KR20020040437A - method of manufacturing a hot-rolled galvanized steel sheets with excellent surfaceappearance - Google Patents

Abstract

PURPOSE: A method for manufacturing a zero spangled hot dipped galvanized iron with superior surface quality is provided which has superior plating cohesion over a period of time lapsed even on a minimized spangle, and has neither flow marks nor pitting defects due to manufacturing of the minimized spangle. CONSTITUTION: The method for manufacturing a hot dipped galvanized hot rolled strip with superior surface quality comprises the steps of plating the pickled hot rolled strip by dipping a pickled hot rolled strip into a plating pot comprising 0.18 to 0.25 wt.% of Al, 0.01 wt.% or less of Pb, 0.05 to 0.075 wt.% of Sb and a balance of Zn and inevitable impurities; manufacturing a minimized spangle by spraying an ammonium hydrogenphosphate solution onto the hot rolled strip dipped into the plating pot just before the hot zinc is solidified; and skin passing the surface of the plating layer to a reduction force of 150 tons or more using a shot blast texturing dull roll.

Description

Method for manufacturing a hot-rolled galvanized steel sheet with excellent surfaceappearance

The present invention is a hot-dip galvanized hot-rolled galvanized hot-rolled zinc plated hot dip galvanized hot-rolled hot rolled steel (HGI) plated layer of the hot-rolled hot-rolled hot-rolled steel (HGI) plated surface defects, such as flow pattern, time-lapse peeling, pitting The present invention relates to a manufacturing method, and more particularly, to the surface quality of the hot-dip galvanized hot-rolled steel sheet by optimizing the components of the galvanizing bath and temper rolling conditions. It relates to a manufacturing method.

In general, since HGI uses hot rolled steel as a plating material, the demand for the surface treatment of steel is more economical than hot dip galvanized steel (CGI), which uses a conventional cold rolled steel as a plating material. In particular, since the HGI material, which is a thick post-plating material, is widely used for construction materials, the surface of the plating layer is exposed to the outside as it is, so that a beautiful surface appearance without surface defects is required. However, the HGI material, which is mainly a thick post-plating material, causes the flow pattern defects of wavy shape during solidification due to the flow of zinc in the molten state immediately after plating by high latent heat. In addition, the Regular Spangle (SR) and Mini Spangle (SM) materials are produced in the same plating bath using Pb-plated baths. In order to prevent pangling, a large amount of injection solution must be injected at high pressure in a semi-melt state, which causes pitting of white spots on the surface. In addition, there is a problem that the plating with time to peel off the plating layer during processing after a certain period of time by using a Pb-added plating bath.

Measures to improve the surface appearance of HGI materials include 1) adjustment of plating conditions in the plating process, minimization of vibration of the steel plate during the solidification process after plating, and prevention of flow pattern defects by preventing the formation of oxide film on the surface of the plating layer, and 2) in the plating bath. 3) prevents over-plating peeling due to Pb concentration limitation, and 3) prevents pitting defects by increasing the cooling rate of sprayed solution after plating. There is no comprehensive surface quality improvement plan.

That is, as in Korean Patent Application No. 1997-77443, the Al concentration of the plating bath is managed from 0.28% to 0.20% to 0.25% from the existing 0.18% to 0.20%, and the plating bath is adjusted by adjusting the plating conditions such as the plating bath temperature and the steel plate deposition temperature. And improving the fluidity of the plating layer to reduce the fluidity difference between the oxide film formed on the surface of the plating layer and the molten zinc inside 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 250 mm to 200 mm. Minimized vibration of the steel plate caused to reduce the flow pattern. Also, as in US Pat. No. 4,330,574, the entire 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 uses a high purity nitrogen. The molten zinc plated layer was kept in an inert atmosphere to the upper part of the air knife to prevent oxidation of the surface of the plating layer, and to prevent flow pattern defects by preventing the difference in flowability between the surface of the plating layer and the molten zinc in the plating layer.

