EP3205741A1

EP3205741A1 - Continuous hot-dip metal plating method, hot-dip zinc-plated steel strip, and continuous hot-dip metal plating equipment

Info

Publication number: EP3205741A1
Application number: EP15848228.1A
Authority: EP
Inventors: Yu TERASAKI; Hideyuki Takahashi; Masaru Miyake; Yusuke YASUFUKU; Takumi Koyama; Atsushi Inaba; Masato Sasaki
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2014-10-08
Filing date: 2015-09-16
Publication date: 2017-08-16
Anticipated expiration: 2035-09-16
Also published as: TW201619411A; KR101910756B1; EP3205741A4; MX2017004585A; KR20170048549A; EP3205741B1; CN106795614A; CN106795614B; TWI561675B; JPWO2016056178A1; WO2016056178A1; JP6011740B2

Abstract

To provide a continuous hot-dip metal coating method that uses a gas wiping nozzle to control the coating weight and prevents occurrence of coating surface defects so that high-quality hot-dip metal-coated steel strips can be stably manufactured at a low cost. A galvanized steel strip and a continuous hot-dip metal coating facility are also provided.

A continuous hot-dip metal coating method includes continuously immersing a steel strip into a molten metal bath and blowing gas from a gas wiping nozzle onto the steel strip immediately after the steel strip is withdrawn from the molten metal bath so as to control a coating weight, in which a temperature T of wiping gas to be injected from the gas wiping nozzle is controlled on a basis of a D/B value, which is a ratio of a distance D between a tip of the gas wiping nozzle and the steel strip to a gap B of the gas wiping nozzle.

Description

Technical Field

The present invention relates to a continuous hot-dip metal coating method, a galvanized steel strip, and a continuous hot-dip metal coating facility.

Background Art

In a continuous hot-dip coating process, as shown in Fig. 4, a steel strip 1 from inside a snout 2 enters a molten metal 4 in a coating tank 3, has its direction changed by a sink roll 5, and is withdrawn from the coating tank 3. Then a gas wiping nozzle 6 disposed above the coating tank 3 wipes away excessive molten metal so that the coating weight is controlled to a predetermined level and the molten metal adhering on the surfaces of the steel strip 1 is leveled out in the strip transversal direction and strip longitudinal direction. The gas wiping nozzle 6 is usually configured to be longer than the steel strip width in order to accommodate various steel strip widths and to manage misalignment in the transversal direction during withdrawal of the steel strip, and so on, and extends outward beyond the transversal end portions of the steel strip 1.
In such a gas wiping method, the steel strip undergoes minute vibrations and the coating layer flows in an irregular manner due to blowing of the wiping gas, which frequently causes formation of a wavy pattern called bath wrinkles (also known as bath sagging) in the coating surface. If a coated steel sheet with bath wrinkles is used in outer panels such that the coating surface is used as a paint base surface, the surface properties of the paint film, smoothness in particular, are adversely affected. Thus, coated steel sheets with bath wrinkles cannot be used in outer panels that should be suitable for a painting process that gives excellent appearance, and the yield of the coated steel sheets is significantly affected.
To address the above-described issues, the following methods have been proposed to prevent bath wrinkle defects, which are appearance defects, of hot-dip metal-coated steel sheets.
A method described in Patent Literature 1 involves changing the surface properties of a temper rolling roll and rolling conditions during performance of temper rolling, which is the step subsequent to the steel sheet coating step, so that bath wrinkles are less conspicuous. A method described in Patent Literature 2 involves adjusting the surface roughness of a steel sheet according to the coating weight by using a skin-pass mill and a tension leveler, etc., prior to introducing the steel sheet into the molten zinc bath so as to reduce occurrence of bath wrinkles. Patent Literature 3 describes a method that involves setting the line speed and the height of the wiping nozzle from the bath surface that are optimum for the sheet thickness so as to reduce occurrence of bath wrinkles.

