KR20010060423A - a method of manufacturing a hot dip galvanized steel sheets with excellent surface appearance - Google Patents

Abstract

PURPOSE: A method for manufacturing a hot galvanized steel sheet having superior external surface appearance is provided to remove ripple marks of the zero spangle hot galvanized steel sheet which are not removed even by optimizing plating and facility conditions. CONSTITUTION: In a method for manufacturing a zero spangle hot galvanized steel sheet, the method comprises the process of carrying out skin pass by adjusting reduction conditions so that an average roughness of the surface of a plating layer is 1.10 to 1.40 microns after plating the steel sheet, wherein the skin pass is performed with a reduction force of 200 to 300 tons in case of a plated steel sheet having a thickness of the steel sheet of 0.3 to 0.7 mm and a width of the steel sheet of 1000 mm or less, the skin pass is performed with a reduction force of 300 to 400 tons in case of a plated steel sheet having a thickness of the steel sheet of 0.7 to 1.6 mm and a width of the steel sheet of 1000 to 1200 mm, and the skin pass is performed with a reduction force of 400 to 450 tons in case of a plated steel sheet having a thickness of the steel sheet of 1.6 mm or more and a width of the steel sheet of 1200 mm or more.

Description

Method of manufacturing a hot dip galvanized steel sheets with excellent surface appearance

The present invention is a method of manufacturing a hot-dip galvanized steel sheet having excellent surface appearance to prevent ripple marks, which are typical surface defects of zero spangle hot-dip galvanized steel sheet, and more specifically, the plating bath Al concentration. The mechanical rolling method, which is a mechanical method, is used to remove defects in the flow patterns which are not removed by the optimization of the plating conditions such as the distance between the air knife and the steel plate, the steel plate deposition temperature and the plating bath temperature and the steel plate vibration. The present invention relates to a hot-dip galvanized steel sheet manufacturing method having excellent flow appearance and eliminating defects in flow patterns.

Recently, due to advances in hot dip galvanizing technology, hot dip galvanized steel sheet is in demand as a high grade steel sheet such as steel sheet for home appliances and automobiles as well as for existing building materials. In particular, since the hot-dip galvanized steel sheet used for the exterior of automobiles is finally coated, it is required to have a zerospangle in which the spangles of the surface of the plating layer are not visually observed. There is a need for a beautiful surface appearance that does not occur. The flow pattern defects of the hot-dip galvanized steel sheet are small sag lines on the surface of the plating layer, and look like an ocean wave. In addition, flow pattern defects are surface irregularities due to the difference in thickness of galvanized plate, and have a certain pitch and amplitude. Therefore, the transfer pattern defects are transferred to the surface during painting, which not only hinders the surface appearance but also increases the surface roughness and decreases the glossiness. As this progresses, sunspots become dark with time, thereby inhibiting the surface appearance.

The cause of the flow pattern of the plating layer is generally 1) when the fluidity of the plating bath is not properly matched with the line speed, 2) when the steel plate vibrates during the solidification process after plating, and 3) an oxide film on the surface of the plating layer. This is a case where the fluidity difference between the molten zinc inside the overcoat layer becomes large. The prior art therefore concentrates research on minimizing or eliminating these factors.

In the known patent 97-77443, the Al concentration of the plating bath is managed to be high from 0.18 to 0.28% from the existing 018 to 0.20%, and the fluidity of the plating bath is improved by adjusting the plating conditions such as the plating bath temperature and the steel plate deposition temperature. In order to prevent the flow pattern by reducing the difference in fluidity between the oxide film formed on the surface of the plating layer and the plating layer. In addition, the distance between the steel plate and the air knife lip should be less than 5 mm to 7 mm, and the distance from the plating bath surface to the air knife lip should be less than 200 mm to less than 200 mm. The method of minimizing the vibration of the steel plate and thickening the arm thickness of the structure in the plating bath, or improving the wear resistance of the sleeve and the bush of the sink roll, The flow pattern was reduced by minimizing vibration and minimizing steel plate vibration by optimizing tension control between sink roll and top roll.

