GB2564365A

GB2564365A - Apparatus for forming nitrogen cloud in order to manufacture hot-dip metal coated steel sheet having excellent surface quality, and method for manufacturing

Info

Publication number: GB2564365A
Application number: GB1817297.3A
Authority: GB
Inventors: Moon Lim Byung; Sun Moon Byung; Keun Song Young; Seok Choi Ick
Original assignee: DONGKUK STEEL MILL CO Ltd
Current assignee: DONGKUK STEEL MILL CO Ltd
Priority date: 2016-05-17
Filing date: 2016-05-27
Publication date: 2019-01-09
Anticipated expiration: 2036-05-27
Also published as: NZ721156A; ES2725126R1; GB2564365B; KR101758717B1; WO2017200134A1; ES2725126A2; ES2725126B1; DE112016006868B4; GB201817297D0; DE112016006868T5

Abstract

Provided is an apparatus, which is provided between the surface of a plating bath for hot-dip plating and air knife equipment for controlling the thickness of plating metal attached to the surface of a strip, so as to form a nitrogen cloud (curtain) along the periphery of the strip rising from the plating bath. The apparatus of the present invention: has a semicylinder-shaped body spaced at a predetermined distance from the surface of the plating bath and of which the bottom surface is opened so as to face the surface of the plating bath; has a slit, through which the strip passes, formed on the upper surface of the body; has a lower gas exhaust part formed along the periphery of the lower end portion of the body, and spraying nitrogen gas toward the surface of the plating bath so as to block external air; includes inner gas exhaust parts arranged at both sides of the strip so as to face each other, while traversing the bottom surface of the body in the width direction of the strip, so as to spray the nitrogen gas toward the strip; and has a plurality of spray nozzles, formed at the inner side of the body, for spraying the nitrogen gas toward the strip.

Description

APPARATUS FOR FORMING NITROGEN CLOUD TO PRODUCE HOT-DIP COATED STEEL SHEET WITH EXCELENT SURFACE QUALITY AND METHOD FOR PRODUCING ZINC-ALUMINUM HOT-DIP COATED STEEL SHEET USING THE SAME

Technical Field

The present invention relates to an apparatus for forming a nitrogen cloud to produce a hot-dip coated steel sheet with excellent surface quality and a method for producing zinc-aluminum hot-dip coated steel sheet using the same, and more particularly, to an apparatus for effectively forming a non-oxidizing atmosphere to block an ambient air for a coated steel sheet in an apparatus for hot-dipping coating a metal such as zinc or aluminum on a steel sheet, and a method for producing a zinc-aluminum hot-dip coated steel sheet using the same.

Background A hot-dip metal coated steel sheet is widely used in an attempt to secure corrosion resistance of a base steel sheet. Typically , a zinc-coated steel sheet (GI) is widely used based on economic efficiency and resource abundance, and is currently a type of the most generally used coated-steel sheet. In addition, a great deal of research to improve the corrosion resistance of zinc-coated steel sheets has been made. In particular, an aluminum-coated steel sheet (so-called “Galvalume”) having an Al-Zn content of 55% was suggested in the late 1960s and exhibits superior corrosion resistance and a beautiful appearance at present.

Such an aluminum-coated steel sheet exhibits superior corrosion resistance and heat resistance, as compared to zinc-coated steel sheets and is thus widely applied to automobile mufflers, household appliances, heat-resistant materials and the like.

For example, lapanese Patent Publication No. 57-47861 discloses an aluminum steel sheet containing Ti in iron, lapanese Patent Publication No. 63-184043 discloses an aluminum-coated steel sheet containing C, Si, Cu, Ni and a small amount of Cr in iron, and lapanese Patent Publication No. 60-243258 discloses an aluminum-coated steel sheet containing 0.01 to 4.0% of manganese, 0.001 to 1.5% of titanium and 3.0 to 15.0% of silicon.

In addition, in order to inhibit growth of a Fe-Al alloy layer or rapid diffusion of aluminum metal into iron by reaction of aluminum with iron, 10% or less of Si is added to an aluminum coating bath. A coated steel sheet produced by this method exhibits relatively superior workability and heat resistance and is widely used for heat-resistant elements such as automobile mufflers, hot water suppliers, heaters, and electric rice cooker inner skins.

However, silicon added to inhibit formation of alloy layers may often cause damage to surface appearance of coated steel sheets and disadvantageously make the surface appearance unclear. In this regard, damage to surface appearance caused by silicon addition is known to be solvable to some extent through addition of a small amount of magnesium (U.S. Patent No. 3,055,771 to Sprowl).

In addition, in recent years, extended lifespan of components used for automobile exhaust gas systems has brought about development of steel sheets obtained by introducing Cr to an aluminum-coated steel sheet. For example, Japanese Patent Publication No. 63-18043 discloses a coated steel sheet containing 1.8 to 3.0% of chromium and Japanese Patent Publication No. 63-47456 discloses a steel sheet containing 2 to 3% of chromium.

Meanwhile, a Zn-Al alloy-coated steel sheet has a disadvantage in that a processed shear portion does not exert sufficient corrosion resistance. This phenomenon is caused by deterioration in corrosion resistance of a surface exposed to the shear portion which results from a decrease in sacrificial corrosion-resistant zinc preventing corrosion of iron through the zinc-aluminum alloy layer. In addition, a Zn-Al alloy-coated steel sheet has a disadvantage of deterioration in corrosion resistance after processing since a coating layer having no heterogeneous alloy phase is formed and an interface surface is vulnerable upon use after a bending or drawing processing and corrosion resistance is thus deteriorated after the processing.

