KR840002329B1 - Process for producing a hot dip galvanized steel strip - Google Patents

Abstract

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Description

Manufacturing method of hot dip galvanized steel sheet

1 is a diagram showing the relationship between magnesium content in an alloy with molten zinc as a parent metal and a galvanized steel sheet with zinc as a parent metal.

2 is a correlation diagram between the amount of aluminum in the zinc metal (hereinafter referred to as zinc alloy) made of a parent metal and the amount of zinc alloy plated on the surface of the steel sheet.

3 shows the correlation between the weight of zinc alloy plated on the surface of steel sheet and the amount of zinc alloy plated on the surface of steel sheet.

4 shows the correlation between the tin and magnesium contents in the zinc alloy plating and the blackening phenomenon of the zinc alloy plating.

5 is a correlation diagram between the contents of tin and aluminum in the zinc alloy plated on the steel sheet and the amount of occurrence of intergranular corrosion in the zinc alloy plating.

FIG. 6 shows the correlation between the molecular weight of oxygen and the amount of dross and metal evaporation generated in three different types of alloys of molten zinc plated on the surface of each steel sheet under oxygen-controlled atmosphere. It is assumed that the plating amount is adjusted to the target amount in an oxygen controlled atmosphere.

Article 7 illustrates the device of the present invention in detail using the correlation as in Article 6.

8 is a correlation diagram of the magnesium content in the hot-dip zinc alloy plated on the surface of the steel sheet and the oxygen molecular weight under the oxygen-controlled atmosphere and the occurrence of skimming in the hot-dip zinc alloy plating.

9 shows the molecular weight of oxygen in an oxygen-controlled atmosphere, the amount of an alloy based on molten zinc that is plated on the steel plate surface, and the loss of depression and fill-form in the plating of molten zinc alloy. ) Correlation with loss occurrence.

FIG. 10 shows the correlation between the molecular weight of oxygen under the oxygen control atmosphere, the weight of other molten zinc alloy plated on the steel sheet, and the occurrence of deflection loss and wheelie-bending loss in the molten zinc alloy plating.

11 is a correlation diagram of the molecular weight of oxygen and the zinc alloy plated on the surface of the steel sheet under an oxygen control atmosphere.

FIG. 12 is a drawing illustrating the apparatus used for the practice of the present invention, showing specific means for controlling the amount of molten zinc alloy plated on the surface of a steel sheet under an oxygen control atmosphere.

FIG. 13 is a detailed view of a device for finishing the solidification process under an oxygen control atmosphere for a zinc alloy plated on a steel plate surface.

FIG. 14 is an explanatory diagram of an embodiment of the present invention, in which an oxygen control atmosphere is divided into two parts, namely, a plating control part 2a and a cooling part 2b.

FIG. 15 is an explanatory diagram of an embodiment of the present invention, in which an oxygen control atmosphere is composed of a space in which a required oxygen molecular weight and an inert gas such as nitrogen are mixed.

FIG. 16 is an explanatory view of an embodiment of the present invention, and a diagram of a cooling gas blower installed to control the weight of a molten zinc alloy plated on a steel plate surface.

FIG. 17 is an explanatory diagram of an embodiment of the present invention, in which a molten zinc alloy to be plated on a steel plate surface is completely solidified under an oxygen control atmosphere composed of a mixture of an inert gas and air.

18 is an explanatory diagram of an embodiment of the present invention, in which a steel plate plated with molten zinc alloy enters into a first oxygen control atmosphere as a means for controlling the amount of molten zinc alloy and then plated with a molten zinc alloy. The figure explains the complete coagulation in the second oxygen-controlled atmosphere. Here, the second oxygen control atmosphere is variable and the first and second atmospheres are each composed of a mixture of air and inert gas.

FIG. 19 is an explanatory diagram of an embodiment of the present invention, in which a plated molten zinc alloy is coated on a steel plate surface under a first oxygen control atmosphere and then under a second cooling atmosphere wrapped in a first atmosphere. Drawing. Here, the mixed gas of air and inert gas is blown toward the zinc alloy plated on the steel plate surface at the outlet of the first atmosphere, and the inert gas is blown toward the zinc alloy plated on the steel plate surface at the outlet of the second atmosphere.

FIG. 20 shows the amount of air in the mixed gas blown into the first oxygen control atmosphere shown in FIG. 19 and the amount of oxygen molecular blown at M point into the first oxygen control atmosphere shown in FIG. Correlation diagram.

FIG. 21 is an explanatory diagram of an embodiment of the present invention, and blows toward the zinc alloy plated on the surface of the steel sheet in order to prevent the gas flow flowing out of the oxygen control atmosphere from occurring in the steel sheet outside of the above-mentioned atmosphere. Picture that seems to become.

FIG. 22 is a relation between the amount of air introduced into the oxygen control atmosphere in the apparatus shown in FIG. 23 and the molecular weight of oxygen at three locations under the oxygen control atmosphere.

FIG. 23 is an explanatory diagram of an embodiment of the present invention, in which gas flow from an oxygen control atmosphere is blown toward a zinc alloy plated on a steel plate surface at an oxygen control atmosphere outlet.

The present invention relates to a method for producing galvanized steel sheet and to a method of significantly increasing corrosion resistance using a molten zinc alloy bath. In the method of manufacturing galvanized steel sheet, the method based on molten zinc has been widely used in buildings, structures, houses, automobile bodies, etc. because of its excellent corrosion resistance. This is because the metal called zinc has good activity and thus the structure increases the corrosion resistance by forming a dense zinc alloy form. The corrosion rate of zinc is highly dependent on the nature and form of the corroded metal body and the surrounding corrosion environment. For example, under a gaseous, acidic or alkali-containing ambient environment, even zinc alloys with good corrosion resistance become solutions that react with sulfide oxides or alkalis, resulting in faster corrosion. Therefore, in order to prevent corrosion of the coating (plating) of the zinc layer on the iron plate, it must be released from the above-mentioned corrosion environment.