In addition, as in US Pat. No. 4,330,574, the pre-plating process is sealed with a chamber of an inert nitrogen atmosphere, and an air knife, which is a coating weight adjusting device, is placed in the chamber, and the wiping gas mainly uses high-purity nitrogen. It is used. This method maintains the molten zinc plated layer leaving the plating bath in an inert atmosphere to the upper part of the air knife to prevent oxidation of the surface of the plating layer, and prevents flow pattern defects by preventing the fluidity difference between the surface of the plating layer and molten zinc in the plating layer. Was intended.

However, the above-described prior arts optimize the plating conditions such as the plating bath Al concentration, the steel plate deposition temperature, the plating bath temperature, the distance between the steel plate and the air knife, the air knife height, and improve the wear resistance of the sleeve / bush of the sink roll and the sink roll and The prevention of flow pattern defects by optimization of equipment conditions such as tension control between the top rolls has some effects, but it is impossible to prevent them completely. In addition, according to the prior art, since the air knife after plating is sealed with an inert atmosphere, it is possible to prevent oxidation of the surface of the plating layer, but since it is oxidized in a section before entering the solution spraying zerospangle manufacturing apparatus, completely preventing the flow pattern It is impossible and economically undesirable because it presupposes additional equipment upgrades and modifications.

In addition, all of these prior arts are concentrated on the prevention of flow pattern defects generated during the plating process, and there is a problem that there is not yet a review of the generated flow pattern prevention method.

It is an object of the present invention to provide a method for removing a flow pattern defect of a zerospangle hot dip galvanized steel sheet that is not removed even by the optimization of plating conditions and equipment conditions as described above.

In order to achieve the above object, the present invention provides a hot-dip zinc with excellent surface appearance characterized by performing a temper rolling by adjusting the reduction force so that the average roughness of the surface of the plating layer is 1.10 to 1.40 µm after plating under ordinary plating conditions. It is characterized by providing a plated steel sheet manufacturing method.

1 is a view showing a change in flow pattern defect index according to the reduction in the rolling force of the temper rolling.

Figure 2 is a view showing a change in the degree of flow pattern generation according to the reduction in the rolling force of the temper rolling.

3 is a view showing a change in the average surface roughness of the coating layer according to the change in the rolling force of the temper rolling.

4 is a view showing a change in average gloss of the plating layer according to the change in the rolling force of the temper rolling.

5 is a view showing a change in the number of cracks generated over time according to the change in the rolling force of the temper rolling.

FIG. 6 is a diagram showing the appropriate temper rolling rolling force according to the steel plate width and steel plate thickness change; FIG.

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

The present inventors have studied how to eliminate the flow pattern defects remaining on the surface of the plating layer without being removed even by the optimization of plating conditions and equipment conditions in the plating process, and as a result, by appropriately adjusting the rolling conditions of temper rolling, It was confirmed that the flow pattern can be removed without affecting.

That is, the flow pattern formed in the continuous hot dip galvanizing process is a surface irregularities caused by the difference in thickness of the plated layer and has a constant interval and amplitude. Therefore, the surface irregularities of the work roll of the temper rolling are transferred to the surface of the plating layer, so that the spacing and the amplitude of the flow pattern are reduced, and the flow pattern defect can be completely removed under the appropriate temper rolling conditions.

In the case where the thickness of the steel sheet is 0.5 to 0.7 mm and the steel sheet width is 1200 mm, as shown in FIG. 1 and FIG. As the rolling force of rolling increased, the flow pattern defect decreased, and it was not visually observed when the rolling force was more than 200 tons.

In this way, the decrease in flow pattern defects as the reduction force of the temper rolling increases is determined by the following action. This is because as the rolling force of the temper rolling increases, the amount of moon roughness of the work roll transferred to the plating layer surface increases. In other words, when the rolling force of the temper rolling is small, only the portion where the height of the flow pattern is relatively high is pressed down by the work roll, and roll indentation is applied to the surface of the plating layer, but when the rolling force is high, the height of the flow pattern is relatively high. The low fine flow pattern is also pushed down by the work roll, so the flow pattern defect is judged to be removed.