Citation List

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2004-27263
PTL 2: Japanese Unexamined Patent Application Publication No. 55-21564
PTL 3: Japanese Unexamined Patent Application Publication No. 9-41113

Summary of Invention

Technical Problem

However, according to the studies made by the inventors of the present invention, although minor bath wrinkles are overcome by the method described in Patent Literature 1, the method has had no effect on severe bath wrinkle defects. According to the method described in Patent Literature 2, a skin-pass mill, a tension leveler, and other facilities are needed to be installed for the process before the molten zinc bath, which poses a problem of cost. Even when these facilities are installed, ideal surface roughness is rarely obtained due to chemical and/or physical changes caused by pickling and recrystallization in pre-treatment facilities and annealing furnaces, which presumably makes it difficult to completely prevent occurrence of bath wrinkles. Moreover, according to the method described in Patent Literature 3, the line speed and the wiping nozzle height cannot immediately follow the changing point of the sheet thickness and thus steel sheets with bath wrinkles are inevitably produced, which leads to yield loss.
The present invention has been made under the circumstances described above and aims to provide a continuous hot-dip metal coating method that uses a gas wiping nozzle to control the coating weight and prevents occurrence of bath wrinkle defects so that high-quality hot-dip metal-coated steel strips can be stably manufactured at a low cost. The present invention also aims to provide a galvanized steel strip and a continuous hot-dip metal coating facility.

Solution to Problem

The gist of the present invention is as follows.

[1] A continuous hot-dip metal coating method including:
- continuously immersing a steel strip into a molten metal bath and
- blowing gas from a gas wiping nozzle onto the steel strip immediately after the steel strip is withdrawn from the molten metal bath so as to control a coating weight,
- wherein a temperature T of wiping gas to be injected from the gas wiping nozzle is controlled on a basis of a D/B value, which is a ratio of a distance D between a tip of the gas wiping nozzle and the steel strip to a gap B of the gas wiping nozzle.
[2] The continuous hot-dip metal coating method according to [1] above, in which the wiping gas injected from the gas wiping nozzle is an inert gas.
[3] The continuous hot-dip metal coating method according to [1] or [2] above, in which the temperature T of the wiping gas is controlled on a basis of the D/B value so as to be within a range indicated by formula (1) below where T_M represents a melting point of molten metal in the molten metal bath:
[Math. 1] $c_{1} \sqrt{{}^{D}{/_{B}}} - c_{2} {}^{D}{/_{B}} + T_{M} - 100 ≦ T ≦ c_{1} \sqrt{{}^{D}{/_{B}}} + c_{3} {}^{D}{/_{B}} + T_{M} + 100$
- T: the temperature of the wiping gas to be injected from the gas wiping nozzle [°C]
- T_M: the melting point of the molten metal [°C]
- D: the distance between the steel strip and the tip of the gas wiping nozzle [m]
- B: the gap of the gas wiping nozzle [m]
- c₁, c₂, and c₃: constants.
[4] A galvanized steel strip produced by the continuous hot-dip metal coating method according to any one of [1] to [3] above, the galvanized steel strip including:
- an Al-Zn based coating layer at a steel strip surface, the Al-Zn based coating layer containing Al: 1.0% to 10% by mass, Mg: 0.2% to 1.0% by mass, Ni: 0.005% to 0.10% by mass, and the balance being Zn and unavoidable impurities.
[5] A continuous hot-dip metal coating facility including a distance meter that measures a distance between a steel strip and a tip of a gas wiping nozzle in a non-contact manner, a control unit that calculates a target temperature T of wiping gas to be injected from the gas wiping nozzle on a basis of a distance D measured by the distance meter and a gap B of the gas wiping nozzle, and a gas heating device that heats the gas to be injected from the gas wiping nozzle to the target temperature T calculated by the control unit.
[6] The continuous hot-dip metal coating facility according to [5], in which the target temperature T is calculated by formula (1) below on the basis of a D/B value, which is a ratio of the distance D measured by the distance meter to the gap B of the gas wiping nozzle:
[Math. 1] $c_{1} \sqrt{{}^{D}{/_{B}}} - c_{2} {}^{D}{/_{B}} + T_{M} - 100 ≦ T ≦ c_{1} \sqrt{{}^{D}{/_{B}}} + c_{3} {}^{D}{/_{B}} + T_{M} + 100$
- T: the temperature of the wiping gas to be injected from the gas wiping nozzle [°C]
- T_M: a melting point of molten metal [°C]
- D: the distance between the steel strip and the tip of the gas wiping nozzle [m]
- B: the gap of the gas wiping nozzle [m]
- c₁, c₂, and c₃: constants.