However, when zinc is plated in a neutral atmosphere, the product zinc alloy becomes a dense structure and may not be able to dissolve (corrosive) in a neutral solution, so that the concentration of chlorine ion is not so high under a medium atmosphere. If so, satisfactory results in corrosion resistance can be obtained. For example, if a lot of salt is sprayed on the road, even if the car body is made of galvanized steel, corrosion is accelerated and cannot be tolerated. In general, galvanized steel sheet has a very large activity of increasing corrosion resistance by sacrificing the zinc plated on the steel sheet when placed under a medium-phase atmosphere. Therefore, when the corrosion rate of zinc is reduced, the corrosion resistance activity due to self-sacrifice is active. For example, even when pure zinc is added to 3% of salt-dissolved brine, the corrosion rate is reduced to 1 / 20-1 / 50 below the original corrosion rate even in the mid-level atmosphere. Accordingly, the corrosion effect in the steel sheet can be satisfactorily obtained. Therefore, if the zinc corrosion rate is reduced to 1 / 20-1 / 50 level compared to the original corrosion rate under the above-mentioned environment, it means that the corrosion resistance is increased by 20 to 50 times by zinc coating (plating). If so, the logic that the galvanizing amount can be reduced to 1 / 20-1 / 50 is established.

In the past, galvanized steel sheet was mainly used for building and structure construction. This is the use that has become more widespread nowadays, and is being used for homes, cars and furniture. So galvanized steel sheet has a need to have properties for the new uses described above, in other words, in addition to very good corrosion resistance,

1) Good adhesion between steel plate and coated zinc.

2) Beautiful appearance and no discoloration.

3) Good surface finish of plated zinc surface.

That is, chemical treatment, organic material coating, painting, etc. should have the property to be beautiful.

The most important thing in this invention is to contain the magnesium element of 0.1-0.2% (weight) of the alloy which uses zinc as a base metal. Magnesium significantly increases the corrosion resistance when plating zinc-based alloys (hereinafter referred to as "lead alloys") on the surface of steel sheets. For example, in the case of FIG. 1, high purity (obtained by electrical production method) of 99.97% is alloyed and plated with magnesium. Other steel sheets also contain 0.22% (weight) aluminum, 0.1% (weight) lead, 0.01% (weight) cadmium and 0.02% (weight) iron as impurities, and 0.1-2.0% (weight) magnesium as shown in FIG. It is also plated with another zinc alloy.

Industrial Standards (KS, JIS) stipulate the test of spraying salt water for 3 days. At this time, the weight loss of each steel sheet is a standard for determining the corrosion rate. FIG. 1 clearly shows that the addition of a small amount of magnesium to zinc significantly reduces the corrosion rate, that is, the corrosion resistance. For example, the corrosion rate of galvanized steel sheets containing 0.5% (by weight) magnesium is only about one seventh that of zinc alloys containing no magnesium or zinc. Also, if you contain 1.0% magnesium, this figure is reduced to about 1/10. Figure 1 clearly shows the effect of magnesium on the zinc alloy until the magnesium is saturated to 2.0% (by weight). (In this case, even if magnesium is contained more than 2.0%, the effect is almost unchanged.) Also, in order to obtain a satisfactory corrosion resistance effect, it is also necessary to make zinc alloy at least 0.1% (weight) or more. Able to know. Therefore, the important point in the present invention is that the magnesium content added to the zinc alloy should be in the range of 0.1 to 2.0% (weight). In addition to the zinc alloy used in the present invention, aluminum of 0.5% (weight) or less is usually added. In other words, the zinc alloy is 0.1 to 2.0% by weight magnesium, and the aluminum balance of 0.5% or less by weight is usually composed of almost zinc.

The addition of aluminum to the zinc alloy enhances the adhesion of the zinc alloy to the steel plate surface. 2 shows the correlation between the aluminum content added to the zinc alloy and the adhesion strength (bonding strength) of the zinc alloy plated on the steel sheet, and each data was added to the molten zinc alloy containing 1.0% (weight) magnesium. The numerical value at the time of plating on different steel plates which added aluminum, 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, or 0.5% (weight), respectively is shown. The zinc alloy plating amount is 50g / m ² here.

In order to measure the degree of peeling of the zinc alloy plating layer, the steel plate is struck with a ball and tested for strength to withstand the impact. The iron ball used here has a diameter of 25 mm and is injected into the zinc alloy layer and attached. Apply the test tape to the zinc alloy plated layer surface, remove it and measure how much it gets off. The zinc alloy adhesion strength is calculated as the ratio of the surface peeled off of the zinc alloy plating layer per total surface area.

FIG. 2 shows that the adhesion of zinc alloy plating increases markedly when 0.1% (weight) or more of zinc alloy is added. Molten zinc alloy solution (A) containing 0.2% (weight) magnesium and 0.2% (weight) aluminium, and 0.5% (weight) magnesium ) Molten zinc alloy solution (B) containing 0.2% (weight) of aluminium, 1.0% (weight) of magnesium, 0.2% (weight) of molten zinc alloy solution (C) and magnesium containing 0.2% (weight) of aluminum ) Were immersed in the molten zinc alloy solution (D). After the removal, the immersed steel sheet is taken out and gas is blown to blow the molten plating layer on the surface of the steel sheet in order to match the next plated amount with the target amount. When the plating process is completed, the adhesion of the plating was measured by a ball impact test with a steel ball, and the result is shown in FIG. 3.

