M
=

0
.
0

⁢
8
⁢
0
⁢
74
×
Vh
×

10

7
.
5

⁢

Th
/

(

Th
+

2
37.3

)

(
2
)

X
=

0.2
×
w
×
S

+
0.4935

(
3
)

- M: amount of moisture contained in the humidified gas that is supplied into the soaking zone (g/min)
- X: parameter regarding an influence of the surface area of the steel sheet
- Vh: flow rate of the humidified gas that is supplied into the soaking zone (Nm³/hr)
- Th: dew point of the humidified gas that is supplied into the soaking zone (° C.)
- w: width of the steel sheet (m)
- S: sheet passing speed (m/s)

Here, the amount of moisture M (g/min) contained in the humidified gas that is supplied into the soaking zone is calculated from the dew point of the introduced humidified gas by using the mole fraction (-) of water vapor contained in the humidified gas. Specifically, the dew point Th (° C.) of the humidified gas that is introduced into the soaking zone is converted to a saturated water vapor pressure and eventually to the mole fraction of water vapor (H₂O) by using the Tetens equation. This conversion equation is given below. In addition, FIG. 4 is a graph obtained from this equation, illustrating the influence of the dew point on the mole fraction of water vapor.

mole
⁢

fraction
⁢

of
⁢

H
2

⁢
O
⁢

(
-
)

=

6.11
×

10

(

7.5
×

Th
/

(

Th
+
237.3

)

/
1013.5

(
A
)

Moreover, from this mole fraction and the flow rate Vh (Nm³/hr) of the humidified gas that is supplied, the above described amount of moisture M contained in the humidified gas that is supplied into the soaking zone is calculated by using Avogadro's law.

M
⁢

(

g
/
min

)

=

mole
⁢

fraction
⁢

of
⁢

H
2

⁢
O
×
Vh
⁢

(

Nm
3

/
hr

)

/
60
⁢

(

min
/
hr

)

×
 
18
⁢

(

mass
⁢

of
⁢

1
⁢

mol
⁢

of
⁢

H
2

⁢
O
:

g
/
mol

)

×
1000
⁢

(

L
/

Nm
3

)

/
22.4

(

volume
⁢

of
⁢

1
⁢

mol
⁢

of
⁢

gas
:

L
/
mol

)

(
B
)

By substituting equation (A) into equation (B), equation (2) is obtained as follows.

M
⁡
(

g
/
min

)

=

0
.
0

⁢
8
⁢
0
⁢
74
×
Vh
×

10

7
.
5

⁢

Th
/

(

Th
+
237.3

)

(
2
)

In the case where the humidified gas described above is supplied and the steel sheet passing conditions are stable with no change, it is preferable that the dew point of the interior of a region from the heating zone to the soaking zone be controlled to be −15° C. to 0° C.

Here, the reason why it is necessary for the humidified gas described above to satisfy expression (1) above is because it is necessary to supply water without excess or deficiency with respect to the surface area of the steel sheet existing in the annealing furnace.

In the case where M/X is 158 or less, since an insufficient amount of water is supplied with respect to the amount of water consumed on the surface of the steel sheet, surface concentration of Si is insufficiently inhibited, which results in bare spot occurrence.

In the case where M/X is 178 or more, since an excessive amount of water is supplied with respect to the amount of water consumed on the surface of the steel sheet, excessive oxidation of the steel sheet substrate occurs due to the excessive amount of water, which results in a pressing flaw defect, which is called pickup, occurring due to the oxides sticking to hearth rolls. Here, M is determined by using equation (2) with which the amount of moisture is calculated from the flow rate of the humidified gas and the dew point of the humidified gas. Similarly, X is determined by using equation (3) which expresses the influence of the surface area of the steel sheet existing in the annealing furnace and which has been derived by using a regression method from past operation results.

FIG. 1 is a schematic diagram illustrating the system supplying the gas mixture into the soaking zone 12. The humidified gas is supplied through upper humidified gas supply ports 36A, 36B, and 36C, middle humidified gas supply ports 37A, 37B, and 37C, and lower humidified gas supply ports 38A, 38B, and 38C, all of which are disposed in the region on the downstream side of the soaking zone, and through humidified gas supply ports 39A, 39B on the exit side of the soaking zone.

In FIG. 1, a portion of the reducing gas or the non-oxidizing gas (dry gas) described above is sent by a gas distributing device 24 to a humidifying device 26, and the remainder is sent to supply ports 42A, 42B, 42C, 44A, 44B, and 44C as a dry gas. The humidified gas is distributed by a gas distributing device 30 to various systems, and the humidified gas is sent through humidified gas pipework 34 and the humidified gas supply ports 36A, 36B, 36C, 37A, 37B, 37C, 38A, 38B, 38C, 39A, 39B into the soaking zone 12. The humidifying device may be disposed for each of the anterior and posterior gas supply systems. In particular, it is desirable that, in the region on the downstream side of the soaking zone where the steel sheet is heated to high temperature, a plurality of supply ports be arranged in the vertical direction.