Advantageous Effects of Invention

According to the present invention, occurrence of coating surface defects called bath wrinkles is suppressed and high-quality hot-dip metal-coated steel strips can be stably produced at a low cost.

Brief Description of Drawings

[Fig. 1] Fig. 1 is a schematic view of a production facility for a continuous hot-dip metal-coated steel strip according to an embodiment of the present invention.
[Fig. 2] Fig. 2 is an enlarged view of a tip of a gas wiping nozzle according to an embodiment of the present invention.
[Fig. 3] Fig. 3 is a graph showing whether bath wrinkles occurred in relation to the relationship between a D/B value and a wiping gas temperature T.
[Fig. 4] Fig. 4 is a schematic diagram of a conventional production facility for a continuous hot-dip metal-coated steel strip.

Description of Embodiments

The present invention will now be specifically described.
A continuous hot-dip metal coating facility according to the present invention is a facility used in continuously dipping a steel strip in a coating bath of a hot-dip metal coating tank so as to conduct a coating treatment, and then withdrawing the steel strip from the coating bath and blowing wiping gas toward the coated steel strip from a gas wiping nozzle installed above the coating bath so as to adjust the coating metal weight. The continuous hot-dip metal coating facility according to the present invention includes a distance meter that measures a distance between a steel strip and a tip of a gas wiping nozzle in a non-contact manner, a control unit that calculates a target temperature T of wiping gas to be injected from the gas wiping nozzle on a basis of a distance D measured by the distance meter and a gap B of the gas wiping nozzle, and a gas heating device that heats the gas to be injected from the gas wiping nozzle to the target temperature T calculated by the control unit.
Fig. 1 illustrates a continuous hot-dip metal coating apparatus according to an embodiment of the present invention. In Fig. 1, 1 denotes a steel strip, 2 denotes a snout, 3 denotes a coating tank, 4 denotes a molten metal, 5 denotes a sink roll, 6 denotes a gas wiping nozzle, 7 denotes a distance meter, 8 denotes a control unit (CU), and 9 denotes a gas heating device. The arrow indicates the direction in which the steel strip 1 is moved. The steel strip 1 from inside the snout 2 enters the molten metal 4 in the coating tank 3, has its direction changed by the sink roll 5, and is withdrawn from the coating tank 3. Then excess molten metal is wiped away by the gas wiping nozzle 6 installed above the coating tank 3 so that the coating weight is controlled to a predetermined weight.
Fig. 2 is an enlarged view of a tip of the gas wiping nozzle 6. In the present invention, the distance between the tip of the gas wiping nozzle 6 and the steel strip 1 is indicated by D for the sake of convenience. The gas wiping nozzle 6 includes an upper nozzle part 6a and a lower nozzle part 6b and the gap in the gas wiping nozzle 6 is indicated by B.
The distance meter 7 is, for example, installed below the gas wiping nozzle 6. The distance meter 7 continuously measures the distance D between the tip of the gas wiping nozzle 6 and the steel strip 1 and inputs the reading to the control unit 8. The control unit 8 calculates a target temperature of the wiping gas to be heated with the gas heating device 9 on a basis of the measurement data of the distance D input from the distance meter 7. The gas heating device 9 heats the wiping gas to the target temperature calculated by the control unit 8 and supplies the heated wiping gas to the gas wiping nozzle 6. The distance meter 7 may be any non-contact meter. The type of the control unit 8 is not particularly limited. The type of the gas heating device 9 is also not particularly limited as long as the device has a function of heating the wiping gas without delay based on the distance D between the gas wiping nozzle 6 and the steel strip 1. The gas heating device 9 may employ any method for heating the wiping gas to be supplied to the gas wiping nozzle 6. Examples of the method include a method of supplying gas heated by a heat exchanger and a method of mixing flue gas from the annealing furnace and air.
In the present invention, the temperature T of the wiping gas injected from the gas wiping nozzle 6 is controlled on a basis of a D/B value, which is the ratio of the distance D between the tip of the gas wiping nozzle 6 and the steel strip 1 to the gap B of the gas wiping nozzle 6. Since the temperature T of the wiping gas is controlled on the basis of the D/B value, flowability of the molten metal is improved. As a result, the molten metal flows down regularly and an effect of preventing bath wrinkle defects can be fully exhibited.
In the present invention, the wiping gas injected from the gas wiping nozzle 6 is preferably an inert gas. With inert gas, oxidation of the molten metal on the steel sheet surface can be prevented and the flowability of the molten metal can be further improved. Examples of the inert gas include, but are not limited to, nitrogen, argon, helium, and carbon dioxide.
When the temperature of the wiping gas is excessively low, bath wrinkle defects caused by low flowability of the molten metal occur. When the temperature of the wiping gas is excessively high, alloying is promoted and the appearance of the steel sheet is deteriorated. Thus, there is need to select the temperature T of the wiping gas optimum for the D/B value. In this respect, in the present invention, the relationship between the D/B value and the temperature T of the wiping gas for obtaining products free of bath wrinkle defects and having good appearance is determined. Assuming that the temperature of the molten zinc bath is 460°C, the line speed is 100 m/min, the pressure of the nozzle header is 30 kPa, and the gas type is air, a steel strip having a thickness of 1.2 mm and a width of 1000 mm is threaded through the continuous hot-dip metal coating facility. As a result, the relationship shown in Fig. 3 is obtained. In Fig. 3, the evaluation standards related to the state of occurrence of the bath wrinkles were as follows where Wa denotes the arithmetic mean waviness Wa [µm] measured in accordance with the JIS B0601-2001 standard.