Figure 3 shows that zinc alloys (A), (B), and (C) have very good adhesion properties. On the contrary, in the case of (D) with no aluminum added, the adhesion properties are inferior. In general, adding more than 0.1% (weight) or more of aluminum has a great effect in increasing the adhesion of zinc alloy plating on the surface of the steel sheet. However, when 0.5% (weight) or more of aluminum is added, the corrosion resistance during zinc alloy plating sometimes decreases. This is because the addition of tin or lead-containing aluminum to the zinc alloy promotes intergranular corrosion (or intergranular corrosion) in the zinc alloy.

As described above, the addition of less than 0.1% (by weight) of aluminum to the zinc alloy is poor in adhesion to the steel sheet surface. However, if the zinc alloy is plated with tin, nickel or copper before plating on the surface of the steel sheet, the adhesion of zinc alloy containing less than 0.1% (weight) of aluminum is improved than that without it. The same as that of zinc alloy containing. This is because a pre-plated metal layer (hereinafter referred to as "pre-plating") prevents unnecessary diffusion between steel and zinc alloys. Pre-plating is carried out by installing a negative electrode made of steel in a plating bath containing tin, nickel and copper ions, and the plating weight is in the range of 0.001 to 1 g / m ² . In another process, a negative electrode made of steel sheet is placed in a plating bath containing metal ions to be plated, and subjected to process for degreasing and, if necessary, pickling.

This method is very effective for simultaneously processing the preplating process and the degreasing process. The preplating process may be used as long as the metal to be preplated can be firmly attached to the surface of the steel sheet.

The pre-plated tin and nickel copper layers can be used to enhance galvanized adhesion to steel plates made of high concentrations of silicon, manganese and, in some cases, aluminum and other steels such as high-strength steel, killed steel or continuous cast steel, which impair plating. Give a great effect.

Surfaces of steel sheets coated with zinc alloys containing only magnesium or containing both magnesium and aluminum may discolor over time due to surface oxidation. The present invention has been studied to prevent such blackening by adding tin to zinc alloy.

The first table shows data obtained by plating zinc alloys containing magnesium, aluminum, and tin on seven different steel sheets.

Each plated steel sheet was treated with a 2% (weight) solution to which chromic anhydride was added to the surface, followed by drying by blowing hot air.

The chromium weight on the surface of the plated steel sheet thus obtained is dispersed in the range of 10-25 mg / m ² .

For each plate, the chromic acid treated part and the untreated part were put in an airtight chamber at a constant temperature of 38 ° C. and tested in the following manner. That is, in each steel plate, half of the chromic acid treated part and the untreated part were placed on the other steel plate, wrapped in waterproof paper, and the other half was directly exposed to the air to be separated separately.

TABLE 1

As a result, the seals required for the appearance of black sides on the surface of each steel sheet are shown in Table 1. Table 1 shows blackening within 3 months when magnesium and aluminum plates were added without adding tin, such as zinc alloy plating Nos. 3 and 4. On the contrary, the addition of only aluminum without magnesium and tin, such as zinc alloy 1, is less discolored. In the case of the zinc alloy 5, 6, 7, which is also added to aluminum magnesium and tin, it can be seen that it is simply stubborn against discoloration. This is to say that the addition of both magnesium and aluminum to the zinc alloy is very effective in eliminating blackening. 4 shows the correlation between the blackening phenomenon and the magnesium and tin content added to the zinc alloy. In this case, when the zinc alloy is added in the same amount as magnesium and tin, the plated parts are discolored when they are kept at a temperature of 38 ° C. for up to 6 months. However, the addition of more than 0.1% (weight) of tin to the zinc alloy containing magnesium significantly reduces this blackening phenomenon.

The present invention is an invention for preventing unnecessary intergranular corrosion in the zinc alloy, the amount of tin added in the zinc alloy is the most appropriate amount derived from the following equation at least 0.1% (weight). Namely, tin addition upper limit A (%) = 1.07-1.33 × B where B represents aluminum content (%). Corrosion tests are carried out on each steel plate coated with a molten zinc alloy containing 0.5% magnesium and aluminum and tin content as shown in Figure 5, usually steam filled at 100 ° C for 2 weeks. The plated steel sheet was placed in the experiment.

After the test, it was measured how much the intergranular corrosion occurred on the zinc alloy plating. The result is shown in FIG. 5, where the content of aluminum or tin is in the hatched portion, and as a result, intergranular corrosion occurs. To avoid intergranular corrosion, the amount of tin A and the amount of aluminum B must satisfy the above equation. Considering the corrosion resistance, adhesion to the surface of the steel sheet, resistance to black deformation, and resistance to intergranular corrosion, it can be seen that magnesium, aluminum and tin amounts are the determining factors for zinc alloys. A recommended zinc alloy is an alloy containing 0.1-2.0% magnesium by weight 0.1-5.0% aluminum by weight and at least 0.1% by weight tin by up to A% by weight. Where A (%) = 1.07-1.33 × B (B denotes aluminum content percentage)

As described above, tin is extremely effective in preventing blackening in zinc alloy plating. Bismarth and silicon also prevent blackening, but the effect is comparable to that of tin. Then, in the present invention, the zinc alloy can be at least replaced by a group consisting of bismuth and silicon instead of tin to be added at least. In the present invention, the magnesium contained in the zinc alloy was observed to be a factor in creating a sparkling property with a shiny pattern on the surface of the product.

In some cases, however, it is necessary to promote sequining properties in zinc alloy plating. In this case, it is necessary to add lead, tin, antimony or bismuth which serves to promote sequinability. 0.1 to 0.25 lead component is added to a zinc alloy generally used for this purpose. However, even though a small amount of lead is contained, it plays a role in accelerating intergranular corrosion during zinc alloy plating. Therefore, in order to prevent intergranular corrosion, it is preferable that the amount of lead added in the zinc alloy is better, so that it is preferably 0.01% (weight) or less.