Since there is a gas flow in which the dry gas which has been supplied into the cooling zone flows into the soaking zone and passes therethrough to the heating zone side, it is possible to provide humidification throughout the heating zone and the soaking zone by using the gas supplying method described above.

The dew point of the interior of the soaking zone is monitored by using dew point meters disposed at 46A, 46B, and 46C. 46A is a position at which the representative dew point on the downstream side of the soaking zone is monitored, 46B is a position at which the dew point in the vicinity of rolls in a lower part of the soaking zone is monitored, and 46C is a position at which the dew point of the gas flowing from the cooling zone 14 to the soaking zone is monitored.

In the case where the dew point of the whole soaking zone is increased to about 0° C., there is, particularly, an increase in time taken to decrease the dew point of the anterior part of the soaking zone when the steel grade is changed to one for which humidification is not necessary. Here, in the case where the dew point of the soaking zone is higher than 0° C., since a phenomenon which is called pickup and in which the oxides of the steel sheet stick to hearth rolls occurs, a pressing flaw-like defect occurs.

Although examples of the humidifying device include devices which humidify the dry gas by using a bubbling method, a membrane exchange method, a high-temperature steam addition method, or the like, it is preferable that a membrane exchange method be used form the viewpoint of the stability of the dew point when the flow rate is changed. In the humidifying device 26, there is a humidifying module having a fluorocarbon- or polyimide-based hollow fiber membrane or flat membrane, and the dry gas flows inside the membrane while pure water whose temperature is adjusted to a predetermined temperature by using a circulation thermostatic water tank 28 is circulated outside the membrane. The fluorocarbon- or polyimide-based hollow fiber membrane or flat membrane is a kind of ion-exchange membrane having an affinity for water molecules. When there is a difference in water concentration between the inside and the outside of the hollow fiber membrane, since a force to eliminate such a difference is generated, water permeates through the membrane to the side on which the water concentration is lower by using the generated force as a driving force. The temperature of the dry gas changes in accordance with seasonal and daily variations in atmospheric temperature. However, in this humidifying device, since it is also possible to perform heat exchange due to a sufficient contact area between the gas and the water with the water vapor-permeable membrane therebetween being achieved, the dry gas is humidified so as to become a gas having a dew point equal to the predetermined water temperature regardless of whether the temperature of the dry gas is lower or higher than the temperature of the circulated water, which makes it possible to control the dew point with high accuracy. It is possible to control the dew point of the humidified gas to any temperature in the range of 5° C. to 50° C. In the case where the dew point of the humidified gas is higher than the atmospheric temperature around the pipework, since dew condensation occurs, there may be a case where the dew condensation water directly enters the interior of the furnace. Therefore, the pipework for the humidified gas is heated and held at a temperature higher than or equal to the dew point of the humidified gas.

(Snout)

Gas nozzles are arranged over the entire perimeter of the inner wall of the snout, nitrogen gas or a nitrogen-hydrogen gas mixture is supplied through the gas nozzles downward along the inner wall, and at least two exhaust ports are disposed in an upper part of the snout to discharge the gas that is supplied through the gas nozzles described above. By discharging 70 volume % or more of the amount of the gas that is supplied through the gas nozzles, since it is possible to avoid ash deposition in the snout, it is possible to prevent an ash defect with more certainty.

The reason why the gas nozzles are arranged over the entire perimeter of the inner wall of the snout and nitrogen gas or a nitrogen-hydrogen gas mixture is supplied through the gas nozzles downward along the inner wall is because this makes it possible to efficiently send zinc vapor (or fine zinc powder) to the outside of the snout from the whole area of the liquid surface in the snout. In addition, the reason why nitrogen gas or a nitrogen-hydrogen gas mixture is supplied through the gas nozzles downward along the inner wall is because this makes it possible to prevent the oxidization of the liquid surface in the snout.

The reason why at least two exhaust ports are disposed in the upper part of the snout is because this makes it possible to efficiently discharge zinc vapor (or fine zinc powder) which is floating in the snout to the outside of the snout.

The reason why discharging 70 volume % or more of the amount of the gas that is supplied through the gas nozzles is effective is because this makes it possible to efficiently discharge zinc vapor (or fine zinc powder) which is floating in the snout to the outside of the snout without allowing the zinc vapor (or fine zinc powder) to remain in the snout. In the case where the amount of the discharged gas is less than 70 volume % of the amount of the gas that is supplied through the gas nozzles, since the zinc vapor (or fine zinc powder) is stuck and deposited on the inner wall of the snout or the like, the stuck and deposited substances drop onto the surface of the steel sheet or the liquid surface, which may result in poor surface appearance due to the substances sticking to the steel sheet.