In other words, as the rolling force of the temper rolling increased, the degree of flow pattern defects decreased drastically, and the flow pattern defects were not visually observed when the rolling force was 200 tons or more. However, at a rolling force of 300 tons or more, not only the flow pattern defect but also the normal plating layer is reduced, thereby damaging the plating layer, thereby affecting the surface quality of the plating layer.

This can be seen in Figure 3 showing the surface roughness change as the rolling force of the temper rolling increases. That is, the average surface roughness increases because the indentation of the work roll is transferred to the surface of the plated layer as the rolling force increases, but the average surface roughness shows a constant value when the rolling force is 300 tons or more. This means that when the rolling force is 300 tons or more, the roughness transfer rate of the work roll reaches a limit value. Therefore, the reduction force of 300 tons or more represents the same meaning as cold rolling instead of temper rolling on the surface of the plating layer.

Further, as shown in FIG. 4, which shows the surface glossiness of the plating layer according to the rolling force of the temper rolling, the surface glossiness decreases in proportion to the increase in the rolling force, and particularly, drops more than 100 at 300 tons or more. In addition, the crack of the plating layer is generated more than 70 per 10mm as shown in Figure 5 serves to promote the aging of the plating layer over time.

Therefore, it can be seen that the rolling force of the temper rolling of the flow pattern defect is eliminated and the plating surface quality is good is 200 to 300 tons. However, since the rolling force of the temper rolling varies depending on the roughness of the work roll, the thickness of the steel sheet, and the width of the steel sheet, there are various restrictions on the use of the temper rolling conditions. That is, since the degree of flow pattern defects depends on how much the indentation of the work roll transferred to the surface of the plating layer depends on the temper rolling conditions, it is preferable to indicate the degree of flow pattern defects by what the average surface roughness of the plating layer is. have.

Therefore, the average surface roughness of the flow pattern defect is removed in the present invention and the plating surface quality is excellent is 1.10 ~ 1.40㎛.

The rolling force of the temper rolling to give the average surface roughness of 1.10 ~ 1.40㎛ to remove the flow pattern defect on the surface of the plating layer is varied depending on the thickness and width of the steel sheet. That is, when the thickness of the steel sheet is 0.5 to 0.7 mm and the steel sheet width is 1200 mm, the rolling reduction force of the temper rolling is 200 to 300 tons. However, it can be seen that as the thickness and width of the steel sheet increases, the rolling force of proper temper rolling increases as shown in FIG.

The work roll for temper rolling used herein is a shot blast dull roll having an average roughness of 1.5 to 3 μm. However, the method of applying illuminance to the work roll is not only a shot blast roll, but also a roll processed by laser light (Laser Texturing), an electro-discharge texturing (EDT) moon roll, and an electron beam (Electron beam). Texturing, EBT) Any month processing rolls do not matter.

(Example)

Hereinafter, embodiments of the present invention will be described.

A cold rolled steel sheet having a thickness of 0.5 to 1.6 mm and a width of 900 to 1500 mm was used as a test piece. The test piece was immersed in an alkaline solution to be degreased, then heat treated at an annealing reduction temperature of 760 ° C., then cooled to 460 ° C., and then deposited in a plating bath having an Al concentration of 0.20% for 3 seconds to give a 150 g / m2 cross-sectional coating amount. The plated was successively passed through a solution spraying type zerospangle manufacturing apparatus to produce a zerospangle in which no spangles on the surface of the plated layer were visually observed.

After plating, the steel sheet was subjected to temper rolling using dahlol having an average surface roughness of 1.5 to 3 탆 under temper rolling conditions as shown in Table 1.

Surface roughness, surface gloss, plating layer hardness, number of cracks after time-lapse peeling test were measured for the surface quality of the produced zinc-plated hot-dip galvanized steel sheet. ) And the height measurement results and the degree of flow pattern defects are shown in Table 1 divided into five grades as follows.

-Grade 1: No flow pattern occurs

-Level 2: Flow pattern is minute and does not appear horizontally, but appears faint when tilted

-Level 3: Visual classification is possible, but the goal and floor are discontinuous

-Level 4: Visual classification is possible, and the goal and the floor are continuous.