In order to solve these phenomena, Korean Patent No. 0586437 discloses coating a Zn-Al-Mg-Si alloy-coated steel sheet material with superior corrosion resistance in a coating bath containing 45 to 70% by weight of Al, 3 to 10% by weight of Mg, 3 to 10% by weight of Si, and the balance of Zn and inevitable impurities, and Korean Patent No. 0928804 discloses a Zn-Al-Mg alloy-coated steel sheet with superior corrosion resistance and workability.

Surface quality of a hot-dip metal coated steel sheet may be relied on a technique of controlling a surface of a steel plate from a plating bath, as well as a composition of the coating bath. Components of a hot dip coating layer, for example, zinc (Zn), aluminum (Al), and magnesium (Mg) are bonded with oxygen in the air to form an oxide film which degrades surface quality of coated steel sheet. In particular, a coated steel sheet product obtained by adding magnesium (Mg) to a coating bath has a problem with outer appearance quality of a surface compared with a case of general GI or GL coating bath in many cases, and the problem is caused due to oxidation as characteristics of the Mg element. Mg is an element having a high oxidation, and oxidation reactivity of Mg is particularly increased in a coating bath having a high temperature, and due to this, an oxide or fine Mg oxidation material bonded with other elements is confined in a strip to degrade quality of the surface of the coated steel sheet.

In an effort to solve the problem, a related art method of forming a nonoxidation atmosphere for preventing oxidation in a section in which a strip deposited in a molten metal coming from a coating bath (port) is exposed and cooled in the air to perform coating, and an apparatus thereof have been well known.

Examples of the related arts include International Publication WO2011/102434 (DI), lapanese Laid-Open Patent Publication 55-141554 (D2), lapanese Laid-Open Patent Publication 2010-202951 (D3), lapanese Laid-Open Patent Publication 2002-348651 (D4), US4,444,814(D5), US4,502,408(D6) and the like.

However, existing methods and apparatuses for forming the non-oxidation atmosphere in the section in which the strip is deposited in a molten metal and subsequently exposed in the air have several problems.

That is, as illustrated in the drawings (see FIG. 2 of DI, FIG. 2 of D2, FIG. 2 of D3, and FIG. 3 of D4) of the aforementioned related arts, the related art apparatuses for forming the non-oxidation atmosphere are configured as a box type covering the entirety from a surface of the coating molten metal to an upper air knife device.

When the coated steel plate is manufactured, a temperature of each coating bath is about 460 °C (general zinc-aluminum coated steel sheet coating bath), about 600 °C (galvalume steel sheet coating bath), and about 650 °C (aluminum coated steel sheet coating bath), and here, due to the sealed box/vessel form, an internal heated air having a high temperature cannot be discharged properly in the air and increases an internal temperature of the box/vessel.

The related art method and structure causes many problems in an actual process as follows: - Deformation of a structure due to heat in the limited space. : Structures such as Air Knife Rip, Rip, Sink roll Arm, or the like are thermally deformed. - Malfunctioning of an electric device for driving air knife, such as various sensors or a motor attached to the air knife : In order to prevent this, a cooling device needs to be separately provided to prevent an increase in temperature of various electric devices. Further, lifespan of the various electric devices is also reduced. - It is not easy to control spangles after controlling coating and an attachment amount : Micronizing a spangle size on a surface of the coated steel sheet significantly affect product quality, and in order to micronize spangles, cooling should be quickly performed after an attachment amount is controlled, but in the case of the box type, cooling efficiency is degraded due to internal latent heat. In order to increase a cooling speed of the strip after coating, various other techniques such as spraying mist or spraying metal powder, in addition to a cooling technique of spraying air, are actually used, but the box type structure is a structure or method which rather hinders cooling after coating. - It is not easy to remove top dross generated on an upper portion of a coating bath. : The purpose of forming a non-oxidation atmosphere by spraying a nitrogen gas is to suppress generation of oxidation hot-dip and adsorption of generated oxide to the strip, but the box type has a structure making it difficult to remove top dross generated on the surface of the molten metal. : A considerable amount of oxide is actually generated on a surface of the strip even under the non-oxidation atmosphere, which is to be removed periodically using personnel or a robot device but the box type structure having a sealed form needs to have an opening and closing type door and the opening and closing type door needs to be repeatedly opened and closed for an operation of removing the oxide from the surface of the strip. In this case, the repeated opening and closing causes difficulty in maintaining a stable nitrogen atmosphere within the box. - Increase in cost of nitrogen gas : There are two types of methods for filling the interior of the box type structure with nitrogen, that is, a method for filling the interior of the box type structure using nitrogen sprayed for controlling a coating attachment amount from an air knife and a method of supplying nitrogen through a different supply line from the outside. : A nitrogen amount sprayed from the air knife of an actual continuous zinc coating line is generally about 3000 to 6000 m3/hr, which is insufficient for filling oxygen within the box type structure with nitrogen, and as mentioned above, in order to outwardly discharge heat due to a temperature of molten metal, a nitrogen gas should be additionally supplied from the outside. To this end, nitrogen of about 3000 to 4000 m3/hr needs to be additionally supplied in addition to the nitrogen supplied from the air knife, which is twice or more of a general nitrogen usage amount and occupies a considerable portion of manufacturing cost.