In the present invention, the sequinability of zinc alloy containing no lead is accelerated by adding tin, bismuth or antimony. It is also noteworthy that the addition of antimony is very effective in promoting sequining, although it does not prevent blackening in zinc alloys. Then at least some of the comments can be used as antimony instead. As mentioned above, magnesium has a very pronounced effect in improving the corrosion resistance of zinc alloy against corrosion. It is also effective in promoting finish-coating properties. The galvanized steel sheet in this invention shows the outstanding finish plating property. Normally, before finish plating of galvanized steel sheets, the zinc alloy plating surface is treated with a phosphate solution under the same conditions as those applied to conventional zinc alloy plating containing no magnesium to promote adhesion between the finish plating and the zinc alloy plating. As a result of the treatment, a hard and stable phosphate film is obtained on the zinc alloy plating surface. As a result, the phosphate solution treated zinc alloy plating can be firmly obtained by any conventional finishing plating method.

Examples include cathode electrodeposition plating, anode electrodeposition plating, and baking plating. The former is applied to top and bottom coats and top coats of car paint and color steel plates. The covering method is used for housing. In the present invention, the following experiments were conducted to obtain excellent plating properties of the galvanized steel sheet. That is, various steel sheets were plated with molten zinc alloys containing aluminum, magnesium, tin and iron in the amounts shown in Table 2.

Each galvanized steel sheet was pretreated with zinc phosphate containing a pre-treating agent in the usual way. After the coating, the coating was coated with a cathode electrode precipitation method and the coating thickness was about 20 microns. Next, the coating layer of each test piece of the steel sheet is cut transversely so that the cut portion of the steel sheet is exposed to the atmosphere. Then, the test piece was sprinkled with brine for 1000 hours as defined by the Industrial Standards (KS, JIS). As a result, the periphery of the cut surface was corroded. The measurement results for the width (mm) of the corroded parts are shown in Table 2.

Separately, each test piece of the coated steel sheet may be sprayed with brine. Continue until 50% of the entire surface of the steel sheet is covered with red rust.

The results are shown in Table 2.

TABLE 2

Note: * 1 is plated by the galvannealing method.

In Table 2, specimens 3, 4, and 5 are consistent with the process of the present invention and show very high corrosion resistance, and paint coating is very firmly fixed on the zinc alloy plating of the steel sheet surface. The addition of magnesium to zinc alloy plating brings several advantages as described above. At the same time, however, it can also cause a number of uncomfortable results, such as increased cross-formation, difficulty in controlling the weight of the molten zinc alloy, creating unnecessary schemes, and promoting undesired blacking or discoloration. The result is that the plating is uneven and a bad effect such as coarse sequins is produced. Therefore, when zinc-containing zinc alloy is plated by the conventional melt immersion plating method, the process is difficult and the quality is unsatisfactory.

In view of the above, the present invention is a result of research on eliminating the above-mentioned disadvantages using a molten zinc alloy containing magnesium. An important point in plating steel sheet with magnesium-containing molten zinc alloy is that the molten zinc alloy bath atmosphere is the oxygen atmosphere and the molten zinc alloy plated on the steel sheet is controlled to reach the limit value. Magnesium usually reacts very strongly with oxygen. Oxidized magnesium produces dross in molten zinc alloy jade. In the conventional hot dip galvanizing method, a steel sheet is immersed in a hot dip zinc alloy bath, taken out of a bath, and then sprayed with a hot dip zinc alloy to control the total coating amount. Magnesium is controlled to oxidize in the atmosphere. However, this method sometimes results in undesirable splashing phenomena and oxidation results in the formation of dorothes in molten zinc. Therefore, as described above, the conventional hot dip galvanizing method cannot be used as it is in the zinc alloy method containing magnesium. In order to prevent the dross formation, a method of using a gas containing no oxygen instead of air injection was attempted, but since no oxygen was present, a large portion of the zinc alloy melt was evaporated. This evaporated metal is accumulated on the inner surface of the melt immersion apparatus. This causes a variety of hazards, which means that gases that do not contain oxygen will eventually not contribute to the prevention of dross production.

For example, when the steel sheet is melt-immersed by zinc alloy containing 0.5% (weight) magnesium at a speed of 80 M / min using a melt immersion plating apparatus as shown in FIG. The steel plate 1 of 7 degrees is immersed in a galvanized pot containing molten zinc and passed through the snout 6 while maintaining 150 degrees Celsius. The steel sheet passes through a sink roll 5 in a molten zinc bath and is then removed from the bath. Next, the chamber is placed under oxygen control atmosphere. Oxygen molecule concentration under an oxygen control atmosphere is adjusted to a predetermined target value. The oxygen control atmosphere in the sealed chamber 2 consists of a first section in which the weight of the molten zinc alloy is injected onto the plating layer or adjusted to a target value and a second section cooling the plated to a predetermined temperature. In the sealed chamber 2, a pair of nozzle is arranged. The gas is sprayed on both surfaces of the plated steel sheet through the nozzle 7 to control the weight of the molten zinc alloy on the surface of the steel sheet.

