KR100406414B1 - A method for manufacturing the galvannealed steel sheet having a high quality - Google Patents

Abstract

The present invention relates to the production of high quality galvannealed (GA) cold rolled steel sheet used in automotive steel plates, etc., using an endothermic reactant solution having excellent cooling ability in place of water alloyed heat-treated plated steel sheet 50 ~ 350 ℃ / sec It is an object of the present invention to provide a method for producing a steel sheet with excellent powder resistance and surface quality by cooling to a temperature of less than 300 ℃ at a cooling rate of.

Accordingly, the method for producing a GE steel sheet according to the present invention is a method of manufacturing a hot-dip galvanized steel sheet in which a cold-rolled steel sheet is plated by passing it through a plating bath, and the plated steel sheet is heated by alloying heat treatment and then cooled. The idea of the technical idea is to rapidly cool the plated steel sheet to a temperature of 300 ° C. or lower at a cooling rate of 50 to 350 ° C./sec by spraying a concentration endothermic solution onto the steel sheet.

Description

A method for manufacturing the galvannealed steel sheet having a high quality}

The present invention relates to the production of high quality galvannealed (GA) cold rolled steel sheet used in automotive steel sheet, and more particularly, in the manufacture of continuous hot dip galvanized cold rolled steel sheet spraying endothermic reaction solution to the alloy heat-treated steel sheet The present invention relates to a method for manufacturing a cold rolled steel sheet having excellent powder resistance and surface quality by super fast cooling.

Due to its excellent paintability and corrosion resistance after coating, the demand of GE steel sheet has been increasing rapidly in recent years as steel sheet for building materials, home appliances and automobiles. In the manufacture of these GE steel sheets, the steel sheets plated in the continuous hot dip galvanizing process are usually produced by alloy heat treatment. In detail, when the steel sheet is first galvanized, when the steel sheet is continuously heated to a temperature range of about 470 to 560 ° C., the iron component of the base iron and the zinc component of the plating layer are mutually diffused, ζ phase, δ1. Fe-Zn-based intermetallic compounds such as phases and Γ phases grow to form an alloyed hot dip galvanized layer. The alloyed hot-dip galvanized steel sheet is excellent in spot weldability, corrosion resistance and paint adhesion after coating, but in the case of the above-alloyed steel sheet, there is a problem in that a powdering phenomenon in which the plating layer falls into powder during processing is generated. Such powdering deteriorates with increasing iron concentration of ferroalloy, but improves adhesion and weldability.

Therefore, considering the above product characteristics, the optimal iron concentration range of the plated layer of GEE steel sheet is relatively narrow to about 8 ~ 12%, so it is common to have δ1 phase as MAIN PHASE. When reaching the upper roll of the cooling table, the technology to control the cooling below 300 ℃ is very important. That is, as shown in FIG. 1, when considering the state diagram according to the alloying temperature, the alloying starts at a temperature of 300 ° C., and the л phase is formed first. From 425 ° C., the ζ phase having an iron content of 8 wt% or less is formed, and in the 530 to 665 ° C. range. It can be seen that the Γ and Γ 1 phases having an iron content of 12 wt% or more are formed at δ having an iron content of 8 to 12 wt% and a temperature of 665 ° C. or higher. In addition, as shown in Figure 2, in view of the physical properties of each phase, the iron content of the most quality-preferred plating layer can be said to be 8 ~ 12wt%, the reason is that when the iron content ratio is less than 8wt% (Hv = 45), the plating layer is pushed when pressed, whereas when it exceeds 12wt%, the plating layer is hardened (Hv = 421), thereby facilitating formation of a capital gamma layer vulnerable to workability, and also plating layer outer layer. This is because there is a disadvantage to fall off in the form of powder.