Summary

The present invention has been made in view of the above problems, and it is, therefore, a primary object of the invention to provide an apparatus for forming a nonoxidizing atmosphere on a steel sheet coming from a surface of a coating bath in producing a hot-dip coated steel sheet, and a method for producing a zinc-aluminum hot-dip coated steel sheet using the same.

According to an aspect of the present invention, there is provided an apparatus installed between a surface of a coating bath for hot-dip coating and an air knife facility for controlling a thickness of a coating metal attached to a surface of a strip to form a nitrogen cloud (curtain) around the strip coming from the coating bath.

Specifically, the apparatus of the present invention is spaced apart from the surface 10 of the coating bath by a predetermined distance, the apparatus includes: a body 3 having a semi-cylindrical shape, in which a lower surface thereof is opened toward the surface 10 of the coating bath, a slit 32 formed on an upper surface of the body 3 for allowing the strip 100 to pass therethrough, lower gas discharge parts 33 formed at a circumference of a lower end portion of the body 3 for jetting a nitrogen gas toward the surface 10 of the coating bath to block an ambient air, and inner gas discharge parts 31 disposed on both sides of the strip 100 in a facing manner across a lower surface of the body 3 in a width direction of the strip 100 for jetting a nitrogen gas toward the strip 100, wherein a plurality of injection nozzles 34 for jetting a nitrogen gas toward the strip 100 are formed within the body 3.

In the apparatus of the present invention, the steel sheet may be a zinc-aluminum-based alloy coated steel sheet, and the coating bath may include 33 to 55 wt% of zinc, 0.5 to 3 wt% of silicon, 0.005 to 1.0 wt% of chromium, 0.01 to 3.0 wt% of magnesium, 0.001 to 0.1 wt% of titanium, the balance consisting of aluminum and inevitably contained impurities. According to this composition, the produced coated steel sheet has excellent surface appearance and corrosion resistance.

Further, in the apparatus of the present invention, the coating bath may further include 1 to 10 wt% of calcium with respect to an overall weight of magnesium.

According to the apparatus of the present invention, by forming a nitrogen cloud (air curtain) around the steel sheet coming from the coating bath, the steel sheet (strip) is prevented from being in contact with an ambient air before reaching an air knife facility. In particular, in the apparatus of the present invention, a nitrogen gas is jetted into a body to fill the entire interior of the body, an opening of a lower surface of the body is blocked from an ambient air by the nitrogen gas through lower gas discharge parts formed on the lower surface of the body, and the nitrogen gas is jetted toward a strip coming from a surface of the coating bath through inner gas discharge parts from both sides of the strip, so that a coated metal attached to the strip is blocked from the ambient air from the moment the strip starts to come from the surface of the coating bath.

Meanwhile, the apparatus of the present invention may further include an upper gas discharge part for jetting a nitrogen gas toward the strip passing through the slit, on both sides of the slit.

Accordingly, the ambient air including oxygen is prevented from being introduced into the body through the slit.

Here, the inner gas discharge part includes a circular pipe body on which a plurality of nozzles for discharging a nitrogen gas are formed to be spaced apart from one another at a predetermined interval in a length direction, a housing in which a groove is formed in a length direction to accommodate one side of the pipe body, and fixing blocks each having a recess corresponding to the pipe body to allow the pipe body to be mounted thereon at both end portions of the pipe body, wherein one or more passages in which a nitrogen gas moves are formed in the pipe body and the housing, respectively.

As described above, according to the present invention, the steel sheet coming from the coating bath is prevented from being in contact with an ambient air before the steel sheet passes through the air knife, enhancing the quality of the hot-dip metal coated steel sheet.

In addition, heat transmitted from the coating bath and the strip into the apparatus can be easily exhausted to the outside of the apparatus.

Brief Description of the Drawings

The above and other objects and features of the present invention will become apparent from the following description of embodiments, given in conjunction with the accompanying drawings, in which:

Fig. 1 is a side view according to an embodiment of the present invention.

Fig. 2 is a partially enlarged view according to an embodiment of the present invention.

Fig. 3 is a perspective view of a body part according to an embodiment of the present invention.

Fig. 4 is a partially enlarged view of a body part according to another embodiment of the present invention.

Fig. 5 is a perspective view of a body part according to another embodiment of the present invention.

Fig. 6 is a perspective view of an inner gas discharge part according to another embodiment of the present invention.

Fig. 7 is an exploded perspective view of an inner gas discharge part according to another embodiment of the present invention.

Fig. 8 is a cross-sectional view taken along line A-A’ of Fig. 6.

Fig. 9 is a bottom view of a body part according to another embodiment of the present invention.

Detailed Description

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

Referring to Figs. 1 to 3, an apparatus according to an embodiment of the present invention is spaced apart from a surface 10 of a coating bath at a predetermined distance and positioned below an air knife facility 2. The apparatus of the present invention has a body part 3. The body part 3 has a semi-cylindrical dome shape, and a slit 32 for allowing a coated strip 100 to pass therethrough is formed to extend in a length direction of the body part 3 on an upper surface thereof. The slit 32 is formed to be larger than a thickness and a width of the strip 100. The body part 3 may be formed of iron plate. A lower surface of the body part 3 is open toward the surface 10 of the coating bath. Further, a lower gas discharge part 33 for jetting a nitrogen gas toward the surface 10 of the coating bath is formed on a rectangular edge of the lower surface of the body part 3. The lower gas discharge part 33, which is similar to a device called an air curtain, downwardly jets a compressed nitrogen gas through a slit 33a (see Fig. 9) to form an air curtain to block an interior and an exterior of the lower gas discharge part 33 from an ambient air.