The oxygen molecular concentration in the sealed chamber 2 was measured by a high-sensitivity oxygen accumulator by collecting gas at 100 mm from the top and bottom of the nozzle 7, and the results are shown in FIG. Here, molten zinc alloy composed of the oxygen molecular weight in the oxygen controlled atmosphere of the curve I sealed chamber (2) and the electric zinc having 0.5% (weight) of magnesium in the first zone of the oxygen controlled atmosphere and high purity of 99.99% of the remaining components. This shows the correlation with the amount of evaporation generated at. The evaporation amount of the metal in each first zone under the oxygen control atmosphere is expressed as the ratio of the evaporation amount of the metal in the first zone to the atmosphere obtained at an oxygen molecular weight of 10 p.p.m. Curve I shows that metal evaporation does not occur when the oxygen molecular concentration in the first zone is above 100 pipmnia. Curve II in FIG. 6 is the oxygen molecular concentration in zone 1 under oxygen controlled atmosphere and 0.2% (weight) aluminum in zone 1. Correlation with the amount of metal evaporation generated in a molten zinc alloy bath consisting of 0.1% lead by weight, 0.01% by weight cadmium 0.015% by weight iron and the remaining zinc is shown. In the case of the aluminum-containing zinc alloy bath described above, metal evaporation does not occur when the concentration of oxygen molecules in the first zone is 50 ppm or more. Curve III in FIG. 6 shows the correlation between the oxygen molecular concentration in the first zone under the oxygen control atmosphere and the amount of dross produced in the same zinc alloy bath containing aluminum as described above in curve II.

The amount of dross produced in each first zone is calculated as the ratio of the amount of metal vapor generated in each first zone to that produced in the atmosphere. Curve III shows that dross is not produced when the molecular weight of oxygen in the first zone is 1000 ppm or less. However, when it is more than 1000PM, as the concentration of oxygen molecular weight increases, the dross also increases significantly.

From the results shown in Figure 6, it can be seen that the weight of zinc alloy formed on the steel sheet can be arbitrarily changed by adjusting the oxygen molecular weight concentration to a low target value. In addition, by adjusting the oxygen molecular weight concentration to a low value in advance, it is possible to prevent the generation of unnecessary dross. Therefore, if the oxygen molecular weight concentration is set to a low value, the molten zinc metal can be highwayd. Conventional speed is less than 150M / min per minute because the high speed of work to produce dross in the molten zinc alloy bath and at the same time unnecessary splash phenomenon occurs. However, by reducing the oxygen molecular weight concentration in the first zone, even when magnesium is added to the molten zinc alloy bath, the molten zinc plating rate can be increased to 150 M / min or more. In the air, the conventional hot-dip galvanizing method uses the injection gas used to perform zinc alloy plating in the air and to adjust the weight (thickness) of the zinc alloy as air, and increases the injection pressure of the injection gas pressure to reduce the zinc alloy plating weight. Was effective. However, increasing the injection pressure only increased the unnecessary spray phenomenon, making the work difficult. In the conventional hot dip galvanizing method, when working without adjusting the oxygen small molecule concentration in a cooling atmosphere, a large amount of dross or metal evaporation occurs in the zinc alloy bath. However, by adjusting the concentration of oxygen molecules in the first zone under the oxygen control atmosphere to a limit value, the thickness of the hot-dip zinc alloy can be reduced without any difficulty.

Reducing the concentration of oxygen molecules in the first zone under an oxygen control atmosphere is effective in reducing the zinc alloy plating weight without increasing the injection pressure of the ejected gas. For example, the zinc alloy plating weight generated at the oxygen molecular concentration of 100 ppm in the first zone under the oxygen control atmosphere is 80% of that produced in the atmosphere. That is, according to the practice of the present invention, it is possible to work by making the molten zinc alloy plating thickness thin at a high speed of 150 m or more per minute.

In the conventional hot dip galvanizing method, the cooling and blowing gas is made of air, and the surface of the zinc alloy plating is easily oxidized. The result is a solid coating layer that covers the remaining hot dip zinc alloy plating in flow. Spraying the blowing gas here causes wrinkles to the solid coating layer. Even after the zinc alloy plating layer is completely solidified, wrinkles remain on the surface, and the resulting solid coating layer is an obstacle to adjusting the zinc alloy plating weight to a target value.

For example, each steel plate is plated with a molten zinc alloy containing magnesium at a temperature of 450 degrees Celsius and a working speed of 80 m / min in the direction shown in FIG. 8, and the ejected gas containing oxygen molecules is shown in FIG. At a pressure of 1.0 kg / cm ² , a skimming phenomenon was observed when spraying the zinc molten plated surface through a jetting gas slit hole 10 mm away from the hot dip zinc alloy plated surface and having a thickness of 0.6 mm. As a result, it can be seen that there is a correlation between the skimming phenomenon under the oxygen molecule concentration and the amount of magnesium added to the zinc alloy as shown in FIG. 8 (the skimming phenomenon is indicated by the diagonal line in FIG. 8). In other words, if the molten zinc alloy contains less than 2.0% magnesium by weight, no skimming occurs at all in the first zone under an oxygen control atmosphere with an oxygen molecular weight of 1000 ppm or less, Reducing the amount of% (weight) does not cause any skimming at all, even in an oxygen-controlled atmosphere with an oxygen molecular weight of 3000 ppm.

In other words, in the production of hot dip galvanized steel sheet made by hot dip galvanizing, the addition of magnesium to the hot dip zinc alloy ensures a smooth surface, so that the oxygen molecular concentration can be reduced to 1000 p.m. or less under an oxygen controlled atmosphere. It is necessary to make magnesium addition amount 2.0% (weight) or less.