On the other hand, in the manufacture of GE steel sheet, the galvanized layer was heated to a δ phase formation temperature to maintain the iron content at 8 to 12 wt%. However, when simple air cooling is performed after the alloying heat treatment, alloying continues to be effected by the latent heat of the material. Therefore, the development of high alloying results in the formation of the capacitive gamma tissue, which is vulnerable to machinability, and the powdering phenomenon that the plating layer falls off in the form of powder during processing, resulting in a fatal adverse effect on machinability. Cooling of the plated steel sheet is required at the following temperatures. In addition, the steel sheet proceeds to the upper top roll after the alloying heat treatment, but if the plated steel sheet is not cooled below 300 ° C, the adhesion of the zinc particles is incomplete, and the plating layer is also peeled off and adhered to the top roll surface (hereinafter referred to as zinc pickup). As a result, the pick-up mark is caused on the product to damage the surface appearance.

As described above, it is very important in the manufacture of GE steel sheet to control the cooling of the plated steel sheet to 300 ° C. or lower after reaching the upper rollet after the alloying heat treatment. Conventionally, the air cooling method was simply used for such cooling, but there was a disadvantage in that the steel sheet temperature in the upper roll could reach only about 400 ° C. In addition, this method was generally used only for the manufacture of thin plated cold rolled steel sheets with a thickness of 1.0 mm or less, or a plated amount of less than about 60 g / m ² on a one-sided basis. This can't be avoided.

Another method for solving the problem of the air cooling method is a method of manufacturing a GE steel sheet by a fog (FOG) cooling method of mixing water and air is disclosed in Korean Patent No. 88580, the apparatus conforming to this method 100 is shown in FIG. 3. The method is the air installed in the upper portion of the plating bath 110 after hot-plating the thick material (2.0 ~ 4.0mm) hot rolled steel sheet 120 in the zinc plating bath 110 containing 0.08 ~ 0.16wt% Al. The plating amount is adjusted through the knife 150. And continuously heated in the alloying heat treatment furnace 130 so that the temperature of the hot rolled steel sheet is 480 ~ 550 ℃, and maintained for a predetermined time in the temperature range in the air cooling zone 140 and the first fog cooling stand 160 and The second fog cooling stand 170 is a method of thermally treating the alloy so that the steel sheet temperature in the upper roll 180 is 300 ° C. or less by rapidly spraying air and water at a high pressure in parallel at a cooling rate of 20 to 40 ° C./sec. However, the method is superior to the conventional air cooling method, but the cooling effect is insufficient because the size of the spray particles are very small below 30 microns (average 20 microns), if the alloying heat treatment furnace 130 from the upper roll 180 If the distance to the short or the plate speed of the steel sheet is a high speed, it is difficult to secure the temperature below 300 ℃ as the steel sheet temperature in the upper roll 180 is about 350 ~ 400 ℃. In addition, the thickness of the Γ layer is about 0.5 microns, which is improved compared to 0.7 microns at the time of air cooling, but is still thick, and there is a problem that overalloying or alloying deviation in the plating layer may occur. Further, the method and then the cooling water use efficiency of the cooling water have not contributed to the cooling due to the lower 50% or less may result in poor alloy to fall into a lower heat treatment, the material after more than 3.0mm thickness to more than muljae coating weight 50g / m ² In order to achieve a target cooling temperature of 300 ° C. in the upper rolls during the production of the plating material, the length of the cooling stand must be sufficiently long by 50 m or more, and high speed production of 90 mpm or more is not possible.

Therefore, the present invention has been made to solve the above problems, by using a heat absorption agent solution having excellent cooling ability to heat the alloyed heat-treated plated steel sheet at a temperature of less than 300 ℃ at a cooling rate of 150 ~ 350 ℃ / sec It is an object of the present invention to provide a method for producing a GE steel sheet having excellent cooling resistance and surface quality.

1 is a state diagram showing zinc-iron alloying;

2 is a texture characteristic diagram according to the iron content of the steel sheet plated steel;

3 is a schematic configuration diagram of an apparatus 100 for manufacturing a conventional GE steel sheet.