Also, as in Japanese Patent Laid-Open No. 61-13371, in order to prevent intergranular corrosion caused by formation of localized cells of Pb and zinc, which are precipitated in the spangle grains, which cause plating over time, Sb is 0.02 to 3 By weight (hereinafter, referred to as%) or by limiting the Pb content in the plating bath to 0.007% or less to prevent over time plating peeling.

In addition, pitting defects were prevented due to the reduction of injection amount due to the improvement of endothermic reactivity of solution spray for producing SM materials.

However, the prior art 1) optimizes 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, and the air knife height, and seals the air knife after the plating with an inert atmosphere. Although it has some effect on the prevention of flow pattern defects by minimizing and preventing the oxidation of the surface of the plating layer, it is impossible to prevent it completely, and it is economically undesirable because it presupposes additional equipment supplement and modification. In the prior art 2) in which Sb is added instead of Pb of the plating bath, the Sb has a smaller surface tension than Pb, and thus, the spangle becomes larger during natural air cooling. Therefore, in order to make this SM, more injection quantity and larger injection pressure are required, so that the occurrence of fitting defects is further increased. In addition, simply limiting the Pb concentration encourages the generation of fine flow patterns on the surface of the plating layer due to the lack of fluidity, and above all, has the problem that it is impossible to produce SR materials. In addition, the prior art 3) to prevent pitting defects by increasing the cooling capacity of the spray solution is not preferable because the cost increase is greater than the cooling capacity improvement effect by containing expensive cooling components.

As such, the surface quality of HGI materials is not a fragmentary problem but a complex effect on each other. Therefore, the conventionally proposed method is to produce HGI materials with excellent surface quality without surface defects such as flow patterns, delamination over time, and pitting. It implies that it is impossible.

Therefore, the present invention has been made in order to solve the above problems, excellent in time-plated adhesion even in SM materials, and there is no flow pattern defect, excellent surface quality without the occurrence of fitting defects according to the manufacture of SM materials An object of the present invention is to provide a hot dip galvanized hot rolled steel sheet production method.

1 is a view showing the changes in plating adhesion over time according to the plating bath component changes.

Figure 2 is a view showing a change in flow pattern defect index according to the method of applying the rough rolling reduction force and the work roll roughness.

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

The gist of the present invention is that after the pickled hot-rolled steel sheet is plated by plating in a plating bath composed of Al 0.18 to 0.25%, Pb 0.01% or less, Sb0.05 to 0.075% and the balance of Zn and unavoidable impurities, the molten zinc is solidified. The SM material is prepared by spraying the aqueous ammonium hydrogen phosphate solution immediately before, and then hot-rolled hot-dip galvanized hot rolled surface is rolled using a shot blast texturing dull roll at a pressure reduction force of 150 tons or more. Steel sheet manufacturing is possible.

Here, the Al concentration of the plating bath is set to 0.18 to 0.25%. This is because the HGI material, which is a thick post-plating material, has a high latent heat, and thus, when the Al concentration in the plating bath is 0.18% or less, a Zn-Fe-based alloy layer, which is vulnerable to the interface between the base iron and the plating layer, is formed. do. In addition, when the Al concentration of the plating bath is 0.25% or more, dross generation is promoted, and formation of a thick oxide film on the surface of the plating layer facilitates the generation of flow patterns due to the fluidity difference between the surface layer of the plating layer and the molten zinc in the plating layer.

The Pb concentration of the plating bath is made 0.01% or less. This is because over time peeling occurs when the Pb concentration of the plating bath is 0.01% or more, as shown in FIG. Since Pb has almost no solid solution with Zn at room temperature, Pb precipitates mostly at the grain boundaries of zinc to form a local battery with Zn, causing grain boundary corrosion, and plating peeling occurs over time. Therefore, as shown in FIG. 1, the plating plating does not occur at any temporal acceleration time of 48 hours or less at the Pb concentration of 0.01% or less, which is allowed in the plating bath. Is occurring. In general, it is known that no cross-plating corrosion occurs for more than 1 year in a normal use environment unless plating plating peels off over time for 48 hours.

