is not impaired. In the present invention, the Al concentration in the bath is 0.15 ± 0.01 wt%.
It is performed by suppressing dross coarsening by keeping within the range described above, and there is no limitation on the base steel sheet of the alloyed hot-dip galvanized steel sheet. In addition, there are no conditions for the alloying processing such as the temperature of the sheet entering the plating tank during the production, the temperature of the molten zinc bath, the amount of plating applied, the alloying processing temperature, the alloying processing time, and the type of the alloying processing apparatus. No restrictions. The alloying treatment is advantageously performed subsequent to the plating, but may be performed in another facility. The present invention suppresses the coarsening of dross and obtains an excellent surface appearance by keeping the dross in the bath fine, and does not reduce the amount of dross generated. Absent. Dross in the bath, which is actually harmful,
Since it exceeds a certain size, it is harmless even if a large amount of fine dross having a size of, for example, 50 μm or less is present in the bath. (Example 1) In CGL, the average particle size was 100 μm.
An alloyed hot-dip galvanized steel sheet was continuously manufactured while maintaining a zinc bath containing m dross at 460 ° C., and changes over time in the particle diameter of dross contained in the zinc bath were investigated. During this time, local changes in bath temperature and bath composition have occurred due to unavoidable factors such as supply of a zinc bath, heating for maintaining the bath temperature, and cooling on the bath surface. FIG. 1 shows the results of the investigation. In the bath in which the Al concentration was kept outside the range of 0.15 ± 0.01 wt%, the dross became coarse with time. On the other hand, in the bath in which the Al concentration was kept within the range of the boundary concentration of 0.15 ± 0.01 wt%, dross was refined due to local fluctuations in bath temperature and bath composition inevitably occurring during plating. When the steel sheet is plated with the finely divided zinc bath, an alloyed hot-dip galvanized steel sheet having an excellent surface appearance can be obtained. (Example 2) In the CGL, a hot-dip galvanizing bath having no dross in the bath was prepared in advance, the Al concentration in the bath was adjusted to various compositions, and an alloyed hot-dip galvanized steel sheet was continuously manufactured. One week later, the average particle size of dross contained in the collected zinc bath was examined. The above experiment was performed at three different levels while changing the holding temperature. Due to unavoidable factors such as supply of a zinc bath, heating for maintaining the bath temperature, and cooling on the bath surface, local fluctuations in bath temperature and bath composition have occurred. FIG. 2 shows the results of the investigation. Compared with a bath in which the Al concentration was kept in a range outside of 0.15 ± 0.01 wt%, the Al concentration was changed to the boundary concentration of 0.15 ± 0.01 wt%.
In the bath maintained within the range, the dross was fine due to local bath temperature and bath composition fluctuations inevitably occurring during plating, and coarsening was suppressed. When a steel sheet is plated in the finely divided zinc bath, an alloyed hot-dip galvanized steel sheet having an excellent surface appearance can be obtained. As described above, according to the present invention,
Since dross coarsening can be suppressed without modifying existing manufacturing equipment, an alloyed hot-dip galvanized steel sheet having an excellent surface appearance can be manufactured without reducing productivity due to dross treatment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a change in dross particle diameter with time. FIG. 2 is a diagram showing a relationship between Al concentration in a bath, bath temperature and dross particle diameter.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-8-20854 (JP, A) JP-A-5-186857 (JP, A) JP-A 4-272163 (JP, A) (58) Field (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] [Claim 1] After a steel sheet is continuously passed and immersed in a hot-dip zinc bath in a plating tank to perform hot-dip plating, the steel sheet is heated to form a plated layer by alloying. Production of alloyed hot-dip galvanized steel sheet with excellent surface appearance characterized by maintaining Al concentration in the bath within the range of 0.15 ± 0.01 wt% for 1 day or more Method.