The apparatus of the present invention includes a plurality of nozzles 34 for jetting a nitrogen gas into the body 3. The nozzles 34 jet a nitrogen gas toward the center of the body 3 to form a nitrogen cloud S within the body 3.

Further, the apparatus of the present invention includes inner gas discharge parts 31 disposed on both sides of the strip 100 in a facing manner across a lower surface of the body 3 in a width direction of the strip 100 and jetting a nitrogen gas toward the strip 100. The inner gas discharge part 31 starts to jet a nitrogen gas from the moment the strip 100 comes from the surface 10 of the coating bath, to fundamentally prevent oxygen from affecting a surface of the strip 100. In addition, an effect of outwardly discharging heat emitted from the strip 100 can be obtained. That is, since the apparatus of the present invention is installed to be spaced apart from the surface 10 of the coating bath and a lower surface thereof is open, heat emitted from the strip 100 and a molten metal 1 may be easily discharged outwardly by the nitrogen gas jetted from the nozzle 34 and the inner gas discharge part 31.

The pressure of the nitrogen gas jetted from the nozzle 34, the lower gas discharge part 33, and the inner gas discharge part 31 is variably adjusted depending on a movement speed, a coating attachment amount of the strip 100, or the like, which is obvious to a person skilled in the art.

In general, jetting nitrogen to a steel sheet coming from a coating bath to form a non-oxidizing atmosphere is an already known technique (DI to D6) as discussed above. However, as mentioned in the related art, when a closed space (like a box form) including a facility for adjusting an amount of coating of a steel sheet is formed and nitrogen is then injected thereto, heat from a molten metal is not discharged outwardly, damaging and malfunctioning machines and various sensors therein. In addition, since the closed space needs to be opened to perform an operation of periodically removing oxide produced on a surface of the coating bath, the operation of the apparatus should be stopped or an introduction of ambient air degrades quality.

The apparatus of the present invention is installed below the air knife facility 2 for adjusting an amount of coating metal attached to the strip 100. Further, in a state where the dome-shaped body 3 covers and an ambient air is blocked by nitrogen jetted from the lower gas discharge part 33 formed at the edge of the lower end portion of the body 3, the inner gas discharge part 31 directly jets nitrogen toward the steel sheet 100 and the remaining space is filled with nitrogen discharged from the injection nozzle 34, thus forming the space S in a non-oxidizing atmosphere formed by nitrogen as illustrated in Fig. 1.

In the state where the air curtain S is formed by the nitrogen gas, heat may be easily discharged outwardly, while preventing the surface of the strip 100 from being in contact with oxygen, so that various components (not shown) are not affected, and thus, not damaged or malfunctioned.

In addition, since the apparatus of the present invention is spaced apart from the surface of the coating bath at a predetermined distance, it is easy to insert equipment for removing oxide from the surface of the molten metal by personnel or a device, and even during the removing operation, nitrogen may be continuously j etted, having an advantage that the apparatus is not required to be stopped.

Further, even with the closed space, conventionally, it is impossible to completely prevent attachment of a molten metal surface oxide formed on the surface of the molten metal to the steel sheet or formation of fine oxide coating due to magnesium when the steel sheet is released from the molten metal. In contrast, in the present invention, nitrogen jetted from the inner gas discharging part 31 is jetted in a downward direction toward the steel sheet 100, generating a force to act to push the surface oxide of the molten metal outwardly from the steel sheet 100, so that introduction of the molten metal surface oxide to the steel sheet 100 or generation of a fine oxide coating on the steel sheet 100 can be effectively suppressed.

In Figs. 1, 2, and 4, unexplained reference numeral 50 denotes meniscus formed by the molten metal attached to both surfaces of the steel sheet 100 and coming from the surface of the coating bath. An amount of the molten metal included in the meniscus corresponds to a thickness of the coated metal attached to the steel sheet 100, which is adjusted by a movement speed of the steel sheet, pressure of the nitrogen gas jetted from the air knife 2, or the like. The inner gas discharge part 31 serves to physically remove dross generated on a surface of the meniscus such that it is not attached to the coated steel sheet 100, or suppress an oxidation atmosphere in which dross may be generated.

Figs. 4 and 5 illustrate an apparatus according to another embodiment of the present invention. Referring to Figs. 4 and 5, a pair of upper gas discharge parts 35 for jetting a nitrogen gas toward the slit 32 formed on an upper surface of the body 3 of the apparatus of the present invention from both sides of the slit 32 are additionally formed.

The air knife facility 2 positioned above the apparatus of the present invention adjusts an amount of coating of the steel sheet 100, while jetting nitrogen with relatively high pressure, and here, the nitrogen gas jetted with high pressure may be mixed with ambient air to cause formation of an eddy current. The eddy current is likely to enter the body 3 through the slit 32. In order to prevent this, the upper gas discharge parts 35 are formed above the slit 32, through which the nitrogen gas is jetted to prevent oxygen mixed with the eddy current from flowing into the body 3 through the slit 32.