FIG. 2 is a diagram illustrating the structure of the snout 18 and the gas flow. In the snout, gas nozzles 60 are arranged over the entire perimeter of the inner wall of the snout, and nitrogen gas or a nitrogen-hydrogen gas mixture is injected through the gas nozzles 60 downward along the inner wall of the snout. The expression “gas nozzles 60 are arranged over the entire perimeter of the inner wall of the snout” denotes a case where the gas nozzles 60 are arranged over the entire perimeter of the interior of the snout at a position where a plane perpendicular to the steel sheet in the snout intersects with the inner wall of the snout. The dew point of the atmosphere gas is measured by using a dew point meter which is disposed at a position 65 located above the gas nozzles 60, and the dew point is controlled to be −50° C. to −35° C. In the case where the dew point is more than −35° C., since the oxides of Zn and Al are formed on the liquid surface of the galvanizing bath, such oxides are entrained into the bath due to the steel sheet passing, which results in bare spots. On the other hand, in the case where the dew point is lower than −50° C., since zinc fume is markedly generated, it is difficult to control the amount of zinc fume by utilizing the gas flow rate through the gas nozzles, which results in a significant deterioration in the surface appearance of the product due to occurrence of ash defects.

Generally, since the temperature of the steel sheet is higher than the temperature of the atmosphere gas in the snout, updraft occurs in the vicinity of the steel sheet P. The gas injected through the gas nozzles 60 carries zinc fume and flows along the steel sheet to the upper part of the snout. The atmosphere gas in the snout containing zinc fume is discharged through exhaust ports 61 which are disposed in the upper part of the snout. At this time, by controlling the amount of gas which is discharged through the exhaust ports 61 to be 70% or more of the amount of the gas injected through the gas nozzles, since it is possible to avoid ash deposition in the snout, it is possible to prevent an ash defect from occurring with more certainty.

FIG. 3 is a diagram illustrating one example of the constitution of continuous hot-dip galvanizing apparatus including an annealing furnace and a coating equipment.

EXAMPLES

By using the continuous hot-dip galvanizing apparatus shown in FIG. 1 to FIG. 3, three kinds of steel sheets were annealed under various annealing conditions and thereafter subjected to a hot-dip galvanizing treatment and an alloying treatment. The main chemical compositions of steel sheets A to C are given in Table 1. The constituents other than the main chemical compositions given in Table 1 are optional constituents and a balance of Fe and incidental impurities.

TABLE 1

(mass %)

Target Temperature

Steel
Chemical Composition
in Soaking Zone

Code
C
Si
Mn
P
S
Exit Temperature (° C.)

A
0.10
0.2
2.4
0.02
0.001
800 ± 15

B
0.10
0.9
2.8
0.01
0.001
850 ± 15

C
0.11
1.5
2.7
0.01
0.001
830 ± 15

balance: Fe and incidental impurities

The examples of the present invention were manufactured by using the humidifying system shown in FIG. 1. As the dry gas, a gas having a chemical composition containing hydrogen in an amount of 10 volume % and a balance of nitrogen and incidental impurities (having a dew point of −50° C.) was used. A portion of this dry gas was humidified by using a humidifying device having a humidifying unit of a hollow fiber membrane type to prepare a humidified gas. The humidifying unit of a hollow fiber membrane type was composed of 10 membrane modules, and the circulation water flowed at a flow rate of 20 L/min at maximum. The circulation thermostatic water tank was used in common, and with this, it was possible to supply pure water in an amount of 200 L/min in total. The humidified gas supply ports were disposed at positions shown in FIG. 1. The dry gas, which was not humidified, was supplied through the supply ports in the lower part of the furnace.

In the snout, the gas nozzles were arranged over the entire perimeter of the inner wall of the snout, nitrogen gas or a nitrogen-hydrogen gas mixture was supplied through the gas nozzles downward along the inner wall, and at least two exhaust ports were disposed in the upper part of the snout to discharge the atmosphere gas in the snout. 64 volume % to 92 volume % of the amount of the gas that was supplied through the gas nozzles was discharged, and the coating appearance was evaluated.

In the examples in which hot-dip galvannealed steel sheets (GA) were manufactured, the temperature of the galvanizing bath was 460° C., the Al concentration in the galvanizing bath was 0.130 mass %, and the coating weight was adjusted to be 50 g/m²per side by using a gas wiping method. In addition, after a hot-dip galvanizing treatment had been performed, an alloying treatment was performed by using an alloying furnace of an induction heating type so that the alloying degree (Fe content) of the coating film was 10 mass % to 13 mass %. The alloying temperature at this time is given in Table 2.

