JPH0681105A - Production of galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To rapidly and correctly apply heating to a strip up to the prescribed temp. at alloying treatment at the time of producing a galvannealed steel sheet. CONSTITUTION:At the time of continuously immersing a strip (S) into a molten metal bath 2 to apply plating, coating weight is regulated by means of gas wiping nozzles 5 and then the strip is directly electrified by using touch rolls 6, 8 as electrodes and heated. By this method, strip temp. can be rapidly controlled according to the changes in the size of the strip and line speed, and alloying treatment can be securely performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot-dip galvanizing method capable of accurately carrying out a diffusion reaction (alloying) between a base steel sheet and a galvanized layer in continuously producing a galvannealed steel sheet. The present invention relates to a method for manufacturing a galvanized steel sheet.

[0002]

2. Description of the Related Art Hot-dip galvanized steel sheets, which have excellent corrosion resistance and are relatively inexpensive, are widely used in all industrial fields. Among them, those that have been alloyed after being plated are Because of its excellent corrosion resistance, it is preferably used for automobile exteriors. This alloyed hot-dip galvanized steel sheet is produced by continuously immersing annealed steel strip in a hot-dip galvanizing bath, adjusting the plating thickness (unit weight) with a gas wiping device, and then heating it to approximately 500 to 600 ° C. To do. Heating is generally performed by burning fuel such as coke oven gas (C gas) in a furnace to heat it, but a high frequency induction heating method has also been proposed.

Although the heating furnace system is excellent in running cost, the heating rate is limited to 20 to 30 ° C./s. When rapid heating is required or when the strip passing speed is increased, the temperature of the heating furnace must be maintained at a high temperature, but this is often difficult due to the structure of the apparatus. In addition, unstable operation is inevitable when a line trouble occurs or the strip size is changed.

The induction heating system is capable of rapid heating and is significantly excellent in response to changes in heating temperature. Therefore, while stable operation is possible even when line troubles or strip size changes, the running cost is high, and uneven processing (unevenness in alloying processing) is likely to occur due to warp and vibration of the strip. There's a problem.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve the problem of producing an alloyed hot-dip galvanized steel sheet of good quality at a low running cost, and its specific purpose is to enable high-speed heating. It is an object of the present invention to provide a method for producing an alloyed hot-dip galvanized steel sheet, which is characterized by a heating method excellent in responsiveness to changes in alloying conditions.

[0006]

Means for Solving the Problems The present invention provides a method for producing an alloyed hot-dip galvanized steel sheet in which a steel strip is continuously immersed in a hot dip zinc bath for plating and then heated to alloy a galvanized layer. A touch roll above the gas wiping nozzle, and a method of manufacturing an alloyed hot dip galvanized steel sheet, characterized in that the strip is directly energized with a pair of more than one touch roll provided above the electrode to heat the strip, Is the gist.

FIG. 1 is a schematic view of a hot dip galvanizing apparatus for explaining the principle of the method of the present invention. As shown,
The strip (plating original plate) S annealed in an annealing furnace (not shown) passes through a snout 1 and a plating bath (molten zinc bath) 2
to go into. In the bath, the turn roll 3 changes the direction upward, and the guide roll 4 is passed to the outside of the bath. Next, the molten zinc attached by the gas wiping nozzle 5 is squeezed to a predetermined thickness. After that, it is heated to a predetermined temperature in the heating furnace 7 for alloying treatment, and further passes through the heat retention furnace 9 and the cooling zone 10 to become a product.

The feature of the method of the present invention is that the touch roll 6 and
The strip S is heated by directly energizing the strip S with a pair of rolls provided above it as electrodes.
The touch roll 6 may also serve as a touch roll provided for stabilizing the path of the strip in the continuous hot-dip galvanizing apparatus, or a touch roll for energization may be provided separately. The power supply used may be a DC power supply or an AC commercial power supply.

The other electrode (touch roll 8 in FIG. 1)
May also serve as a roll for controlling strip passing, which is usually installed in a heating furnace, or one or more pairs of touch rolls may be installed separately for electrodes.

According to the method of the present invention, the conventional heating furnace is not always necessary, and it is possible to sufficiently raise the temperature to a predetermined temperature by directly energizing the strip between the touch rolls 6 and 8. . However, it is practical to use the conventional heating furnace as it is, and perform fine adjustment of the temperature and high-speed temperature increase by direct electric heating.