Figs. 6 to 8 illustrate a configuration of the inner gas discharge part 31. Referring to Figs. 6 to 8, the inner gas discharge part 31 includes a circular pipe body 3 la on which a plurality of nozzles 311 for discharging a nitrogen gas are formed to be spaced apart from one another at a predetermined interval in a length direction, a housing 3 lb in which a groove 314 is formed in a length direction to accommodate one side of the pipe body 31a, and fixing blocks 31c each having a recess 330 corresponding to the pipe body 31a to allow the pipe body 31a to be mounted thereon at both end portions of the pipe body 31a.

Here, one or more nitrogen supply holes 315 and 312 for providing a passage for supplying a nitrogen gas are formed in the pipe body 31a and the housing 31b, respectively.

The housing 31b, the pipe body 31a, and the fixed blocks 31c are fixed by a fixing bolt 318 passing through screw holes 316 and 317.

In addition, caps 313 having an outer diameter greater than that of the pipe body 3 la are formed at both end portions of the pipe body 31a. If necessary, in order to adjust a jetting angle of the nozzle 311, an operator may unwind the fixing bolt 318 and grip the cap 313 to rotate the pipe body 3la at a predetermined angle.

In Fig. 8, it is illustrated that a plurality of nozzles 311 are formed on the pipe body 31a, but a nitrogen gas may also be discharged through a slit extendedly cut out like the air curtain.

Fig. 9 illustrates a state where the slit type inner gas discharge part 31 is formed on the body 3. Fig. 9 is a bottom view of the body 3, in which the inner gas discharge part 31 having slits 3 lf for jetting a nitrogen gas are fixed to the body 3 by a support bridge 39.

In the above description and drawings, a component for supplying a nitrogen gas to the gas discharge parts 31, 33, and 35 and the nozzles 34 and 311 of the apparatus of the present disclosure from the outside is not specifically illustrated, but it is a matter of design obvious to a person skilled in the art.

Advantages of the device for forming a nitrogen cloud of the present invention described above will be described compared with the related art apparatuses DI to D6 . 1) Since the device of the present invention forms the nitrogen cloud only in a partial space at a lower end of the air knife, the structure is not deformed due to latent heat generated by the related art box type and there is no factor hindering micronization of spangles due to a degradation of a cooling rate after coating. : The method and device of the present invention relate to a method (or structure) of forming nitrogen DAM by forming a nitrogen curtain (nitrogen cloud) using a nozzle in a section of a lower end portion of the air knife(2) in which oxidation may first occur or in which a dross may adsorbed to the strip(lOO) on the surface of the coating molten metal, rather than having such a box type as that in the cited inventions in which the entirety of an air knife for controlling a coating attachment amount from the surface of the coating molten metal is covered. : Since the nitrogen cloud(S) is formed using the nitrogen nozzle at upper and lower portions of the section of the lower end portion of the air knife and the interior thereof is maintained under a nitrogen atmosphere, rather than the method of filling the closed space with nitrogen, a gas may be smoothly flow from the interior of the apparatus outwardly, and thus, latent heat is not maintained. : As can be seen in the drawings, since the nitrogen cloud(S) of the present invention is formed only in the partial space of the lower end of the air knife, it does not affect any structure (component) other than the surface of the coated molten metal or the strip on which coating is performed. Thus, a possibility of deforming the structure due to heat generated by the related art box type or generating an error due to heat of an electric device for driving the air knife such as various sensors or a motor is low. 2) The top dross may be easily removed : Since the manufacturing apparatus of the present invention is spaced apart from the surface of the coating molten metal by a predetermined distance, rather than an atmosphere of being directly in contact with the surface of the coating molten metal or deposited, dross may be removed by personnel or a robot through the space without being interfered. Also, since the cloud in the form of a nitrogen curtain sprayed through the nozzle is constantly maintained even when the apparatus or a tool is inserted into the separated space to remove the top dross, it may also be effective in maintaining the nitrogen atmosphere. 3) Effect of preventing adsorption of top dross of the upper portion of the coated molten metal to strip : Even though the port portion of the coated molten metal is fdled with nitrogen in manufacturing an Mg-added alloy coated steel sheet, it is not possible to actually perfectly prevent a fine oxide film by the partial top dross and Mg having high oxidation. However, since the amount can be considerably reduced, the manufacturing method of spraying the nitrogen gas is applied. : In the present invention, in order to suppress a fine oxide film at the upper portion of the coated molten metal and the top dross, the nitrogen atmosphere is formed and adsorption of the top dross and fine oxide film to the strip may also be physically prevented. : In the present invention, when a nitrogen is sprayed downwardly from the lower nitrogen discharge portion 33, a nitrogen cloud is formed in a side direction of the coating port (see FIG. 1). This generates an effect of physically preventing movement of the top dross and fine oxide film floating in an upper portion of the coating bath to the vicinity of the strip to thus prevent adsorption thereof to the strip. : Thus, the present invention obtaining the effect of preventing adsorption to the strip after coating simultaneously when the nitrogen atmosphere is formed is different from the related art device for suppressing an oxide by forming only the nitrogen atmosphere. 4) Reduction in cost for nitrogen gas : Since the device of the present invention forms the nitrogen atmosphere only at the required partial space at the lower end of the air knife, the nitrogen cloud may be maintained only with a small amount of nitrogen coming from the lower nitrogen discharge portion 33, and is more effective compared with the prior box type for supplying nitrogen while maintaining a pressure higher than normal pressure. : Thus, the present invention is able to reduce a nitrogen usage amount, compared with the related art method for filling the interior of the box type with nitrogen. Also, the manufacturing method of the present invention is a manufacturing method which can exhibit a considerably effective oxide generation suppressing and adsorption preventing effect, compared with the related art method, even with the same amount of nitrogen.

