JP5125198B2 - Method for hot metal plating of steel sheet using continuous hot metal plating equipment - Google Patents

Description

  The present invention comprises a plating pot for containing a molten metal for plating and dipping a steel plate, and a melting pot for dissolving a plating metal ingot for providing the molten metal for plating to the plating pot. The present invention relates to a molten metal plating method for a steel sheet using the continuous molten metal plating apparatus provided.

  As a general method for continuous molten metal plating of steel sheets, after pretreatment such as degreasing, the steel sheet is heated with a predetermined heating pattern, cooled, and immersed in the molten metal for plating contained in the plating pot, and the amount of plating adhered There is a method of cooling to room temperature with a predetermined cooling pattern after controlling the above. In this method, in order to compensate for the molten metal for plating that has been reduced by being plated on the steel plate, an ingot of the metal for plating is charged into the plating pot and melted.

  However, while the steel plate is immersed in the molten metal for plating, the iron eluted from the steel plate reacts with the metal for plating, such as zinc and aluminum added in a small amount to form an intermetallic compound, so-called dross. Adheres to and induces surface defects. In particular, dross is likely to be generated at a low temperature location in the molten metal for plating, and a large amount is generated around the ingot of the metal for plating. Therefore, a plating pot for dipping the steel sheet and a melting pot for melting the plating metal ingot are provided, and the surface defects of the steel sheet due to dross are compensated by connecting the two to make up the molten metal in the plating pot. Methods for preventing it have been proposed.

  For example, in Patent Document 1, a subpot for dissolving a solid metal (ingot) for replenishing a plating bath (molten metal) is provided on the side surface of a main pot for continuously hot-plating steel sheets, and the subpot and the main pot are bathed. Each of the molten metal flow paths is connected with a “molten metal introduction path to the main pot” provided at an upper position below the surface and a “melt metal lead-out path from the main pot” provided at a lower position below the bath surface. There has been proposed a continuous molten metal plating facility in which a direct resistance heating means and an induction heating means are provided side by side. This facility enables good temperature control and suppresses the occurrence of dross in the main pot.

Patent Document 2 contains a zinc-based molten metal containing 0.05 wt% or more of aluminum, a plating pot for plating by dipping a steel strip, a melting pot for melting an ingot used for plating, and the plating pot And a melting zinc plating apparatus comprising a flow path connecting the melting pot on the same bath surface, and heating the molten metal in the melting pot to the melting pot above the average temperature of the molten metal bath in the plating pot There is disclosed a hot dip galvanizing apparatus for steel strip in which a heating means capable of heating is further provided and a heating means is provided in a flow path. With this equipment, dross in the plating pot is melted in contact with the hot molten metal in the flow path, and iron diffuses toward the hot molten metal, so that dross in the plating pot is reduced. become.
Japanese Patent Laid-Open No. 5-222500 Japanese Patent Laid-Open No. 11-286761

  However, in recent years, as the line speed of continuous molten metal plating equipment has increased, the amount of iron eluted from the steel sheet into the molten metal has increased. A large amount of dross is generated in subpots or melting pots where dross is likely to occur due to a decrease in the molten metal temperature, making it difficult to remove dross.

  An object of the present invention is to provide a molten metal plating method for a steel sheet that can suppress dross generation in a melting pot using a continuous molten metal plating apparatus provided with a melting pot separately from a plating pot.

The purpose is to accommodate a molten metal for plating, a plating pot for immersing a steel plate and plating, a melting pot for melting an ingot of a metal for plating to supply the molten metal for plating to the plating pot, An average of plating pots using a continuous molten metal plating apparatus that has a connecting portion that connects the plating pot and the melting pot and is connected so that the liquid level of the molten metal for plating in the connecting portion matches the plating pot and the melting pot. When the temperature is T1 (K), the average temperature of the melting pot is T2 (K), and the volume expansion coefficient of the molten metal is β (K ^-1 ), the steel plate is plated to satisfy the following formula (1): It is achieved by a hot-dip metal plating method for a steel sheet characterized by being applied.
| T1-T2 | × β ≦ 7 × 10 ^-4 ... (1)
In particular, the present invention is effective when molten zinc is used as the molten metal.

  The molten metal plating method for a steel sheet according to the present invention prevents the molten metal having a high iron concentration from flowing into the melting pot from the plating pot, and can reduce the amount of dross generated in the melting pot. Therefore, the labor spent for the dross removal work in the melting pot can be greatly reduced, and the operability can be remarkably improved.

  FIG. 1 shows an example of a continuous molten metal plating apparatus used in the molten metal plating method for steel sheets according to the present invention. When the molten metal 4 is continuously plated on the steel plate 6 entering from the snout 9 using the continuous molten metal plating apparatus provided with the melting pot 2 separately from the plating pot 1, the molten metal 4 is used in the plating pot 1. Is attached to the steel plate 6 and consumed, it is necessary that the ingot 5 of the plating metal is charged in the melting pot 2 and melted, and the molten metal 4 needs to be supplied to the plating pot 1 through the connecting portion 3.