Usually, the zinc alloy plating amount is controlled by spraying on the surface of the ejected gas when the zinc alloy plating is in the flow state as described above. The work at this time takes place directly on the surface of the molten zinc alloy bath. After this is done, the alloy plating is exposed to an oxygen-controlled atmosphere to solidify. During the solidification process sometimes unnecessary depression loss and wheely-bending loss occur. For example, in each hot dip galvanizing, each steel plate is made of 0.51% magnesium or 1.0% (weight) aluminum 0.2% (weight), lead 0.1% (weight), cadmium 0.01% (weight) and iron 0.01% (weight). When galvanizing with a molten zinc alloy containing molten zinc alloy, the molten zinc alloy is solidified under an oxygen-controlled atmosphere having an oxygen molecular weight concentration as shown in FIG. 9 and the plating amount is as shown in FIG. Controlled and deflection losses and wheelie-bending losses were observed at each zinc alloy plated surface, as shown in FIG.

Except for the zinc alloy containing 0.3% (weight) tin as an additional element, the result of the same operation as described above is shown in FIG. Referring to FIG. 9 and FIG. 10, it can be seen that the diagonal line is located at the top of the figure. Depression loss is the best known phenomenon in zinc alloy plating operations. In comparison, wheelie-bending loss is only a phenomenon during zinc alloy plating containing magnesium.

In FIG. 9, if the oxygen molecule concentration under the oxygen control atmosphere is adjusted in the range of 100 to 1000 ppm, depression and wheely-bending loss do not occur. However, when tin is added to the zinc alloy as shown in FIG. 10, it can be seen that even in an oxygen-controlled atmosphere containing an oxygen molecular weight of 100 to 100,000 ppm, depression and wheely-bending loss do not occur. When the hot-dip zinc alloy weight is less than 200 g / m ² and the oxygen molecular weight in the oxygen controlled atmosphere is more than 100,000 Pm, wheely-bending loss does not occur.

The surface of the zinc alloy plated steel sheet as in FIG. 10 shows the same gloss as in FIG. Referring to FIG. 11, the plated steel sheet produced when the oxygen molecular concentration is 300 ppm or less under the oxygen control atmosphere has a spherical-free mirror-like luster. However, when it is over 500PM, the gloss becomes normal. In the intermediate level of 300 to 500PM, the gloss has an intermediate property, so to adjust the glossiness of the plated steel freely, it is necessary to control the oxygen molecular concentration under the oxygen control atmosphere.

This important property in magnesium-containing zinc alloy plating operation is not known at all prior to the present invention. As can be clearly seen from the important characteristics shown in Figs. 6 to 8, in order to obtain a zinc alloy coated steel sheet having a desired quality level, at least an oxygen-controlled atmosphere, in particular the zinc alloy plating weight, It is important to make a restriction adjustment so that the concentration of oxygen molecules reaches a specific value in the first zone around the injection nozzle of the jetting gas to be controlled. The range of plating weight and oxygen control atmosphere limiting control area is determined by the amount of splash generation of the hot dip zinc alloy, the target thickness of the zinc alloy plating, the composition of the hot dip zinc alloy and the plating operation speed. It is usually preferred that the plating weight control zone in an oxygen controlled atmosphere is at a position of 1000 mm immediately above the molten zinc alloy bath and above the position of the gas injection nozzle. Oxygen molecule concentration in the plating amount control zone of the oxygen control atmosphere is preferably set to less than 1000PM, preferably 50 to 1000PM. When the zinc alloy plating amount is less than 50g / m ² , the oxygen molecule concentration is in the range of 10 to 1000PM.

The present invention accomplishes such a task using a device as shown in FIGS. 12 to 23. Referring to FIG. 12, the oxygen control atmosphere is made in a sealed chamber 2a, which is located above the molten zinc alloy bath 4 including the galvanizing pot 3. A pair of gas injection nozzles 7 are located directly on the surface of the molten zinc alloy bath 4.

The steel sheet 1 is guided through the inlet 6 and immersed in the molten zinc alloy bath 4 where it passes through the sink roll 5 vertically upwards and passes between the pair of gas spray nozzles 7. At this stage, the molten zinc alloy plating is in a flow state (viscous state). The blowing gas is injected from the nozzle 7 to adjust the thickness of the molten zinc alloy on the surface of the steel sheet to a target value. The steel plate 1L plated with the predetermined zinc alloy thickness is completely solidified on the sealed chamber while leaving the sealed chamber 2a. In other words, a plated steel sheet 1S in which zinc alloy plating is solidified is obtained.

In the apparatus shown in FIG. 13, the upper part of the sealed chamber 2a is connected to the lower part of the sealed chamber 2a, and in this upper sealed chamber 2b, the hot-dip zinc alloy plated layer on the surface of the steel sheet has a predetermined solidification rate. solidify completely at a rate.

The oxygen molecule concentration under the oxygen control atmosphere is 100 ppm or less in the upper closed chamber 2b. In the apparatus diagram as shown in FIG. 14, the upper hermetic chamber 2b is separately placed above the lower hermetic chamber 2a, and the molten zinc alloy plating is completely solidified at a predetermined solidification rate. In the apparatus diagram as shown in FIG. 15, the sealed chamber 2a is composed of an outer wall 2c-1 and an inner wall 2c-2, and a passage is formed between the outer wall 2c-1 and the inner wall 2c-2. Make it. The lower end of the inner wall has a space from the molten zinc alloy bath 4 to the passage 2c-3 to the inner space 2c-4. A pair of blowing gas injection nozzles 7 is placed in the inner space 2c-4 and this nozzle is also connected to the inert gas supply source 9a. This inert gas is injected from the nozzle 7 toward the molten zinc alloy plating layer on the surface of the steel sheet.

The passage 2c-3 is connected to a gas supply source 10 having a predetermined molecular weight of oxygen such as air and a conduit 8 connected to the inert gas supply source 9b. The inert gas is mixed with the oxygen-containing gas and the mixed gas is led to the inner space 2c-4 to create an oxygen control atmosphere through the passage 2c-3.

