JP3385970B2 - 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 -Zn-based and / or Fe-Al-based intermetallic compounds are 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 due to a difference in specific gravity with the molten zinc. However, since the molten zinc bath is constantly stirred by the passage of the steel plate to be plated and the rotation of a roll in the bath such as a sink roll, some dross floats in the molten zinc bath and adheres to the steel plate to be plated. 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. There have been proposed many techniques for cleaning a molten zinc bath in a bath and returning the bath to a plating bath. 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 problems BRIEF SUMMARY OF THE INVENTION may, after hot dipping the steel sheet by continuously Tsuban-immersed in the molten zinc bath in the plating tank, wherein In a method for producing an alloyed hot-dip galvanized steel sheet in which a steel sheet is heated to alloy a plating layer, when the hot-dip zinc bath temperature is T (° C.) and the boundary Al concentration defined by the following formula is Cz, temperature
T (° C) within the range of 430 to 500 ° C and Al in the bath
A method for producing an alloyed hot-dip galvanized steel sheet having excellent surface appearance, wherein the concentration is maintained within a range of Cz ± 0.01 wt% for 1 day or more . However, Cz = -0.0015 * T + 0.76 (wt%). DETAILED DESCRIPTION OF THE INVENTION The present invention will be described below.
After the dross in the bath is completely removed, the operation is started in a clean bath and the production of the hot-dip galvanized 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 molten zinc bath maintained at 465 ° C, dross generated at an Al concentration of about 0.14% or more in the bath is Fe-Al based,
At a lower Al concentration in the bath, the dross formed is Fe-Zn
Dross [delta] _1-phase 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. Regardless of the dross phase in which the bath temperature and the Al concentration in the bath are maintained in a stable region, the dross increases and coarsens over time, 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 δ ₁ phase is present is reduced to change the ζ phase to a stable composition, the dross in the bath changes to the ζ phase. At this time, dross ζ phase, grow into nucleated bath direction on the surface of the dross [delta] ₁ phase. Further, dross plurality of ζ phase from the dross one [delta] ₁ phase occurs. That is, the bath conditions are changed to change the stable dross phase, and when the dross phase changes, one dross is divided into a plurality of dross particles. A similar phenomenon occurs again when the bath composition is changed in the opposite direction. The molten zinc bath temperature T at which such a change occurs
The relationship between (° C.) and the Al concentration in the bath is represented by the following equation, and the Al concentration Cz in the bath will be referred to as a boundary Al concentration in the future. Cz = -0.0015 * T + 0.76 (wt%) When the molten zinc bath temperature T (° C.) exceeds 500 ° C., the temperature exceeds the peritectic temperature of the ζ phase and the phase transformation between the ζ phase and the δ ₁ phase The molten zinc bath temperature T (° C.)
It must be below 500 ° C. In addition, the molten zinc bath temperature T
If the temperature (° C.) is lower than 430 ° C., the molten zinc tends to solidify and the plating operation becomes difficult.
(° C) must be 430 ° C or higher. When the operation is performed while maintaining the Al concentration in the bath at a composition exceeding the boundary Al concentration Cz, the region becomes a region where the δ ₁ phase is stable, and dross of the δ ₁ phase grows with time. On the other hand, if the operation is performed below the boundary Al concentration Cz, the phase becomes a phase-stable region, and the dross of the phase-II grows with time.