4 is a schematic diagram showing an example of an apparatus 200 conforming to the method for producing a GE steel sheet of the present invention;

5 is a tax target diagram,

Figure 5 (a) is an enlarged detail of the solution spray bar,

5 (b) is an enlarged detail view of the super rapid cooling stage;

6 is a schematic diagram showing another apparatus 300 in accordance with the method for producing a GE steel sheet of the present invention.

* Explanation of simple symbols in the drawing *

201 ....... Plated steel sheet 210 ....... Plating tank

220 ....... 230 ° ...... alloyed by heat treatment of alloy

240 ....... solution supply means 250 ....... top roll

Solution suction section 270 Solution spray bar

In the present invention for achieving the above object, in the method for producing a hot-dip galvanized steel sheet to pass the cold-rolled steel sheet plated through a plating bath, and to heat the plated steel plate and then to heat the alloying heat treatment, the alloyed heat-treated plated steel sheet The present invention relates to a method for producing a high quality steel sheet, characterized in that the super-cooling of the plated steel sheet to a temperature of 300 ° C or less at a cooling rate of 50 ~ 350 ° C / sec by spraying the endothermic reactant solution.

Hereinafter, the present invention will be described.

In the manufacture of conventional GE steel sheet, the alloyed heat-treated plated steel sheet was cooled to 300 ° C. or lower by fog cooling, but insufficient cooling resulted in deterioration of plating characteristics such as powder resistance. In order to solve the above problems, the present inventors have found that there is an excellent cooling effect by spraying the plated steel sheet with an endothermic reagent solution that does not affect the plating layer instead of water as a refrigerant, and suggests the present invention. will be. That is, the present invention is characterized in that, in the manufacture of a GE steel sheet, by cooling the endothermic reagent solution to the alloyed plated steel sheet at a cooling rate of 150 ~ 350 ℃ / sec at a temperature of 300 ℃ or less.

When the endothermic reactant solution is sprayed on a high temperature steel sheet, the supercooling degree of the steel sheet is increased. In other words, the cooling driving force for cooling the steel sheet varies greatly depending on the refrigerant. The cooling driving force is greater because water cooling by water rather than air cooling by air and endothermic reagent solution, which is a refrigerant rather than water cooling by general water, are large. Subcooling will increase. Therefore, when a refrigerant having a high cooling rate is selected, rapid cooling to the target cooling temperature is possible. For example, considering the endothermic ammonium hydrogen phosphate (2NH ₄ H ₂ PO ₄ ) solution as the refrigerant, the ammonium phosphate first becomes phosphoric acid (2H ₃ PO ₄ ) by a de-2NH ₃ reaction, and the phosphoric acid is the temperature of the steel sheet. Is increased to around 200 ℃, 300 ℃ and 500 ℃, and it becomes Pyrophoric Acid, Meataphosphoric Acid and Phosphorous Pentoxide while evaporating water (H ₂ O), respectively, and it is possible to quench the hot steel sheet by increasing the supercooling of the steel sheet. .

Therefore, for the super fast cooling of the alloyed plated steel sheet, the endothermic reactant solution of the present invention is 70 ~ 100wt% NH ₄ H ₂ PO ₄ and Na ₂ HPO ₄ or Na ₄ P ₂ O ₇ as an additive It is preferably provided by an endothermic reactant including 30 wt% or less.

In addition, the endothermic reactant of the present invention is preferably made up to include 70 to 100wt% Na ₂ HPO ₄ and 30 wt% or less Na ₄ P ₂ O ₇ as an additive.

The endothermic agent of the present invention is also preferably using either CuSO ₄ or NaNO ₃ as an endothermic agent without additives.

As described above, in the endothermic reaction agent of the present invention, when the additive is included, it is added at a composition ratio of 30 wt% or less, for the following reason. That is, the additive in the endothermic reactant of the present invention is catalytically thermally activated to activate the main endothermic reactant (ie NH ₄ H ₂ PO ₄ , Na ₂ HPO ₄ ). This is because there is no benefit of reaction activation.