The Sb concentration of the plating bath is set to 0.05 to 0.075%. Unlike Pb, Sb does not cause the problem of delamination over time due to grain boundary corrosion as well as the segregation of the segregation as well as the entire coating layer, but when the Sb concentration is 0.05% or less, the spankle size of SR material is 5 ~ 10㎜. It is impossible to secure the size of SR material spangle because it is smaller than 10 ~ 15mm, which is the size of spangle in the case of natural cooling after plating in conventional Pb plating bath. Especially, when coexist with Pb of plating bath, Pb-Sb compound is formed. As a result, it precipitates at Pb and grain boundaries, and thus promotes grain boundary corrosion, resulting in poor plating adhesion over time, and serves to further promote the generation of flow patterns on the surface of the plating layer due to the fluidity difference between Pb and Sb. In addition, when the Sb concentration is 0.075% or more, the surface tension becomes very low, and the latent heat of the HGI material, which is a thick post-plating material, is added, resulting in an excessive flow pattern on the surface of the plating layer. Due to the need for the working liquid and greater spray pressure, many pitting defects are generated on the surface of the plating layer. Therefore, the Sb concentration is preferably 0.05 to 0.075%.

The plating in the component system of the present invention does not cause the problem of delamination over time, but the fine flow pattern that is not removed due to the high latent heat of the HGI material, which is a thick post-plating material, and the pitting defects generated during manufacturing of the SM material are mechanical methods. It was confirmed that it can be removed by optimizing the temper rolling operating conditions.

Here, the reason why the roughness providing method of temper rolling is limited to shot blast dull roll is as follows. As shown in FIG. 2, which shows the flow pattern defect occurrence rate according to the rolling reduction force of the temper rolling and the method of applying the work roll roughness, the shot blasting roll applied the roughness to the work roll was polished to the grindstone even under the same variation of the rolling reduction under the temper rolling. It can be seen that the flow pattern defect can be removed even at a lower rolling force than that of Britroll. This is because the shot blast roll is easier to remove flow pattern defects because the roughness transfer rate of the work roll transferred to the surface of the plating layer is higher than that of the bright roll having no roughness applied to the work roll.

Also, limiting the rolling force of the temper rolling to 150 tons or more is as shown in FIG. 2, which expresses the degree of flow pattern defect according to the rolling force of the temper rolling, and in the case of shot blasting roll, as the reduction of the temper rolling increases. This is because the pattern defect is reduced, and the rolling reduction is not visually observed at 150 tons or more. On the contrary, the bright trolley should be rolled to more than 250 tons to remove the flow pattern.

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 the amount of pull roughness transferred to the surface of the plating layer increases as the rolling force of the temper rolling increases. In other words, when the rolling force of the temper rolling is small, only the floor where the height of the flow pattern is high is pushed down by the work roll, and roll indentation is given. However, when the rolling force is high, even the fine flow pattern having the relatively low height of the flow pattern is applied to the work roll. It is assumed that this is because it is pressed down by.

In particular, Sb-added HGI materials do not cause plating plating over time, so it is possible to perform temper rolling at a high pressure of 300 tons or more, so that a large amplitude flow pattern can be completely removed, and fittings are formed on the valleys as well as the valleys. Even defects can be removed.

The method for applying roughness to the shot blasting roll or the work roll of the temper rolling work roll used here is not only shot blasting but also laser processing (Laser Texturing, LT) and discharge (Electro-Discharge Texturing, EDT) moon processing roll, electron beam (Electron beam Texturing, EBT) moon processing roll

Hereinafter, embodiments of the present invention will be described.