-Grade 5: When visual classification is possible and the bone and floor are visible and can be confirmed by touch

As shown in Table 1, when temper rolling was performed so that the surface average roughness satisfies the range proposed in the present invention (Nos. 5 to 7, 11 and 14), there was no flow pattern defect and hot dip galvanized with excellent surface appearance. It can be seen that steel sheet can be produced. On the other hand, when the surface roughness is rolled so that the average surface roughness is out of the scope of the present invention, the flow pattern remains and is clearly observed by the naked eye, or the surface glossiness is lowered to 100 or less, and time-lapse peeling of the plating layer occurs.

No. Steel plate thickness (mm) Steel plate width (mm) SPM Condition Plating Quality Evaluation Flow pattern evaluation Remarks Rolling force (ton) Elongation (%) Average surface roughness (Ra, ㎛) Surface glossiness Plating layer hardness (Hk) Number of cracks over time (10/10 mm) Pitch (mm) Height (mm) Incidence (5: deterioration, 1: excellent) One 0.5 to 0.7 1200 0 0 0.55 145 44 15 4.2 2.85 5 Comparative example 2 " " 50 0 0.66 130 44 20 3.8 2.6 5 Example 3 " " 100 0.1 0.85 122 47 34 3.6 2.75 4 Comparative example 4 " " 150 0.23 0.95 115 49 42 3.3 2.4 3 " 5 " " 200 0.72 1.10 112 57 47 2.9 0.75 2 Inventive Example 6 " " 250 1.53 1.20 114 60 48 2.2 0.70 One " 7 " " 300 2.17 1.4 100 64 51 1.6 0.68 One " 8 " " 350 3.24 1.5 80 66 72 1.5 0.68 One Comparative example 9 " " 400 3.9 1.5 67 66 76 1.8 0.70 One " 10 <0.7 <1000 200-300 3.5 to 4.2 1.10 to 1.40 140-120 55-65 40-50 2.5 to 1.2 0.65 to 0.55 One Inventive Example 11 " " <200 <3.5 <0.95 > 140 <55 - > 2.5 > 0.65 4 Comparative example 12 " " > 400 > 4.5 > 1.45 <90 > 65 - <1.2 <0.55 One " 13 0.8-1.6 > 1200 400-500 3.8 to 4.8 1.10 to 1.40 130-100 55-65 45 to 60 2.2 to 1.6 0.70 to 0.50 One Inventive Example 14 " " <400 <3.5 <0.10 > 130 <55 - > 2.2 > 0.7 4 Comparative example 15 " " > 500 > 4.2 > 1.50 <80 > 65 - <1.6 <0.5 One "

As described above, it is possible to eliminate the flow pattern defects that are not removed even by adjusting the plating conditions and equipment conditions by performing temper rolling, which is a mechanical method. It has the effect that it is possible to produce high-quality steel sheet, such as for home appliances and automotive exterior.

Claims (4)

In the method for manufacturing a zero-span zinc hot-dip galvanized steel sheet, after the steel plate is plated, hot-rolled zinc having excellent surface appearance is subjected to temper rolling by adjusting the rolling conditions so that the average roughness of the surface of the plating layer is 1.10 to 1.40 µm. Method of manufacturing plated steel sheet. The hot-dip galvanizing with excellent surface appearance according to claim 1, wherein in the case of a plated steel sheet having a thickness of 0.3 to 0.7 mm and a steel sheet width of 1000 mm or less, temper rolling is performed at a rolling force of 200 to 300 tons. Steel plate manufacturing method. The plated steel sheet having a steel sheet thickness of 0.7 to 1.6 mm and a steel sheet width of 1000 to 1200 mm is subjected to temper rolling at a rolling reduction of 300 to 400 tons. Method of manufacturing galvanized steel sheet. The hot-dip galvanized steel sheet having excellent surface appearance according to claim 1, wherein the coated steel sheet having a thickness of 1.6 mm or more and a steel sheet width of 1200 mm or more is subjected to temper rolling with a rolling force of 400 to 450 tons. Manufacturing method.