[0011]

The strip S after being immersed in the plating bath 2 and having its weight per unit area adjusted by the gas wiping nozzle 5 continues to travel to the alloying zone, as in the production of normal galvanized steel sheets. The wiping operation itself may be an ordinary method. The gas used may be air, N ₂ , Ar or any other commonly used gas, and may be used by heating the gas to a predetermined temperature.

In the alloying zone, the strip after plating is heated to promote the diffusion of elements between the plating layer and the original plate to alloy the plating layer, but the temperature control is important. In particular, when the line speed or the strip size is changed, it is necessary to change the temperature promptly. According to the method of the present invention, this temperature control can be performed quickly and accurately by adjusting the amount of current supplied to the strip between the touch rolls 6 and 8. As described above, it is also possible to heat the inside of the heating furnace with a combustion gas such as C gas and use the above-mentioned electric heating for fine adjustment of the temperature.

The temperature of the steel plate (S) running between one electrode (touch roll 6) and the other electrode (touch roll 8) rises substantially linearly and rapidly. Then, it is possible to accurately adjust to a predetermined alloying temperature by controlling the power supplied. Such an effect cannot be expected by gas combustion heating. Further, in this direct energization method, even if the strip is warped or vibrated to some extent, there is no uneven heating, and this is superior to the induction heating method.

[0014]

EXAMPLE An alloyed hot-dip galvanized steel sheet was manufactured using a steel strip having a thickness of 0.8 mm and a width of 1000 mm as a base plate in the actual production line having the structure shown in FIG.

After the above strip was annealed at 830 ° C. for 40 seconds soaking, it was immersed in a hot dip galvanizing bath 2 at a temperature of 460 ° C. at a strip running speed of 100 m / min or 200 m / min. The wiping condition was set so that the basis weight was constant at 60 g / m ^{2 on} one side.

Table 1 shows the temperature (furnace temperature) of the heating furnace 7 adjusted by C gas combustion, the output of the direct current heating of the strip by the power source 11, the strip temperature, the rate of temperature rise, and the alloying of the obtained plated steel sheet. Indicates the degree. A commercial AC power source (frequency 60 Hz, 200 V) was used as the power source 11.
The strip temperature was measured directly above the touch roll 8.

Nos. 1 to 5 in Table 1 have a line speed of 100 m / min.
In the example of No. 1, Nos. 1 and 2 correspond to the conventional method in which electric heating is not performed. When the furnace temperature is raised above 1120 ℃ like No.1.
A heating rate of about 15 ° C / s can be obtained, but at a furnace temperature of about 900 ° C
In No. 2, the rate of temperature rise was as low as 5.8 ° C / s, the strip temperature was low, and alloying was insufficient.

Among Nos. 6 to 10 having a line speed of 200 m / min, No. 6 corresponding to the conventional example has a strip temperature of only about 460 ° C even though the furnace temperature is 1121 ° C. . When the furnace temperature is lowered as in No. 7, the strip temperature further decreases. Both No. 6 and 7 are insufficiently alloyed.

Nos. 3 to 5 and Nos. 8 to 10 of the invention example
Is a furnace temperature similar to No. 2 and No. 7, but a large heating rate can be obtained by using direct current heating together, the strip temperature is sufficiently high, and good alloying is achieved. is there. In particular, the fact that a good alloying treatment can be performed even at a high line speed of 200 m / min demonstrates that the method of the present invention greatly contributes to the improvement of the productivity of equipment.

[0020]

[Table 1]

[0021]

The method of the present invention can be carried out not only by a slight modification of the conventional plating line, but also the operating cost is low because the energy efficiency of direct current heating is high. It is also a method that can promptly respond to alloying conditions that vary depending on the line speed, strip size, and the like. According to the method of the present invention, an excellent quality alloyed hot-dip plated steel sheet can be manufactured at low cost.

[Brief description of drawings]

FIG. 1 is a schematic view of a hot-dip metal plating line for explaining the method of the present invention.

[Explanation of symbols]

S: Strip, 1: Snout, 2: Plating bath (molten metal bath), 3: Turn roll, 4: Guide roll,
5: Gas wiping nozzle, 6, 8: Touch roll,
7: Heating furnace, 9: Insulating furnace, 10: Cooling zone, 11: Power supply

Claims (1)

[Claims] 1. A touch roll above a gas wiping nozzle in the production of an alloyed hot dip galvanized steel sheet in which a steel strip is continuously immersed in a hot dip zinc bath for plating and then heated to alloy a galvanized layer. And a pair of touch rolls provided above the electrode as electrodes to directly energize the strip to heat the strip to heat the galvannealed steel sheet.