  At this time, in order to reduce the amount of dross generated in the melting pot 2, it is necessary to prevent the molten metal 4 in the plating pot 1 having a high iron concentration from flowing into the melting pot 2 from the connecting portion 3. As a result of intensive studies by the inventors, as shown in FIG. 1, in the connecting portion 3, a flow divided into two upper and lower layers is generated due to the difference in specific gravity of the molten metal 4 in both pots, and the melting pot 2 to the plating pot The upper flow (a) to 1 and the lower flow (b) from the plating pot 1 to the melting pot 2 occur simultaneously, and the molten metal 4 in the plating pot 1 with a high iron concentration flows into the melting pot 2. all right. Here, FIG. 1 shows a case where the average temperature T1 of the plating pot 1 is lower than the average temperature T2 of the melting pot 2, but the average temperature T1 of the plating pot 1 is higher than the average temperature T2 of the melting pot 2. In this case, the flow (a) from the melting pot 2 to the plating pot 1 is the lower layer, and the flow (b) from the plating pot 1 to the melting pot 2 is the upper layer. Arise. Therefore, in either case, the molten metal 4 in the plating pot 1 having a high iron concentration flows into the melting pot 2 and the amount of dross generated increases.

The product of the temperature difference between both pots | T1-T2 | and the volume expansion coefficient β of the molten metal | T1-T2 | × using the continuous molten metal plating apparatus of FIG. If β is 7 × 10 ⁻⁴ or less, the flow separation between the upper and lower layers is weakened, so that the amount of dross generated can be reduced. In particular, when molten zinc is used as the molten metal, it is easy to realize such conditions.

  The steel plate 6 plated in this way is changed in direction by the sink roll 7 and goes out of the plating pot 1 while being supported by the support roll 8.

In an actual continuous molten metal plating apparatus, it is difficult to separate and observe the flow from the plating pot to the melting pot and the flow from the melting pot to the plating pot. The weight (kg), that is, the flow rate (kg / s) of the molten metal flowing in per unit time (s) was evaluated by flow simulation using a numerical fluid analysis model. In the simulation, molten zinc having a volume expansion coefficient β of 1.37 × 10 ⁻⁴ K ⁻¹ was used as the molten metal. The capacity of the plating pot 1 was 250 t, the capacity of the melting pot 2 was 40 t, and the shape of the connecting portion 3 shown in FIG. 1 was a rectangular parallelepiped having a cross-sectional area of 0.06 m ² as shown in FIG.

FIG. 3 shows the relationship between | T1-T2 | × β and the flow rate of molten zinc from the plating pot to the melting pot. The flow rate of molten zinc from the plating pot 1 to the melting pot 2 is small if | T1-T2 | × β ≦ 7 × 10 ^-4 and rapidly increases with 7 × 10 ^-4 <| T1-T2 | × β. I understand that.

  Since the flow of the connecting portion 3 is a symmetric flow with respect to the width direction of the connecting portion 3, the same phenomenon is observed for the connecting portion 3 having any width. Further, the shape of the connecting portion 3 is not necessarily a rectangular parallelepiped as shown in FIG. 2, and may be a shape like a triangular prism as shown in FIG.

It is a figure which shows an example of the continuous molten metal plating apparatus used with the molten metal plating method of the steel plate which is this invention. It is a figure which shows an example of the shape of a connection part. It is a figure which shows the relationship between | T1-T2 | × β and the flow rate of molten zinc from the plating pot to the melting pot. It is a figure which shows another example of the shape of a connection part.

Explanation of symbols

1 Plating pot
2 Melting pot
3 Connecting part
4 Molten metal
5 Ingot
6 Steel plate
7 Sync roll
8 Support roll
9 Snout

Claims (2)

A plating pot for containing a molten metal for plating and dipping a steel plate;
A melting pot for melting a plating metal ingot for supplying a molten metal for plating to the plating pot;
A connecting portion for connecting the plating pot and the melting pot;
With
Using a continuous molten metal plating apparatus connected so that the liquid level of the molten metal for plating in the connecting portion of the plating pot and the melting pot match,
When the average temperature of the plating pot is T1 (K), the average temperature of the melting pot is T2 (K), and the volume expansion coefficient of the molten metal is β (K ^-1 ), the following equation (1) is satisfied. A method for hot metal plating of a steel sheet, wherein the steel sheet is plated;
| T1-T2 | × β ≦ 7 × 10 ⁻⁴ (1).   2. The molten metal plating method for a steel sheet according to claim 1, wherein molten zinc is used as the molten metal.

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