On the other hand, the endothermic reactant is mixed with water to form the endothermic reactant solution of the present invention, wherein the concentration of the endothermic reactant is preferably limited to 0.05 ~ 30.0%. This is because, if the concentration is less than 0.05%, it is impossible to secure a predetermined cooling rate, so that super fast cooling of the plated steel sheet is impossible, and thus excellent plating characteristics and surface quality cannot be secured. When the concentration exceeds 30.0%, when NH ₄ H ₂ PO ₄ is used as the basic endothermic reaction agent, pin holes are generated during coating with NH ₄ gas, and staining occurs on the surface of the steel sheet by phosphate precipitation, and the Na ₂ HPO ₄ This is because the corrosion resistance deteriorates in the case of chromium after treatment with Na residue. In addition, even when CuSO ₄ or NaNO ₃ is used as an endothermic agent without additives, it may cause deterioration of corrosion resistance and surface precipitation of Na steel sheet.

The endothermic reactant solution of the present invention prepared as described above may be cooled to a temperature of 300 ° C. or less by being sprayed on an alloyed plated steel sheet, and the cooling rate is preferably limited to 50 to 350 ° C./sec. If the cooling rate is less than 50 ℃ / sec, the predetermined cooling rate cannot be obtained, resulting in deterioration of the powder resistance and zinc pick-up of the coated steel sheet, and if it exceeds 350 ℃ / sec, the surface quality of the coated steel sheet This is because the shape of the plated steel sheet is warped due to good but large subcooling, thereby degrading the flatness of the plate. More preferably, the cooling rate is limited to 100 to 200 ° C / sec in view of simultaneous securing of surface quality and plate flatness of the plated steel sheet.

On the other hand, the plated steel sheet cooled as described above is more preferably cooled by the fog cooling method at a temperature of 200 ℃ or less. This is because the plated steel sheet can be secondarily cooled to alleviate plate warping of the plated steel sheet due to subcooling during the primary cooling, and to clean the endothermic reagent solution remaining on the plated steel sheet.

As described above, the present invention is capable of producing a high quality steel sheet having excellent plating properties by cooling the alloyed plated steel sheet using a heat absorbing reaction solution at a temperature of 300 ° C. or below 200 ° C. at a predetermined cooling rate.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

An example of the manufacturing apparatus 200 of the GE steel sheet which corresponds to manufacture of the GE steel sheet of this invention is shown in FIG. As shown in FIG. 4, the apparatus 200 is largely connected to the plating bath 210, the alloy heat treatment furnace 220, the supercooling stage 230, and the supercooling stage 230 to supply a cooling solution. It consists of a supply means 240. In addition, the supercooling cooler 230 is installed between the alloy heat treatment furnace 220 and the upper roll 250, and a blower 235 for transporting the pressurized air is connected to the supercooling cooler 230. . In addition, a solution suction section 260 and a solution spray bar 270 are provided in the super-cooling stage 230. On the other hand, reference numerals 285 and 295 denote separators and heat exchangers, respectively.

The solution spray bar 270 is composed of an inner tube (271B) and an outer tube (271A) as shown in Figure 5 (a), the solution is supplied to the inner tube and the air is supplied to the outer tube. The nozzle 273 and the ripple portion 275 are bolted to and coupled to the injection holes 277A and 277B, and the injection holes are formed to coincide with the holes. In addition, the nozzle 273 has a structure in which air and solution inlets 279A and 279B are embedded therein, so that air and solution are mixed and sprayed onto the plated steel sheet.