Meanwhile, the present invention provides a method for producing a zinc-aluminum-based alloy coated steel sheet having excellent workability and corrosion resistance by coating a zinc-aluminum-based alloy coated steel sheet in the coating bath including 33 to 55 wt% of zinc, 0.5 to 3 wt% of silicon, 0.005 to 1.0 wt% of chromium, 0.01 to 3.0 wt% of magnesium, 0.001 to 0.1 wt% of titanium, and the balance consisting of aluminum and inevitably contained impurities by using the apparatus of the present invention.

In the present invention, by forming a nitrogen dam at a portion spaced apart from the surface of the coating bath at a predetermined distance and at a lower end portion of the air knife facility, a fine oxide coating formed on the surface of the coating bath as oxygen comes into contact therewith is prevented from being adsorbed to the zinc-aluminum-based alloy coated steel sheet coated in the coating bath having the coating composition.

In the method of the present invention, the coating bath includes 35 to 55 wt% of zinc. Zinc has sacrificial protection compared with a base steel sheet, serving to suppress corrosion. Zinc of 35 wt% or greater is required, because, if zinc is less than 35 wt%, flowability of the coating bath is degraded and corrosion resistance is lowered, and if zinc is more than 35 wt%, a temperature of the coating bath is required to be increased, increasing Top dross and causing an obstacle in operation, which leads to a degradation of workability. In addition, if zinc is more than 55 wt%, a ratio of aluminum in the coated steel sheet is increased to increase cost, degrading economic efficiency.

The coating bath of the present invention contains 0.5 to 3.0% by weight of silicon. Silicon functions to inhibit growth of an alloy layer, is effective in improving flowability of the coating bath and imparting gloss thereto and should be added in an amount of 0.5% by weight or more. A key role of silicon in the coating layer is to control formation of an alloy layer of a base steel sheet with aluminum. When an amount of added silicone is 0.5% by weight or less, the function of silicon is limited and workability is considerably deteriorated. On the other hand, when silicone is added in an amount exceeding 3% by weight, Mg2Si serving as a factor which contributes to improvement of corrosion resistance of the coating layer is excessively produced and grown on the surface of the coating layer, the surface thereof is rough, thus causing surface discoloration at an early stage and deteriorating post-treatment coating properties. Accordingly, the amount of added silicon is preferably 0.5 to 3% by weight.

Chromium added to the coating bath functions to form a dense and passive oxide film on the surface of the coating layer, improve corrosion resistance of the aluminum-coated steel sheet and make grains of the coating layer fine since chromium is uniformly distributed in the coating bath.

Chromium functions to form a predetermined shape of an Al-Zn-Si-Cr mixed phase band integrated in the coating layer (FIG. 2). Chromium present in the coating layer reacts with aluminum to form an AlCr2 phase and functions to improve workability and corrosion resistance at a fracture plane after processing. The chromium enables the content of silicon to be controlled to 3% by weight or less and prevents thus excessive precipitation of silicon in the form of a needle in the coating layer.

The content of chromium providing these effects is known to be 0.1 or more (U.S. Patent No. 3,055,771 to Sprowl). However, in the method of the present invention, the content of chromium is 0.005 to 1.0% by weight. When the content of chromium is 0.005% by weight or less, chromium is not readily homogeneously distributed in the coating bath, and when the content thereof is 1.0% by weight or more, an increase in temperature of the coating bath is required due to increase in chromium content, dross increases and appearance is disadvantageously damaged due to the dross attached to the surface of the coated steel sheet.

Accordingly, the content of chromium is preferably 0.005 to 1.0% by weight.

The coating bath of the present invention also contains 0.01 to 3.0% by weight of magnesium.

Magnesium added together with chromium is bonded to oxygen present in the air contacting the coating layer to form a passive film and thus prevents oxygen from diffusing into the alloy layer, and prevents additional corrosion and thus improves corrosion resistance. Presence of a Mg2Si phase (see FIGs. 1 and 2) formed through reaction between magnesium and silicon, and a MgZn2 phase produced through reaction between magnesium and zinc in the coating layer functions to reduce corrosion speed through sacrificial corrosion resistance of zinc during corrosion and formation of a local battery. In addition, magnesium reacts with aluminum, blocks permeation of oxygen and thus considerably improves corrosion resistance of a shear surface.

When an amount of added magnesium is 0.01% by weight or less, dispersibility, and improvement effect of corrosion resistance associated with oxidation properties are low, and when the amount exceeds 3.0% by weight, the coating bath is saturated, a melting point increases, workability is deteriorated, surface qualities are deteriorated due to continuous generation of dross on the surface, production costs increase and problems associated with production processes become serious.

The amount of added magnesium is preferably 0.01 to 3.0% by weight.

The coating bath of the present invention also contains calcium in an amount of 1 to 10% by weight with respect to the weight of magnesium. Calcium added together with magnesium and chromium inhibits formation of magnesium oxide on the interface of a coated melt metal and thus prevents deterioration in appearance qualities by a fine magnesium oxide film adhered to the surface of the coated steel sheet.