For example determined, at Al concentration than Cz, the Al concentration of the zinc bath containing dross [delta] _1-phase, it is varied below Cz beyond the boundaries Al concentration Cz, dross present in the bath by the bath Al concentration In the process of changing to a stable phase, ie, ζ phase. However, greatly changing the bath composition to prevent dross poses 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. Therefore, as a result of further study, by keeping the Al concentration in the bath within the range of Cz ± 0.01 wt% determined according to the bath temperature, the hot-dip galvanized steel sheet was continuously formed under stable bath conditions. It was found that it was possible to suppress the growth of dross while manufacturing, and that the suppression effect was higher. In actual operation, even if an effort is made to keep the bath conditions constant, local unevenness in temperature and bath composition is actually unavoidable. 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 in which the Al concentration in the bath is close to Cz reaches the above-mentioned bath composition and the bath temperature is different, when the bath composition exceeds Cz or Cz changes depending on the temperature, The stable dross phase changes and refinement occurs with the dross phase transformation. Also, when this bath recovers to the same temperature and bath composition as most of the plating bath, dross refining accompanying the dross phase transformation occurs again, and dross refining is continuously performed, as a result This is because dross growth is suppressed in the entire plating tank. In order to suppress the growth of dross by using the unavoidable unevenness of the bath temperature and bath composition in the plating bath, the present inventors have proposed that the bath of the uniform bath occupying most of the plating bath is used. It has been found that the composition is controlled by controlling the composition within the range of Cz ± 0.01 wt%. That is, the present inventors have found that the bath composition is measured by controlling the bath composition measured in the normal operation management within the range of Cz ± 0.01 wt% determined by the bath temperature measured in the same manner. According to this method, most of the bath temperature in the plating tank,
Since the bath composition does not change, the stability is high 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 changed to the boundary Al concentration Cz ±.
By controlling the dross to be coarse by keeping it within the range of 0.01 wt%, there is no limitation on the base steel sheet of the galvannealed steel sheet. In addition, the plating conditions such as the temperature of the plate entering the plating bath during the production, the temperature of the zinc bath, the amount of plating applied, the alloying treatment temperature, the alloying treatment time,
There are no restrictions on the alloying conditions such as the type of the alloying apparatus. The alloying treatment is advantageously performed subsequent to the plating, but may be performed in another facility. The present invention exerts the effect of suppressing the coarsening of dross and obtaining an excellent surface appearance by keeping the dross in the bath fine, and reduces the amount of dross generated. It does not reduce it. Since the harmful dross in the bath exceeds a certain size, it is harmless even if a large amount of fine dross 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 the time-dependent change in the dross particle diameter contained in the zinc bath was investigated. During this time, in the plating bath, local changes in bath temperature and bath composition are caused by inevitable 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 maintained in the range of the boundary concentration Cz, that is, 0.07 wt% to more than ± 0.01 wt%, the dross became coarse with time. On the other hand, the Al concentration is changed to the boundary concentration Cz
In the bath maintained within the range of ± 0.01 wt%, dross was refined due to inevitable fluctuations in bath temperature and bath composition occurring during plating. 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. (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 a continuous galvannealed steel sheet was 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. In the plating bath, local changes in bath temperature and bath composition are caused by unavoidable factors such as supply of a zinc bath, heating for maintaining the bath temperature, and cooling on the bath surface.
Fig. 2 shows the survey results. The Al concentration was compared with a bath in which the Al concentration was kept outside the boundary concentration Cz ± 0.01 wt%, and the Al concentration was changed to the boundary concentration Cz ± 0.01 wt%.
In the bath maintained within the wt% range, dross was fine and coarsening was suppressed due to local bath temperature and bath composition fluctuations inevitably occurring in the plating bath. 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. 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 the relationship among Al concentration in a bath, bath temperature and dross particle diameter.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-9-157821 (JP, A) JP-A-8-53744 (JP, A) JP-A-7-48662 (JP, A) JP-A-4- 272163 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] [Claim 1] After a steel sheet is continuously passed through 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 plating layer by alloying. In the method for producing an alloyed hot-dip galvanized steel sheet, when the hot-dip zinc bath temperature is T (° C.) and the boundary Al concentration defined by the following formula is Cz, the hot-dip zinc bath temperature T (° C.) is 430 to 500. Within the range of ℃, and in the bath A
A method for producing an alloyed hot-dip galvanized steel sheet having excellent surface appearance, wherein the concentration is maintained within a range of Cz ± 0.01 wt% for 1 day or more. However, Cz = -0.0015 * T + 0.76 (wt%)