Example

After pickling a hot rolled steel sheet having a thickness of 2.3 mm, the plate was heated to a heating table temperature of 500 ° C. and a steel plate deposition temperature of 460 ° C., and then the plating bath temperature was 460 ° C., and the plating bath composition was adjusted as shown in Table 1 and Table 2 below. It was immersed for 3 seconds and plated so that the cross-sectional plating deposition amount was 150 g / m <2>. Thereafter, the plating layer was quenched or naturally air cooled to prepare SM and SR materials. In particular, the SM material was prepared by spraying a 2% ammonium hydrogen phosphate aqueous solution in the mist state immediately before solidification (semi-melted state). As a result of evaluation of the surface quality characteristics of the plated steel sheet plated in the plating bath component system as shown in Table 1, the temper rolling was applied to the bright roll (average roughness 0.3㎛) for the plated steel sheet (No.10 ~ 13) manufactured in the plating bath component system with excellent adhesion over time. ) And shot-brayrol (average roughness 2.3 ~ 2.5㎛) were used to perform temper rolling from 50 to 300 tons.

The surface quality characteristics of the zerospangle hot-dip galvanized hot-rolled steel sheet thus prepared were evaluated with time-plated adhesion, flow pattern defect, pitting defect, and plated layer hardness.

1) Evaluation of over time plating adhesion

Over time, the plating peeling property of the plating layer was corroded for 48 hours by accelerated grain boundary corrosion in a high temperature and high humidity atmosphere with a temperature of 95 ° C. and a relative humidity of 95% or higher, followed by the 0T-bend test and the evaluation of taping of the processed part (bending part). It was evaluated based on the time-based coating adhesion index.

Excellent ◎: When there is no peeling of the plating layer

Good ○: When a small amount of weight loss ratio is less than 5%

Normal □: When a small amount of weight loss ratio is more than 5% and less than 10%.

Poor △: Weight loss ratio is more than 10% and less than 30%

Extremely Poor ×: When weight loss ratio is over 30%

2) Flow pattern occurrence degree

The degree of flow pattern occurrence was evaluated based on the following criteria by visually observing the result of measurement of pitch and height of the flow pattern by the surface roughness meter and the degree of flow pattern defect.

Excellent ◎: When no flow pattern occurs

Good ○: When the flow pattern is minute and does not appear horizontally, but appears faint when tilted.

Normal □: Visual classification is possible, but bone and floor are discontinuous

Poor △: Visual classification is possible and valley and floor are continuous.

Extremely Poor ×: When visual classification is possible and the bone and floor are clearly visible and can be confirmed by touch

3) Pitting defect

The incidence of fitting defects was evaluated based on the following criteria by measuring the incidence fraction in the naked eye and image analysis.

Excellent ◎: If no occurrence of pitting

Good ○: If observed visually but occurs less than 10%

Poor △: Observed with the naked eye, but occurred more than 10% and less than 50%

Extremely Poor ×: Observed with the naked eye but with an area of more than 50%

As shown in Table 1, in order to satisfy the plating plating time-lapse of the plating layer, it is necessary to make Pb of a plating bath 0.01% or less and Sb to 0.05 to 0.075%.

In addition, it can be seen that in order to improve the surface quality of the plated steel sheet, it is necessary to rough roll the plated in the component system. That is, as shown in Table 2, when temper rolling was carried out so that the roughness providing method and temper rolling force of the temper rolling were within the range suggested by the present invention (Nos. 8 to 10), the plating adhesion was excellent over time, and the flow pattern It can be seen that there are no defects and fitting defects, and a hot-dip galvanized steel sheet having excellent surface appearance can be produced.

No Plating bath component (%) Temper Rolling Condition Surface quality characteristic evaluation Remarks Al Pb Sb Zn Roughness Rolling force (ton) Over time adhesion Flow Fitting defect One 0.20 0.098 - Remaining amount bright 50 × △ △ Comparative example 2 0.20 0.075 - Remaining amount bright 50 △ △ △ Comparative example 3 0.20 0.065 - