METHOD FOR MANUFACTURING HOT-DIP GALVANIZED STEEL SHEET

Abstract

When a hot-dip galvanizing treatment is performed on a steel sheet containing Si in an amount of 0.2 mass % or more by using a continuous hot-dip galvanizing apparatus including an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order, a snout adjacent to the cooling zone, and hot-dip galvanizing equipment, a humidified nitrogen-hydrogen gas mixture containing moisture in such a manner that a certain expression is satisfied is supplied into a region on the downstream side of the soaking zone, gas nozzles are arranged over the entire perimeter of an 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 the dew point in the snout is controlled to be ?50? C. to ?35? C.

Claims

1. A method for manufacturing a hot-dip galvanized steel sheet, the method comprising performing a hot-dip galvanizing treatment on a steel sheet containing Si in an amount of 0.2 mass % or more by using a continuous hot-dip galvanizing apparatus including an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order, a snout adjacent to the cooling zone, and hot-dip galvanizing equipment, wherein a humidified nitrogen-hydrogen gas mixture containing moisture in such a manner that expression (1) below is satisfied is supplied into a region on the downstream side of the soaking zone, wherein gas nozzles are arranged over the entire perimeter of an inner wall of the snout, wherein nitrogen gas or a nitrogen-hydrogen gas mixture is supplied through the gas nozzles downward along the inner wall, wherein 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, and wherein the dew point in the snout is controlled to be ?50? C. to ?35? C.: $\begin{array}{cc}158<M/X<178,& \left(1\right)\end{array}$ where M denotes the amount of moisture contained in the humidified gas that is supplied into the soaking zone and X denotes a parameter regarding an influence of a surface area of the steel sheet.

2. The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, wherein M and X satisfy equations (2) and (3) below: $\begin{array}{cc}M=0.08074?\mathrm{Vh}?{10}^{7.5\mathrm{Th}/\left(\mathrm{Th}+237.3\right)}& \left(2\right)\end{array}$ $\begin{array}{cc}X=0.2?w?S+0.4935& \left(3\right)\end{array}$ 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 (Nm.sup.3/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)

3. The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, wherein 70 volume % or more of the amount of the gas that is supplied through the gas nozzles is discharged through the exhaust ports in the upper part of the snout.

4. The method for manufacturing a hot-dip galvanized steel sheet according to claim 2, wherein 70 volume % or more of the amount of the gas that is supplied through the gas nozzles is discharged through the exhaust ports in the upper part of the snout.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 is a diagram illustrating one embodiment of a supply route of a furnace gas in a soaking zone.

[0035] FIG. 2 is a diagram illustrating one embodiment of a snout structure and gas pipework.

[0036] 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.

[0037] FIG. 4 is a graph illustrating the influence of the dew point on the mole fraction of water vapor.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0038] First, the constitution of a continuous hot-dip galvanizing apparatus which is used in a method for manufacturing a hot-dip galvannealed steel sheet according to one embodiment of the present invention will be described with reference to FIG. 3. The continuous hot-dip galvanizing apparatus includes an annealing furnace in which a heating zone 10, a soaking zone 12, and cooling zones 14 and 16 are arranged in this order and hot-dip galvanizing equipment adjacent to the cooling zone 16, that is, a hot-dip galvanizing bath 22. The heating zone 10 in the present embodiment includes a first heating zone 10A (anterior part of the heating zone, not illustrated) and a second heating zone 10B (posterior part of the heating zone, not illustrated). The cooling zone includes a first cooling zone 14 (rapid cooling zone) and a second cooling zone 16 (gradual cooling zone). The front end of a snout 18 connected to the second cooling zone 16 is immersed in the hot-dip galvanizing bath 22, and the annealing furnace and the hot-dip galvanizing bath 22 are connected through the snout 18. The continuous hot-dip galvanizing apparatus also includes alloying equipment 23 which is used for heating and alloying a galvanizing layer.

(Heating Zone)

[0039] In the heating zone in the present embodiment, it is possible to indirectly heat a steel sheet P by using a radiant tube or an electric heater. It is preferable that the average temperature of the interior of the heating zone be 500? C. to 800? C. While a gas from the soaking zone flows into the heating zone, a reducing gas or a non-oxidizing gas is separately supplied into the heating zone. Typical examples of the reducing gas used include a nitrogen-hydrogen gas mixture such as a gas having a chemical composition containing hydrogen in an amount of 1 volume % to 20 volume % and a balance of nitrogen and incidental impurities (having a dew point of about-60? C.). In addition, examples of the non-oxidizing gas include a gas having a chemical composition containing nitrogen and incidental impurities (having a dew point of about-60? C.).

(Soaking Zone)

[0040] In the soaking zone 12 in the present embodiment, it is possible to indirectly heat the steel sheet P by using a radiant tube (RT, not illustrated) as heating means. It is preferable that the average temperature of the interior of the soaking zone 12 be 700? C. to 900? C.

[0041] A reducing gas or a non-oxidizing gas is supplied into the soaking zone 12. Typical examples of the reducing gas used include a gas mixture of nitrogen and hydrogen (hereinafter, also referred to as nitrogen-hydrogen gas mixture) such as a gas having a chemical composition containing hydrogen in an amount of 1 volume % to 20 volume % and a balance of nitrogen and incidental impurities (having a dew point of about ?60? C.). In addition, examples of the non-oxidizing gas include a gas having a chemical composition containing nitrogen and incidental impurities (having a dew point of about ?60? C.).

[0042] In the present embodiment, the reducing gas or the non-oxidizing gas which is supplied into the soaking zone 12 is used in two forms, that is, in the form of a humidified gas and in the form of a dry gas. Here, the term dry gas denotes a reducing gas or non-oxidizing gas described above having a dew point of about ?60? C. to ?50? C. which is not humidified by using a humidifying device. On the other hand, the term humidified gas denotes a gas which is humidified by using a humidifying device so as to have a dew point of 0? C. to 30? C. When a high-strength steel sheet containing Si or the like is manufactured, by supplying a humidified gas to increase the dew point in a furnace, internal oxidation of Si or the like is promoted, thereby performing control so that Si or the like is not concentrated on the surface of the steel sheet.

[0043] The amount of the humidified gas which flows into a region on the downstream side of the soaking zone is adjusted so that expression (1) below is satisfied.

[00003]$\begin{array}{cc}158<M/X<178& \left(1\right)\end{array}$

[0044] Here, M denotes the amount of moisture contained in the humidified gas that is supplied into the soaking zone and X denotes a parameter regarding an influence of a surface area of the steel sheet. More specifically, M and X are values which satisfy equations (2) and (3) below.

[00004]$\begin{array}{cc}M=0.08074?\mathrm{Vh}?{10}^{7.5\mathrm{Th}/\left(\mathrm{Th}+237.3\right)}& \left(2\right)\end{array}$$\begin{array}{cc}X=0.2?w?S+0.4935& \left(3\right)\end{array}$ [0045] M: amount of moisture contained in the humidified gas that is supplied into the soaking zone (g/min) [0046] X: parameter regarding an influence of the surface area of the steel sheet [0047] Vh: flow rate of the humidified gas that is supplied into the soaking zone (Nm.sup.3/hr) [0048] Th: dew point of the humidified gas that is supplied into the soaking zone (? C.) [0049] w: width of the steel sheet (m) [0050] S: sheet passing speed (m/s)

[0051] 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 (H.sub.2O) 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.

[00005]$\begin{array}{cc}\mathrm{mole}\mathrm{fraction}\mathrm{of}{H}_{2}O(-)=6.11?{10}^{(7.5?\mathrm{Th}/\left(\mathrm{Th}+237.3\right)}/1013.5& \left(A\right)\end{array}$

[0052] Moreover, from this mole fraction and the flow rate Vh (Nm.sup.3/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.

[00006]$\begin{array}{cc}M\left(g/\min \right)=\mathrm{mole}\mathrm{fraction}\mathrm{of}{H}_{2}O?\mathrm{Vh}\left({\mathrm{Nm}}^3/\mathrm{hr}\right)/60\left(\min /\mathrm{hr}\right)?18\left(\mathrm{mass}\mathrm{of}1\mathrm{mol}\mathrm{of}{H}_{2}O:g/\mathrm{mol}\right)?1000\left(L/{\mathrm{Nm}}^3\right)/22.4\left(\mathrm{volume}\mathrm{of}1\mathrm{mol}\mathrm{of}\mathrm{gas}:L/\mathrm{mol}\right)& \left(B\right)\end{array}$

[0053] By substituting equation (A) into equation (B), equation (2) is obtained as follows.

[00007]$\begin{array}{cc}M\left(g/\min \right)=0.08074?\mathrm{Vh}?{10}^{7.5\mathrm{Th}/\left(\mathrm{Th}+237.3\right)}& \left(2\right)\end{array}$

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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)

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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

[0071] 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-US-00001 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

[0072] 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.

[0073] 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.

[0074] 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/m.sup.2 per 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.

[0075] 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/m.sup.2 per side by using a gas wiping method.

(Evaluation Method)

[0076] 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-US-00002 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.) (Nm.sup.3/hr) (Nm.sup.3/hr) (? C.) M X M/X (1) (Nm.sup.3/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.) (Nm.sup.3/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 ?48.1 2.3 0.92 GA 503 x (Pickup) 791 Fail Comparative Example 6 ?41.2 2.3 0.92 GA 503 ? 798 Pass Example 7 ?42.7 2.3 0.92 GA 564 x (Bare spot) 1022 Fail Comparative Example 8 ?33.5 2.3 0.92 GA 510 x (Bare spot) 1195 Fail Comparative Example 9 ?36.3 2.3 0.92 GA 512 ? 1203 Pass Example 10 ?45.1 2.3 0.92 GA 509 ? 1201 Pass Example 11 ?48.9 2.3 0.92 GA 510 ? (Slight Ash) 1195 Pass Example 12 ?51.2 2.3 0.92 GA 508 x (Ash Defect) 1205 Fail Comparative Example 13 ?46.3 2.0 0.8 GA 510 ? 1033 Pass Example 14 ?42.1 1.8 0.72 GA 513 ? 808 Pass Example 15 ?38.7 1.6 0.64 GA 515 ? (Slight Ash) 1203 Pass Example 16 ?44.9 2.3 0.92 GA 498 x (Bare spot) 950 Fail Comparative Example 17 ?42.1 2.3 0.92 GA 500 ? 992 Pass Example 18 ?42.1 2.3 0.92 GI N/A x (Bare spot) 995 Fail Comparative Example 19 ?41.9 2.3 0.92 GI N/A ? 998 Pass Example 20 ?42.6 2.3 0.92 GI N/A x (Pickup) 1001 Fail Comparative Example 21 ?43.5 2.3 0.92 GI N/A x (Bare spot) 995 Fail Comparative Example 22 ?42.4 2.3 0.92 GI N/A ? 1005 Pass Example

[0077] In addition, the tensile strengths of GI and GA manufactured under the various conditions were measured. In the case of steel code A, which is a high-strength steel, a tensile strength of 780 MPa or higher was determined as satisfactory. In the case of steel code B, which is a high-strength steel, a tensile strength of 1180 MPa or higher was determined as satisfactory. In the case of steel code C, which is a high-strength steel, a tensile strength of 980 MPa or higher was determined as satisfactory. The results are given in Table 2.

[0078] As indicated in Table 2, it was clarified that the coating appearance was good and the desired tensile strength was also achieved in the case where M/X was within the range indicated in expression (1). On the other hand, in the case where M/X was out of the range indicated in expression (1), the coating appearance was poor and, in some cases, the desired tensile strength was also not achieved. In the case of No. 15 where the ratio of the amount of gas discharged from the snout to the amount of gas supplied through the gas nozzles into the snout was 64%, that is, less than 70%, although the appearance was within the acceptable range, slight ash deposition was observed. In the case of No. 11 where the dew point in the snout was ?48.9? C., that is, close to the lower limit (?50? C.), although the appearance was within the acceptable range, slight ash deposition was observed.

INDUSTRIAL APPLICABILITY

[0079] According to the method for manufacturing a hot-dip galvanized steel sheet according to aspects of the present invention, it is possible to achieve high coating adhesiveness and good coating appearance, even in the case where a hot-dip galvanizing treatment is performed on a steel sheet containing Si in an amount of 0.2 mass % or more, and it is possible to inhibit a decrease in tensile strength as a result of decreasing the alloying temperature even in the case where an alloying treatment is performed after a hot-dip galvanizing treatment has been performed. In addition, even in the case where an ordinary steel sheet and a high-strength steel sheet are manufactured continuously, it is possible to avoid operation problems such as pickup or the like.

REFERENCE SIGNS LIST

[0080] P steel sheet [0081] 10 heating zone [0082] 12 soaking zone [0083] 14 first cooling zone (rapid cooling zone) [0084] 16 second cooling zone (gradual cooling zone) [0085] 18 snout [0086] 22 hot-dip galvanizing bath [0087] 23 alloying equipment [0088] 24 gas distributing device [0089] 26 humidifying device [0090] 28 circulation thermostatic water tank [0091] 30 humidified gas distributing device [0092] 32 humidified gas flow rate meter [0093] 33 humidified gas dew point meter [0094] 34 humidified gas pipework [0095] 36A, 36B, 36C humidified gas supply port [0096] 37A, 37B, 37C humidified gas supply port [0097] 38A, 38B, 38C humidified gas supply port [0098] 39A, 39B humidified gas supply port [0099] 42A, 42B, 42C dry gas supply port [0100] 44A, 44B, 44C dry gas supply port [0101] 46A, 46B, 46C dew point measuring position in soaking zone [0102] 60 supply gas nozzle in snout [0103] 61 exhaust port in upper part of snout [0104] 65 dew point measuring position in snout