F: Fail, galvanized steel sheet with large visually recognizable bath wrinkles (1.50 < Wa)
C: Fail, galvanized steel sheet with small visually recognizable bath wrinkles (1.00 < Wa ≤ 1.50)
A: Pass, aesthetically pleasing galvanized steel sheet free of visually recognizable bath wrinkles (0.50 < Wa ≤ 1.00)

The results in Fig. 3 show that the temperature T of the wiping gas is preferably controlled to be in the range indicated by formula (1) below according to the D/B value.
[Math. 1] $c_{1} \sqrt{{}^{D}{/_{B}}} - c_{2} {}^{D}{/_{B}} + T_{M} - 100 ≦ T ≦ c_{1} \sqrt{{}^{D}{/_{B}}} + c_{3} {}^{D}{/_{B}} + T_{M} + 100$

T: the temperature of the wiping gas injected from the gas wiping nozzle [°C]
T_M: melting point of molten metal [°C]
D: distance between the steel strip and the tip of the gas wiping nozzle [m]
B: gas wiping nozzle gap [m]
c₁, c₂, and c₃: constants

Since the constants c₁, c₂, and c₃ in the formula (1) change depending on the size of the wiping nozzle gap B and the nozzle shape, they must be determined preliminarily off-line. Specifically, a thermometer is set to a sheet surface simulating a steel strip and the reading of the thermometer is assumed to be the "temperature of the wiping gas at an impinging position where the wiping gas impinges on the steel strip". Some conditions for the D/B value are changed and the temperature is measured to determine the values of c1 to c3.
Since the wiping gas has substantially no wiping power at a D/B exceeding 60, there is no need to heat the wiping gas. In this respect, the upper limit of the D/B value is preferably 60 or less.
In the present invention, the temperature of the wiping gas at an impinging position where the wiping gas impinges on the steel strip is set to be within a 200°C range below and above the melting point T_M ((T_M ±100)°C) of the molten metal. When the temperature of the wiping gas at the impinging position where the wiping gas impinges on the steel strip is lower than (T_M - 100)°C, the ratio of metal solidified from the molten metal adhering on the steel sheet is significantly increased, the flowability of the molten metal is thus degraded, and bath wrinkle defects occur. For example, when a heat exchanger is used, the temperature of gas or liquid at the heating side may be changed to control the temperature of the wiping gas at the heated side. However, the method of control is not limited to this.
Meanwhile, when the temperature of the wiping gas at the impinging position where the wiping gas impinges on the steel strip exceeds (T_M + 100)°C, alloying of the steel strip and the molten metal is accelerated, appearance of the steel sheet is degraded, and the coating weight becomes higher than what is targeted. Moreover, since extra energy is consumed to heat the gas, energy efficiency is also degraded. As shown by Fig. 5 in Non-Patent Literature 1 (The Journal of the Iron and Steel Institute of Japan, vol. 81 (1995), No. 2, p. 49), it is known that the potential core of the wiping gas attenuates in proportion to the D/B value; hence, an appropriate gas temperature must be set according to the D/B value.
Since the melting point changes with changes in the composition of the molten metal coating (coating layer), the optimum range for the wiping gas temperature T varies.
In the case of a galvanized steel strip having an Al-Zn based coating layer at the steel strip surface, the Al-Zn coating layer containing Al: 1.0% to 10% by mass, Mg: 0.2% to 1.0% by mass, Ni: 0.005% to 0.10% by mass, and the balance being Zn and unavoidable impurities, Mg, which is more readily oxidizable than Al and Zn, is contained. Thus, it has been confirmed that when the wiping gas temperature is low, the flowability of the molten metal is particularly degraded and bath wrinkles easily occur. As a result, the surface appearance is degraded. In this respect, optimizing the wiping gas temperature to achieve a bath wrinkle defect preventing effect is particularly preferable for a galvanized steel strip produced by the gas wiping method of the present invention and having an Al-Zn based coating layer at the steel strip surface, the Al-Zn coating layer containing Al: 1.0% to 10% by mass, Mg: 0.2% to 1.0% by mass, Ni: 0.005% to 0.10% by mass, and the balance being Zn and unavoidable impurities.