The oxygen control atmosphere in the sealed chamber 2a is maintained at a pressure level of 5-10 mmH ₂ O higher than the surrounding atmospheric pressure. Therefore, the air in the atmosphere enters the sealed chamber 2a and is pushed to the upper part of the sealed chamber 2a through the outlet W ₁ . When the gas is injected from the gas supply source 10 without oxygen molecules, the oxygen molecular weight becomes 10 ppm or less under the oxygen control atmosphere.

In the apparatus shown in FIG. 16, the mixed gas is mixed with a gas having a predetermined oxygen molecular weight supplied from the source 10 of the inert gas supplied to the source 9, and the mixed gas is directly led to the inside of the sealed chamber 2a. Supply oxygen control atmosphere.

And a chamber (chamber) is made in the upper part of the sealed chamber 2a, and this chamber surrounds the outlet W ₁ of the sealed chamber 2a, and let only a split (open space | gap) is input. This chamber 2d is connected to a source 9b of inert gas. The inert gas is guided into the chamber 2d and then injected into the steel sheet through the split to pass through the outlet W ₁ to create a curtain of inert gas flow, which is introduced into the closed chamber 2a. Effective for shielding the atmosphere from

As shown in FIG. 17, the inert gas is supplied from the supply source 9a and injected into the sealed chamber 2a through the pair of gas injection nozzles 7. In addition, the gas supplied from the supply source 10 containing the inert gas and the predetermined molecular weight of oxygen enters directly into the sealed chamber 2a. The upper part of the sealing chamber is connected to the lower part of the upper upper part sealing chamber 2b. In this way, the interior of the sealed chamber 2a and the upper sealed chamber 2b are filled with an oxygen control atmosphere. This long upper airtight chamber 2b is effective in preventing unnecessary air pollution by the oxygen control atmosphere in the sealed chamber 2a.

In the apparatus shown in FIG. 18, the closed chamber 2a is as shown in FIG. 16, except that the chamber 2d is connected to a source 10 of gas having a predetermined oxygen molecular weight content. The upper hermetic chamber b away from the hermetic chamber 2a is arranged above the hermetic chamber 2a. The upper sealing chamber (2b) is to be aligned on the way the plated steel sheet (1) passes. The upper hermetic chamber 2b is divided into an upper chamber 2e and a lower chamber 2f each having a slit surrounding the path through which the plated steel sheet 1 passes.

The gas mixture is composed of a mixed gas of an inert gas supplied from the source 9 and a gas having a predetermined molecular weight of oxygen. A part of the mixed gas is directly led into the upper hermetic chamber 2b, the other part is led to the upper chamber 2e, and then sprayed through the slit toward the inlet W ₂ of the upper hermetic chamber 2b. A part of the mixed gas enters the lower chamber 2f and is injected through the slit toward the inlet W ₃ of the upper closed chamber. The hot-dip zinc alloy plated layer on the steel sheet is completely solidified while the steel sheet stays in the upper sealing chamber 2b.

In the apparatus shown in FIG. 19, the inert gas supplied from the source 9a is injected into the inner wall 2c-2 through the pair of nozzles 7. The mixed gas of the inert gas supplied from the source 9 and the gas supplied from the source 10 containing a predetermined oxygen molecular weight is led into an inner upper chamber formed at the top of the inner wall 2c-2, and then It is injected through the slit surrounding the inner inlet W ₁ of the inner wall 2c-2. The passage 2c-3 is made between the inner wall 2c-2 and the outer wall 2c-1.

The lower end of the inner wall 2c-2 has a space between the surfaces of the zinc alloy bath 4 and the passage 2c-3 is connected to the inner space 2c-4 surrounded by the inner wall 2c-2. The outer upper chamber 2d-2 is made on top of the outer wall 2c-1.

The inert gas enters the upper chamber 2d-2 and is injected through a slit surrounding the outer inlet W ₂ of the next passage 2c-4. The injection flow of the inert gas makes the outer curtain to block the passage 2c-3 from the atmosphere. Part of the injection gas mixture flows directly into the interior space 2c-4 and the remaining portion flows through the passage 2c-3 and then enters the interior space 2c-4.

As a working example of the present invention, an apparatus as shown in FIG. 19 is used. The nozzle 7 is placed at a position approximately 150 mm above the surface of the molten zinc alloy bath 4. The inner and outer inlets W ₁ and W ₂ shall be approximately 200mm wide.

A 150mm wide steel plate is plated at 80M / min using the same mechanism as shown in Figure 19.

A mixed gas of an hourly flow rate of 16M ₃ (Q ₂ ) of nitride gas and air having a flow rate of 2.5, 10, 17.5, and 25 liters per hour, respectively, enters the upper chamber 2d-1 and is then injected through the slit. In addition, the nitriding gas is also injected through a nozzle having a slit having a width of 400 mm and a thickness of 0.3 mm under a pressure of 0.5 kg / cm ² , and the nozzles 7 are arranged to be spaced apart from each other by 30 mm. When there is no gas supply, the concentration of oxygen molecules in the atmosphere of oxygen concentration in the internal space 2c-4 becomes 5-15PM.

The relationship between atmospheric flow rate Q ₃ and oxygen molecule concentration is shown in FIG. 20. In other words, if the air flow rate Q ₃ is 2.5, 10, 17.5, and 25 liters per hour, the corresponding oxygen molecule concentrations are 10-250, 100-500, 900-1000, 1500-2000, respectively. Here, the oxygen molecular concentration is measured at point M shown in FIG. 19, and the position M has a space of 30 mm from the inner wall 2c-2 between 50 mm above the nozzle 7.