In the examples in which hot-dip galvanized steel sheets (GI) were manufactured, the temperature of the galvanizing bath was 450° C., the Al concentration in the galvanizing bath was 0.200 mass %, and the coating weight was adjusted to be 60 g/m²per side by using a gas wiping method.

(Evaluation Method)

To evaluate the coating appearance, a test utilizing an optical surface defect detector (for detecting a bare spot defect or a flaw due to roll pickup having a diameter of 0.5 mm or more) and evaluation grading of a variation in alloying by visual observation (in the case of GA) or evaluation grading of an appearance pattern by visual observation (in the case of GI) were performed. A case where all the items were good was denoted as ⊚, a case where the result of the test utilizing the surface defect detector was satisfactory and there was a slight variation in alloying or a slight variation in appearance that caused no quality problem was denoted as o, a case where there was a variation in alloying or a variation in appearance that resulted in a decrease in surface quality grade was denoted as A, and a case where the result of the test utilizing the surface defect detector was unsatisfactory was denoted as x. The results are given in Table 2.

TABLE 2

Snout

Soaking Zone to Connection Part
Nozzle

Sheet
Steel Sheet

Gas

Passing
Temperature
Dry Gas
Humidified
Humidified

Conformity
Supply

Sheet
Speed
Measured at
Flow
Gas Flow
Gas Dew

Expression
to
Flow

Steel
Width
V
Exit
Rate
Rate
Point

(1)
Expression
Rate

No
Code
(m)
(m/s)
(° C.)
(Nm³/hr)
(Nm³/hr)
(° C.)
M
X
M/X
(1)
(Nm³/hr)

1
A
1.0
1.5
803
600
0
—
—
—
—
no
2.5

2
A
1.0
1.5
803
250
350
20
108.2
0.7935
136.3
no
2.5

3
A
1.0
1.5
805
250
420
20
129.8
0.7935
163.6
yes
2.5

4
A
1.0
1.5
800
250
450
20
139.1
0.7935
175.3
yes
2.5

5
A
1.0
1.5
801
250
500
20
154.5
0.7935
194.8
no
2.5

6
A
1.0
1.5
805
250
350
24
138.0
0.7935
174.0
yes
2.5

7
B
1.2
1.5
848
600
0
—
—
0.8535
—
no
2.5

8
B
1.2
1.5
850
250
450
20
139.1
0.8535
163.0
yes
2.5

9
B
1.2
1.5
852
250
450
20
139.1
0.8535
163.0
yes
2.5

10
B
1.2
1.5
851
250
450
20
139.1
0.8535
163.0
yes
2.5

11
B
1.2
1.5
850
250
450
20
139.1
0.8535
163.0
yes
2.5

12
B
1.2
1.5
853
250
450
20
139.1
0.8535
163.0
yes
2.5

13
B
1.2
1.5
848
250
450
20
139.1
0.8535
163.0
yes
2.5

14
B
1.2
1.5
847
250
450
20
139.1
0.8535
163.0
yes
2.5

15
B
1.2
1.5
845
250
450
20
139.1
0.8535
163.0
yes
2.5

16
C
0.9
1.2
829
600
0
—
—
0.7095
—
no
2.5

17
C
0.9
1.2
830
250
400
20
123.6
0.7095
174.3
yes
2.5

18
C
0.9
1.2
830
600
0
—
—
0.7095
—
no
2.5

19
C
0.9
1.2
832
250
400
20
123.6
0.7095
174.3
yes
2.5

20
C
0.9
1.2
832
250
410
20
126.7
0.7095
178.6
no
2.5

21
C
0.9
1.2
832
250
360
20
111.3
0.7095
156.8
no
2.5

22
C
0.9
1.2
832
250
370
20
114.4
0.7095
161.2
yes
2.5

Snout

Material

Exhaust
Exhaust
Alloying Treatment

Strength

Atmosphere
Flow
Flow

Temperature

Tensile
Comprehensive

Dew Point
Rate
Rate/Supply

Alloying
Coating
Strength
Evaluation

No
(° C.)
(Nm³/hr)
Flow Rate
GA/GI
(° C.)
Appearance
(MPa)
Pass/Fail
Class

1
−44.3
2.3
0.92
GA
560
x (Bare spot)
740
Fail
Comparative Example

2
−41.2
2.3
0.92
GA
540
x (Bare spot)
765
Fail
Comparative Example

3
−45.5
2.3
0.92
GA
509
⊚
798
Pass
Example

4
−36.3
2.3
0.92
GA
506
⊚
795
Pass
Example

5