EXAMPLE 1

A galvanized steel strip production test was conducted to study optimum installation conditions and embodiments of the gas wiping nozzle. A gas wiping nozzle having a nozzle gap B of 1.2 mm was used. The gas wiping nozzle height from the molten zinc bath surface was set at 350 mm and the wiping gas injection direction was set to be perpendicular to the steel strip surface. Specifically, a steel strip having a thickness of 1.2 mm and a width of 1000 mm was threaded at a line speed of 100 m/min and the composition of the coating layer, the distance D between the tip of the gas wiping nozzle and the steel strip, the pressure of gas injected from the gas wiping nozzle (nozzle header pressure), the gas set temperature, the gas type, and the coating weight were varied for evaluating the appearance of the steel sheet. The molten zinc bath temperature was set at 460°C. The constants in formula (1) were determined in advance through off-line testing and were c₁ = 45, c₂ = 1.5, and c₃ = 2.5.
The method for supplying gas to the gas wiping nozzle was a method that included heating room-temperature gas to a predetermined temperature with a heat exchanger and compressing the gas to a predetermined pressure by using a blower. The melting point (T_M) of the molten metal was T_M = 420°C when Al = 0.2% by mass and was T_M = 375°C when Al = 4.5% by mass, Mg = 0.5% by mass, and Ni = 0.05% by mass.
The appearance of the steel sheet was evaluated according to the following standards. Wa is the value of the arithmetic mean waviness Wa [µm] measured in accordance with the JIS B0601-2001 standard.

F: Fail, galvanized steel sheet with large visually recognizable bath wrinkles (1.50 < Wa)
C: Fail, galvanized steel sheet with small visually recognizable bath wrinkles (1.00 < Wa ≤ 1.50)
A: Pass, aesthetically pleasing galvanized steel sheet free of visually recognizable bath wrinkles (0.50 < Wa ≤ 1.00)
AA: Pass, highly aesthetically pleasing galvanized steel sheet free of visually recognizable bath wrinkles (0 < Wa ≤ 0.50)

The results are shown in Table 1.

[Table 1]