As shown in FIG. 5 (b), the pitch arrangement between the nozzles is adjusted so that the spraying solutions overlap with each other based on the center thereof to minimize the cooling deviation of the solution sprayed from the nozzle to the steel sheet. It is. In addition, auto valves 233A and 233B are provided for controlling the amount of injection and the spray rate of the super coolant according to the thickness of the steel sheet and the plate speed, and for removing foreign substances from the air supply pipe. In addition, since the solution that does not contribute to the cooling of the plated steel sheet may fall naturally and penetrate into the alloy heat treatment furnace located below the super-cooling stage 230, the alloying failure may be caused, so that the drip pen 235 is directly below the solution suction section. The solution collected therein is configured to be collected at the pit 239 through the drain pipe 237.

First, the cold-rolled steel sheet is plated by being deposited in the plating bath 210, the plated cold-rolled steel sheet 201 is transferred to the alloy heat treatment furnace 220 after the plating amount is controlled through the air knife 205.

In the alloying heat treatment furnace 220, the plated steel sheet 201 is heated to a temperature range of 470 to 560 ° C., and the heated plated steel sheet 201 is transferred to the supercooling stage 230.

The plated steel sheet 201 transferred to the supercooling stage 230 is cooled to 300 ° C. or less by the endothermic reagent solution injected from the solution spray bar 270. At this time, the cooling rate may vary depending on the thickness of the steel sheet and the plate speed, but in the present invention, it is preferable to control the log cooling rate in the range of 50 ~ 350 ℃ / sec.

On the other hand, the plated steel sheet cooled as described above is more preferably secondary cooling below 200 ° C by fog cooling method, according to the manufacturing apparatus 300 of the GE steel sheet is shown in FIG. As shown in FIG. 6, the apparatus 300 is largely connected to the plating bath 310, the alloy heat treatment furnace 320, the supercooling stage 330, and the supercooling stage 330 to supply a cooling solution. It is composed of the supply means 340 and basically has the same configuration as the device 200 shown in FIG. However, FIG. 6 is characterized in that the fog type cooling stand 350 is additionally installed at the upper portion of the super fast cooling stand 330.

Hereinafter, the present invention will be described in detail through examples.

(Example 1)

The cold-rolled steel sheet having a material thickness of about 1.0 mm and a width of about 1219 mm was hot-dipped galvanized so that the amount of single-sided coating was 60 g / m ² , and then alloyed for about 12 seconds at a temperature range of 470 to 560 ° C. Some of the alloy plated steel sheet specimens treated as described above were air-cooled, and some of them were cooled by a conventional fog cooling method to prepare a steel sheet. And the remaining plated steel sheet was cooled by spraying the endothermic reactant solution using the endothermic reagent refrigerant consisting of 70 ~ 100wt% NH ₄ H ₂ PO ₄ and less than 30wt% Na ₂ HPO ₄ or Na ₄ P ₂ O ₇ At this time, the GE steel sheet was manufactured while varying the concentration of the solution.

Meanwhile, in the case of air cooling, the air injection amount per nozzle was 0.1-0.5 Nm ³ / min, and in the case of fog cooling, the air injection amount per nozzle was 0.1-0.5 Nm ³ / min and the cooling water injection amount was 0.1-5.0 L / min. In the case of spraying the endothermic reactant solution, the spray amount of air per nozzle was 0.1-0.5 Nm ³ / min, and the endothermic reactant solution spray amount was 0.05-6.0 l / min.

In order to measure the powder resistance of the steel sheet manufactured as described above, a tape was attached to the manufactured steel sheet and subjected to a 180 ° bending test. Indicated.

In addition, in order to measure the workability, the maximum processing depth was measured and evaluated by a universal testing machine, and the zinc pick-up mark property was evaluated by measuring the degree of occurrence per unit area of the pickup mark trace on the surface of the steel sheet. And alloying uniformity is shown in Table 1 by evaluating by comparing the deviation of the degree of alloying.