In the apparatus of FIG. 21, a pair of air cushion pats 11 are arranged on top of the sealed chamber 2a. A part of the gas contained in the sealed chamber 2a is discharged | emitted from the hole 12 provided in the position M2 corresponding to the center part of the sealed chamber 2a. The blower 12 then enters the air cushion 10 through the conduit 14. The gas discharged from the conduit 14 mixes the oxygen molecular weight supplied from the source 10 with a predetermined gas.

Apart from this, the inert gas is injected through the nozzle 7 and enters the sealed chamber 2a. If necessary, the mixed gas of the inert gas and the oxygen molecules can enter the upper part of the interior of the sealed chamber 2a, in the upper part and the lower part of the case. The mixed gas flow is injected from the air cushion fat 11 to prevent unnecessary vibration of the steel sheet and helps to block the inside of the sealed chamber 2a from the outside air.

According to the working example of the present invention using the apparatus shown in FIG. 21, a steel plate having a width of 150 mm is plated at a speed of 80 M / min, the slit of each air cushion portion is 100 × 100 × 3 mm, and is supplied from the air cushion portion. The flow rate of the mixed gas is 4.4M ³ per minute and the fats are separated between 30mm each.

The nozzles 7 each having a width of 350 mm and a thickness of 0.5 mm are positioned between 20 mm each. Nitrogen gas is allowed to be injected at a pressure of 1.0 kg / cm ² at a flow rate of 3.9 M ³ per minute. And the ratio of the mixed gas supplied to the air cushion fat 11 with respect to the inert gas supplied from the nozzle 7 shall be 1.4: 1 or less. When this ratio is more than 1.4: 1, the pressure inside the sealed chamber 2a has a negative value. This negative pressure results in the intake of air into the airtight chamber 2a. In this case, it is necessary to additionally inhale and fill the nitriding gas and the inert gas.

The concentration of oxygen molecules under oxygen atmosphere in the sealed chamber 2a is as shown in Table 1, and measured at M1, M2, and M3 positions, respectively, and the results are shown in FIG.

In FIG. 22, when no air is supplied, it can be seen that the oxygen molecular concentration under the oxygen control atmosphere in the closed chamber 2a is lowered to about 10 Pm. The correlations between the oxygen flow and the oxygen concentration at 20 liters, 50 liters and 100 liters per minute are as follows.

In the apparatus shown in FIG. 23, the upper airtight chamber 2b is connected to the lower airtight chamber 2a, and the pair of air cushions 11 are arranged at the top of the upper airtight chamber 2b so that the outlet ( W) is between the fats 11. The gas contained in the lower hermetic chamber 2a is ventilated by the closed blower 12 in the hole 13 located in the center of the lower hermetic chamber 2a. And if necessary, the oxygen molecules supplied from the source 10 are mixed.

The mixed gas is injected through the air cushion fat 11. The oxygen control atmosphere in the closed chamber 2a is prepared from the oxygen molecule-containing gas supplied from the source 10 and the inert gas supplied from the source 9.

The apparatus required for carrying out the present invention does not necessarily need to be as described above. In the practice of the present invention, any device may be used as long as the oxygen molecule concentration under the oxygen control atmosphere can be adjusted to a specific value. The following examples are for the purpose of clarifying the purpose of the present invention. However, these examples are only examples and do not limit the practical scope of the present invention.

Example 1-7

In each case from 1 to 7, as shown in Table 2, an appropriate amount of magnesium and tin, 0.15% (by weight) of impurities, 0.2% (by weight) of aluminum, cadmium (0.01%), iron (0.01) Zinc alloys containing% (weight) are melted at 450 degrees Celsius.

50th the concentration of molecular oxygen under controlled oxygen atmosphere by using the same device as in Figure 21 in position pipiem M _1, M ₂ 100 pipiem in location, is adjusted to 500 M ₃ pipiem in position. The steel plates are plated at a rate of 50 m / min in the molten zinc alloy bath.

The zinc alloy plating amount is adjusted to 50 g / m ² by spraying the gas at 1.0 kg / cm ² pressure through a pair of nozzles spaced between 20 mm each. Even with the hot dip galvanizing, there is no difficulty due to dross generation or metal evaporation. The properties of the plated steel sheet are shown in Table 2.

Note: (* 1): Ball-impact test.

(* 2): Specimens are placed on top of each other and wrapped in wrapping paper.

(* 3): According to Industrial Test Standard (JIS Z 2371).

Example 8-14

Except for the apparatus as shown in FIG. 19, the same method as described in each of Examples 1-7 was used, and the oxygen molecular concentration was adjusted to 100 to 250 ppm around the gas injection nozzle placed in the oxygen control atmosphere. .

The plating solution of the zinc alloy is allowed to solidify completely outside the oxygen control atmosphere after plating. As a result, the coated steel sheets in Examples 8-14 showed the same properties as those in Examples 1-7, respectively.

Example 15-16

As in Example 15, except that the aluminum concentration of 0.1% (by weight) contained in the zinc alloy and the aluminum concentration of 0.5% (by weight) in Example 16 are the same as those of Example 6, and are produced as a result. The properties of the plated steel sheet were almost the same as in Example 6.

Claims (1)

Plating at least one surface of the steel sheet with a zinc alloy melt containing 0.1 wt% to 2 wt% magnesium, and adjusting the weight of the hot dip zinc alloy formed on the surface of the steel sheet; ) And at least one of the stages where the molten zinc alloy plating is still in solution, the molten zinc alloy plating is exposed to the oxygen-controlled atmosphere defined by the sealed chamber, and oxygen content therein in the atmosphere. Melt immersion type, characterized in that it is controlled to 50 ~ 1000ppm by introducing a gas or oxygen-containing gas while the weight of the molten zinc alloy plating is controlled by injecting an inert gas over it in the oxygen control atmosphere Method of manufacturing galvanized steel sheet.

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