Addition of Ca, Be, Al or the like to a molten Mg bath is known to considerably inhibit oxidation and combustion of the molten Mg bath even at a high temperature. In accordance with a mechanism which inhibits oxidation of the molten Mg bath through addition of calcium, a combustion temperature of the molten Mg bath increases to 200°C or higher due to addition of calcium. This increase in combustion temperature of a Mg alloy generally causes an oxide layer generally formed on the surface to be changed from a porous oxide layer to a dense oxide layer and efficiently blocks permeation of oxygen.

When the content of calcium is 1% by weight or less with respect to the weight of magnesium, dispersibility is deteriorated and inhibition effect of the MgO oxide film are low, and when the content thereof exceeds 10% by weight with respect to the weight of magnesium, deterioration in coating layer workability caused by formation of a metallic compound of aluminum and calcium may occur. Accordingly, the amount of added calcium is preferably 1 to 10% by weight with respect to the weight of magnesium.

The present invention provides application of a nitrogen spray nozzle-attached dam which enables nitrogen purging and prevents an oxide film from being adhered to strips, to a lower surface of an air knife of the coating bath. Formation of an oxide film is inhibited by purging the lower surface of the air knife which ascends to the interface of the coating bath after a strip is dipped in the coating bath, with a nitrogen atmosphere and nitrogen wiping is performed on the lower surface of the nitrogen dam through a nitrogen curtain nozzle in order to prevent introduction of a fine oxide film formed after contacting the air in an outside of the surface of the molten coating bath into the dam and adhesion thereof to the strip.

Furthermore, the coating bath of the present invention also contains 0.001 to 0.1% by weight of titanium in order to reduce size of spangles which constitute appearance of the coating layer and form a flower shape of the coating layer. When an amount of added titanium is 0.001% by weight or less, dispersibility on the steel sheet is deteriorated, and when the amount thereof is 0.1% by weight or more, dissolution in the coating bath is not easy and titanium does not affect improvement in effects.

The present invention is based on size reduction of spangles which realizes by increasing likelihood of nucleation on a conventional Galvalume coated steel sheet through addition of suitable amounts of chromium, magnesium, calcium and titanium to a coating bath containing aluminum, zinc and silicon.

That is, added components are dispersed in the coating layer to form various nuclei such as Mg2Si, MgZn2 and AlCr2 phases after the steel sheet is coated, and mutual interference between grain boundaries controls growth of the grains.

Accordingly, beautiful surface appearance is secured, corrosion between the grain boundaries is inhibited and corrosion resistance is enhanced. In addition, growth of an alloy layer of aluminum and iron is inhibited and a coating fdm with superior workability is thus formed.

Meanwhile, it is preferable to set a temperature of the base steel sheet bathing in the molten coating bath to 570 to 650°C and a temperature of the molten coating bath to 550 to 650°C.

When the bathing temperature of the base steel sheet is lower than 550°C, flowability of the coating bath is deteriorated, appearance of coating is bad and coating adhesiveness is deteriorated. When the bathing temperature thereof is 650°C or higher, rapid thermal diffusion of the base steel sheet causes abnormal growth of the alloy layer and deterioration in workability and formation of excessive Fe oxide layer in the molten coating bath. A coating amount is preferably 20 to 100 g/m2, on a basis of one side. When the coating amount is 20 g/m2 or less, air pressure of the air knife equipment controlling the coating amount excessively increases, variation in coating amount occurs, and damage to appearance of the film and adhesion of oxide dross thereto occur due to rapid increase in surface oxide in the molten coating bath.

In addition, when the coating amount is 100 g/m2 or more, the alloy layer is excessive formed and workability is considerably deteriorated.

Hereinafter, the present invention will be described in more detail through comparison between Examples and comparative Examples. These examples are provided only to illustrate the present invention in more detail and should not be construed as limiting the scope and spirit of the present invention.

A cold-rolled steel sheet with a thickness of 0.8 mm, a width of 120 mm and a length of 250 mm was coated using a melt-coating simulator according to the apparatus of claim (. As shown in Table 1, a zinc-aluminum-based alloy-coated steel sheet was produced by changing a composition of the coating bath. I

The amount of adhered coating was controlled using an air knife(2) and the amount of coating of the produced zinc-aluminum-based alloy-coated steel sheet evaluated based on one side is shown in Table 1.

Evaluation items were corrosion resistance and workability. Corrosion resistance was compared with an initial rust generation time (5%) under a 35°C NaCl salt spray test atmosphere in accordance with KSD 9504 and evaluated. Workability was compared and evaluated by observing a width (fracture width) of cracks generated after 180° OT bending test in accordance with a KSD 0006 test method using a 30 to 50X stereomicroscope and measuring the width of the fracture surface. Observation of alloy phase was carried out using an X-ray diffraction.