As shown in Table 1, in the case of the conventional method, the cooling effect is not good, it can be seen that the overall measured plating properties are bad. In Comparative Examples 1 and 4, the concentration of the endothermic reagent, which is a refrigerant, was too low to obtain desired level of plating characteristics. In Comparative Examples 2 and 3, the measured plating characteristics were excellent, but It can be seen that defects such as stains occur on the surface of the steel sheet because the concentration of the endothermic agent is too high.

division Treatment condition Alloying degree Measurement result Remarks Refrigerant Solution concentration (%) Cooling rate (℃ / sec) Cooling method Powder resistance Machinability Zinc Pickup Alloy uniformity Inventive Example One NH ₄ H ₂ PO ₄ (70 ~ 100wt%) Na ₂ HPO ₄ (30wt% or less) 0.05-30 200-350 S 9.5-10.3 One One One One 2 NH ₄ H ₂ PO ₄ (70 ~ 100wt%) Na ₂ HP ₂ O ₇ (30wt% or less) 0.05-30 200-350 S 9.6-10.4 One One One One Comparative example One NH ₄ H ₂ PO ₄ (70 ~ 100wt%) Na ₂ HPO ₄ (30wt% or less) Less than 0.05 20-25 S 12.2-13.5 3 3 3 3 2 NH ₄ H ₂ PO ₄ (70 ~ 100wt%) Na ₂ HPO ₄ (30wt% or less) 30-40 250-400 S 10.6 ~ 11.9 One One One One Surface spots and pinholes 3 NH ₄ H ₂ PO ₄ (70 ~ 100wt%) Na ₂ HP ₂ O ₇ (30wt% or less) Less than 0.05 25-35 S 12.7-13.9 3 2 3 2 4 NH ₄ H ₂ PO ₄ (70 ~ 100wt%) Na ₂ HP ₂ O ₇ (30wt% or less) 30-40 230-400 S 9.6-10.7 One One One One Surface spots and pinholes Conventional example One air - 15-25 A 12.9-13.9 4 4 4 4 2 Air + water - 20-30 F 13.6-15.1 2 2 2 3

* Quality evaluation method: 1 (excellent) ↔ 5 (bad)

Cooling method: A means air cooling, F means fog cooling, S means super fast cooling.

On the other hand, it can be seen that the invention examples 1,2 and 3 in which the concentration of the endothermic reactant is in the range of 0.05 to 30% have both excellent plating properties and excellent surface quality.

(Example 2)

Injecting an endothermic reactant solution using an endothermic reactant refrigerant consisting of 70 ~ 100wt% Na ₂ HPO ₄ and 30wt% or less Na ₄ P ₂ O ₇ alloyed plated steel sheet specimens under the same conditions as in Example 1 After cooling, the GE steel sheet was manufactured by varying the concentration of the solution. At this time, the injection amount of the air and the solution in the nozzle was the same as in Example 1.

In order to measure the powder resistance and the like for the steel sheet manufactured as described above was the same experiment as in Example 1, the results are shown in Table 2 below.

Treatment condition Alloying degree Measurement result Remarks Refrigerant Solution concentration (%) Cooling rate (℃ / sec) Powder resistance Machinability Zinc Pickup Alloy uniformity Inventive Example One Na ₂ HPO ₄ (70 ~ 100wt%) Na ₄ P ₂ O ₇ (30wt% or less) 0.03-30 300-350 9.7-10.4 One One One One Comparative example One Na ₂ HPO ₄ (70 ~ 100wt%) Na ₄ P ₂ O ₇ (30wt% or less) Less than 0.05 27-33 10.5-11.6 2 2 3 2 2 Na ₂ HPO ₄ (70 ~ 100wt%) Na ₄ P ₂ O ₇ (30wt% or less) 30-40 320 ~ 420 9.9 to 10.8 One One One One Surface spots and pinholes

* Quality evaluation method: 1 (excellent) ↔ 5 (bad)

As shown in Table 1, Comparative Example 1 can be seen that the concentration of the endothermic reactant as a refrigerant is too low to achieve rapid cooling, the measured plating characteristics are poor, in the case of Comparative Example 2 Too high the measured plating properties, but it can be seen that stains or pinholes occur on the surface of the cooled steel sheet.