	Nozzle header pressure [kPa]	Gas set temperature [°C]	Gas type	Coating layer composition [mass%]				Nozzle-steel strip distance D [mm]	Nozzle gap B [mm]	D/B [-]	Optimum gas temperature range derived from D/B value [°C]	Coating weight [g/m²]	Appearance	Wa [µm]
	Nozzle header pressure [kPa]	Gas set temperature [°C]	Gas type	Al	Mg	Ni	Zn	Nozzle-steel strip distance D [mm]	Nozzle gap B [mm]	D/B [-]	Optimum gas temperature range derived from D/B value [°C]	Coating weight [g/m²]	Appearance	Wa [µm]
Example 1	30	550	Air	0.2	0	0	Balance	25	1.2	20.83	494-777	130	A	0.73
Example 2	30	600	Air	0.2	0	0	Balance	30	1.2	25.00	508-808	125	A	0.64
Example 3	30	590	Air	0.2	0	0	Balance	4	1.2	3.33	397-610	56	A	0.86
Example 4	30	420	Air	0.2	0	0	Balance	6	1.2	5.00	413-633	64	A	0.74
Example 5	30	670	Air	0.2	0	0	Balance	17	1.2	14.17	468-725	102	A	0.88
Example 6	30	740	Air	0.2	0	0	Balance	45	1.2	37.50	539-889	142	A	0.53
Example 7	30	610	Air	0.2	0	0	Balance	56	1.2	46.67	557-944	156	A	0.79
Example 8	30	900	Air	0.2	0	0	Balance	58	1.2	48.33	560-954	167	A	0.68
Example 9	30	550	Nitrogen	0.2	0	0	Balance	25	1.2	20.83	494-777	126	AA	0.08
Example 10	30	550	Nitrogen	4.5	0.5	0.05	Balance	25	1.2	20.83	449-732	127	AA	0.12
Comparative Example 1	30	420	Air	0.2	0	0	Balance	25	1.2	20.83	494-777	155	C	1.41
Comparative Example 2	30	830	Air	0.2	0	0	Balance	25	1.2	20.83	494-777	163	C	1.25
Comparative Example 3	30	770	Air	0.2	0	0	Balance	6	1.2	5.00	413-633	85	C	1.34
Comparative Example 4	30	400	Air	0.2	0	0	Balance	15	1.2	12.50	460-710	114	C	1.30
Comparative Example 5	30	820	Air	0.2	0	0	Balance	20	1.2	16.67	479-745	133	C	1.29
Comparative Example 6	30	960	Air	0.2	0	0	Balance	48	1.2	40.00	545-905	171	C	1.21
Comparative Example 7	30	490	Air	0.2	0	0	Balance	52	1.2	43.33	551-925	173	C	1.44
Comparative Example 8	30	960	Air	0.2	0	0	Balance	7	1.2	5.83	420-643	108	F	1.67
Comparative Example 9	30	860	Air	0.2	0	0	Balance	10	1.2	8.33	437-671	134	F	1.98
Comparative Example 10	30	330	Air	0.2	0	0	Balance	21	1.2	17.50	482-752	136	F	1.72
Comparative Example 11	30	360	Air	0.2	0	0	Balance	55	1.2	45.83	556-939	185	F	1.69
Comparative Example 12	30	420	Air	4.5	0.5	0.05	Balance	25	1.2	20.83	449-732	149	F	1.88

In Example 1, bath wrinkle defects were prevented by performing wiping at an optimum gas temperature for the D/B value. The reasons bath wrinkle defects were prevented are presumably that wiping was performed at a gas temperature optimum for the D/B value and that the temperature of the wiping gas at the impinging position where the wiping gas impinged on the steel strip was set at (T_M - 100)°C or higher, which inhibited the cooling effect of the injected gas and caused the molten zinc on the steel sheet to remain relatively unsolidified and flow down regularly.
In Example 2, D/B was changed and wiping was performed at a wiping gas temperature T optimum for the changed value, thereby preventing bath wrinkle defects as in Example 1. In Examples 3 to 8, the results of changing the wiping temperature relative to the D/B values are shown.
In contrast, Comparative Example 1 and Comparative Example 2 are examples in which the gas temperature was outside the optimum gas temperature range derived from the D/B value. The reasons the coating weight increased in Comparative Example 1 were presumably that wiping was conducted at a temperature outside the optimum gas temperature range for the D/B value and that the temperature of the wiping gas at the steel strip impinging point was below (T_M - 100)°C due to mixing of ambient gas into the wiping gas injected from the nozzle. In Comparative Example 2, the temperature of the wiping gas was increased to be higher than in Example 1 and the coating weight increased. This is presumably because of an excessively high wiping gas temperature, which accelerates alloying of the zinc coating on the steel strip surface layer. Also, due to acceleration of alloying, the steel sheet surface became whitish in color and appearance was degraded. Results under other wiping conditions are indicated in Comparative Examples 3 to 11.
Example 10 and Comparative Example 12 are examples in which the composition of the coating layer was changed. Because the composition of the molten zinc was changed, the melting point of the zinc bath decreased to 375°C and thus the optimum temperature range of the wiping gas also changed. In Comparative Example 12, occurrence of larger bath wrinkles than in Comparative Example 1 was observed. This is probably because Mg in the coating layer composition is readily oxidable and bath wrinkles readily occurred as a result. In Example 10, the wiping gas temperature T was controlled and bath wrinkle defects were prevented as in Example 1.
In Examples 9 and 10, the type of gas was nitrogen, which is an inert gas, and thus better appearance was obtained than in Example 1.
As described above, a bath wrinkle defect preventing effect can be obtained by performing wiping at an appropriate wiping gas temperature.