Detailed test results obtained by the test method are given below. 1. Workability: observed according to crack width level. ©: fracture width of 10 to 20 pm Δ: fracture width of 20 to 30 pm X: fracture width of 40 to 50 pm 2. Dross level: an amount of dross generated in an upper part of coating bath after molten coating specimens according to coating composition. ©: generation of 5% or less of dross with respect to coating bath Δ: generation of 10 to 20% less of dross with respect to coating bath X: generation of 20% or more of dross with respect to coating bath 3. Surface appearance: visibility (clearance) and formation level of spangles of surface appearance of coating layer observed by the naked eye ©: Clear formation of spangles with high gloss Δ: Non-clear formation of spangles X: Little formation of spangles with bad appearance 4. Corrosion resistance of shear surface: ratio of rust generated after salt spray test for 1,000 hours ©: rust ratio of 5% or less Δ: rust ratio of 10 to 20% X: rust ratio of 30% or more 5. Corrosion resistance of flat portion: a ratio of rust generated after salt spray test for 2,500 hours. ©: rust ratio of 5% or less Δ: rust ratio of 20 to 30% X: rust ratio of 30% or more (TABLE 1)

As illustrated in Table 1, it can be seen that, when coating is performed using the apparatus of the present invention, a dross generation amount is low and workability and corrosion resistance of the steel sheet coated according to the example of the present invention are excellent.

That is, Examples of the present invention exhibited a crack (fracture surface) generated after 180° OT bending, of about 10 to about 20 pm and thus superior corrosion resistance, as compared to comparative Examples. Examples of the present invention exhibited generation of a total cross-section rust after 3,000 hours or longer in an amount of adhered coating of 50 g/m2 on a basis of one side and generation of rust on the cross-section after 1,000 hours or longer. These results demonstrate that Examples of the present invention exhibit superior corrosion resistance, as compared to conventional compositions.

As a result of observation by the naked eye, Examples exhibited superior surface appearance, as compared to conventional Examples. This is caused by reduction of spangle size.

Claims

What is claimed is: Claim 1. An apparatus installed between a surface 10 of a coating bath for hot-dip coating and an air knife facility 2 for controlling a thickness of a coating metal attached to a surface of a strip 100 to form a nitrogen cloud (curtain) around the strip 100 coming from the coating bath 1, wherein the apparatus is spaced apart from the surface 10 of the coating bath by a predetermined distance, and the apparatus comprises: a body 3 having a semi-cylindrical shape, in which a lower surface thereof is opened toward the surface 10 of the coating bath, a slit 32 formed on an upper surface of the body 3 for allowing the strip 100 to pass therethrough, lower gas discharge parts 33 formed at a circumference of a lower end portion of the body 3 for jetting a nitrogen gas toward the surface 10 of the coating bath to block an ambient air, and inner gas discharge parts 31 disposed on both sides of the strip 100 in a facing manner across a lower surface of the body 3 in a width direction of the strip 100 for jetting a nitrogen gas toward the strip 100, wherein a plurality of injection nozzles 34 for jetting a nitrogen gas toward the strip 100 are formed within the body 3. Claim
2. The apparatus of claim 1, wherein upper gas discharge parts 35 for jetting a nitrogen gas toward the strip 100 coming from the slit 32 are additionally formed on both sides of the slit 32. Claim
3. The apparatus of claim 1, wherein the inner gas discharge part 31 comprises: a circular pipe body 31a on which a plurality of nozzles 311 for discharging a nitrogen gas are formed to be spaced apart from one another at a predetermined interval in a length direction; a housing 31b in which a groove 314 is formed in a length direction to accommodate one side of the pipe body 31a; and fixing blocks 31c each having a recess 330 corresponding to the pipe body 31a to allow the pipe body 3 la to be mounted thereon at both end portions of the pipe body 31a, wherein one or more nitrogen supply holes 315 and 312 for providing passages in which a nitrogen gas moves are formed in the pipe body 31a and the housing 31b, respectively. Claim
4. The apparatus of claim 2, wherein the inner gas discharge part 31 comprises: a circular pipe body 31a on which a plurality of nozzles 311 for discharging a nitrogen gas are formed to be spaced apart from one another at a predetermined interval in a length direction; a housing 31b in which a groove 314 is formed in a length direction to accommodate one side of the pipe body 31a; and fixing blocks 31c each having a recess 330 corresponding to the pipe body 31a to allow the pipe body 3 la to be mounted thereon at both end portions of the pipe body 31a, wherein one or more nitrogen supply holes 315 and 312 for providing passages in which a nitrogen gas moves are formed in the pipe body 31a and the housing 31b, respectively. Claim
5. A method for producing a zinc-aluminum-based hot-dip coated steel sheet with superior workability and corrosion resistance using the apparatus according to claim 3, the method comprising coating a steel sheet in a coating bath comprising 35 to 55% by weight of zinc, 0.5 to 3% by weight of silicon, 0.005 to 1.0% by weight of chromium, 0.01 to 3.0% by weight of magnesium, 0.001 to 0.1% by weight of titanium, and the balance of aluminum and inevitable impurities. Claim
6. A method for producing a zinc-aluminum-based hot-dip coated steel sheet with superior workability and corrosion resistance using the apparatus according to claim 4, the method comprising coating a steel sheet in a coating bath comprising 35 to 55% by weight of zinc, 0.5 to 3% by weight of silicon, 0.005 to 1.0% by weight of chromium, 0.01 to 3.0% by weight of magnesium, 0.001 to 0.1% by weight of titanium, and the balance of aluminum and inevitable impurities. Claim
7. The method according to claim 5, wherein the coating bath further comprises 1 to 10% by weight of calcium, based on the total weight of magnesium. Claim
8. The method according to claim 6, wherein the coating bath further comprises 1 to 10% by weight of calcium, based on the total weight of magnesium.