On the contrary, in the case of Example 1 of the present invention, it can be seen that not only has excellent plating characteristics but also surface quality.

(Example 3)

The alloyed plated steel sheet specimens treated under the same conditions as in Example 1 were cooled by spraying an endothermic reagent solution using CuSO ₄ or NaNO ₃ as an endothermic refrigerant, and the GE steel sheet was changed at different concentrations of the solution. Prepared. At this time, the injection amount of the air and the solution in the nozzle was the same as in Example 1.

In order to measure the powder resistance and the like for the steel sheet manufactured as described above was the same experiment as in Example 1, the results are shown in Table 3 below.

Treatment condition Alloying degree Measurement result Remarks Refrigerant Solution concentration (%) Cooling rate (℃ / sec) Powder resistance Machinability Zinc Pickup Alloy uniformity Inventive Example One CuSO ₄ 0.05-30 250-300 9.9-10.7 One One One One 2 NaNO ₃ 0.05-30 250-300 10.2 ~ 11.0 One One One One Comparative example One CuSO ₄ Less than 0.05 15-20 12.3 ~ 13.3 3 3 2 3 2 CuSO ₄ 30-40 250-400 10.6 ~ 11.3 One One One One Corrosion resistance deterioration 3 NaNO ₃ Less than 0.05 15-20 12.2-13.0 3 2 2 3 4 NaNO ₃ 30-40 250-400 10.0-11.0 One One One One Surface Na Precipitation

* Quality evaluation method: 1 (excellent) ↔ 5 (bad)

As can be seen from Table 3, Comparative Examples 1 and 3 can be seen that excellent plating properties can not be obtained because the concentration of the heat absorbing plate is a refrigerant is too low. In addition, Comparative Example 1 has good plating properties but poor corrosion resistance, and in Comparative Example 4 it can be seen that the surface quality is degraded by the precipitation of Na on the steel sheet surface.

On the contrary, in the case of Examples 1 and 2 of the present invention, it can be seen that not only the plating characteristics are excellent but also the surface quality.

As described above, the present invention, by spraying the endothermic reactant solution to the alloyed plated steel sheet can ensure not only the plating characteristics such as powder resistance, but also excellent surface quality, and also excellent productivity compared to the conventional method according to the super fast cooling It is to have a useful effect in the manufacture of a steel sheet.

Claims (6)

In the method of manufacturing a hot-dip galvanized steel sheet by passing the cold rolled steel sheet through a plating bath, and heating the plated steel sheet to heat the alloyed heat treatment and then cooling it, 50 to 350 by spraying the endothermic reactant solution on the alloyed heat-treated plated steel sheet. Method for producing a high quality steel sheet, characterized in that the super-cooling of the plated steel sheet to a temperature of less than 300 ℃ at a cooling rate of ℃ / sec The endothermic solution of claim 1, wherein the endothermic solution comprises 70-100 wt% of NH ₄ H ₂ PO ₄ and Na ₂ HPO ₄ or Na ₄ P ₂ O ₇ as an additive or less at 30 wt% Method characterized in that the prepared by the reactive agent The method according to claim 1, wherein the endothermic solution is prepared by an endothermic agent comprising 70 to 100 wt% of Na ₂ HPO ₄ and 30 wt% or less of Na ₄ P ₂ O ₇ as an additive. The method according to claim 1, wherein the endothermic solution is prepared by an endothermic agent of any one of CuSO ₄ or NaNO ₃ . The method according to any one of claims 2 to 4, wherein the endothermic reactant constitutes the endothermic reactant solution at a concentration of 0.05 to 30%. The method of claim 1, wherein the cold-treated plated steel sheet is subjected to secondary cooling to a temperature of 200 ° C or less by fog cooling.