Reference Signs List

1: steel strip
2: snout
3: coating tank
4: molten metal
5: sink roll
6: gas wiping nozzle
6a: upper nozzle part of the gas wiping nozzle
6b: lower nozzle part of the gas wiping nozzle
7: distance meter
8: control unit (CU)
9: gas heating device

Claims

A continuous hot-dip metal coating method comprising:
continuously immersing a steel strip into a molten metal bath and

blowing gas from a gas wiping nozzle onto the steel strip immediately after the steel strip is withdrawn from the molten metal bath so as to control a coating weight,

wherein a temperature T of wiping gas to be injected from the gas wiping nozzle is controlled on a basis of a D/B value, which is a ratio of a distance D between a tip of the gas wiping nozzle and the steel strip to a gap B of the gas wiping nozzle.
The continuous hot-dip metal coating method according to Claim 1, wherein the wiping gas injected from the gas wiping nozzle is an inert gas.
The continuous hot-dip metal coating method according to Claim 1 or 2, wherein the temperature T of the wiping gas is controlled on a basis of the D/B value so as to be within a range indicated by formula (1) below where T_M represents a melting point of molten metal in the molten metal bath:
[Math. 1] $c_{1} \sqrt{{}^{D}{/_{B}}} - c_{2} {}^{D}{/_{B}} + T_{M} - 100 ≦ T ≦ c_{1} \sqrt{{}^{D}{/_{B}}} + c_{3} {}^{D}{/_{B}} + T_{M} + 100$

T: the temperature of the wiping gas to be injected from the gas wiping nozzle [°C]

T_M: the melting point of the molten metal [°C]

D: the distance between the steel strip and the tip of the gas wiping nozzle [m]

B: the gap of the gas wiping nozzle [m]

c₁, c₂, and c₃: constants.
A galvanized steel strip produced by the continuous hot-dip metal coating method according to any one of Claims 1 to 3, comprising:
an Al-Zn based coating layer at a steel strip surface, the Al-Zn based coating layer containing

Al: 1.0% to 10% by mass,

Mg: 0.2% to 1.0% by mass,

Ni: 0.005% to 0.10% by mass, and

the balance being Zn and unavoidable impurities.
A continuous hot-dip metal coating facility comprising:
a distance meter that measures a distance between a steel strip and a tip of a gas wiping nozzle in a non-contact manner,

a control unit that calculates a target temperature T of wiping gas to be injected from the gas wiping nozzle on a basis of a distance D measured by the distance meter and a gap B of the gas wiping nozzle, and

a gas heating device that heats the gas to be injected from the gas wiping nozzle to the target temperature T calculated by the control unit.
The continuous hot-dip metal coating facility according to Claim 5, wherein the target temperature T is calculated by formula (1) below on a basis of a D/B value, which is a ratio of the distance D measured by the distance meter to the gap B of the gas wiping nozzle:
[Math. 1] $c_{1} \sqrt{{}^{D}{/_{B}}} - c_{2} {}^{D}{/_{B}} + T_{M} - 100 ≦ T ≦ c_{1} \sqrt{{}^{D}{/_{B}}} + c_{3} {}^{D}{/_{B}} + T_{M} + 100$

T: the temperature of the wiping gas to be injected from the gas wiping nozzle [°C]

T_M: a melting point of molten metal [°C]

D: the distance between the steel strip and the tip of the gas wiping nozzle [m]

B: the gap of the gas wiping nozzle [m]

c₁, c₂, and c₃: constants.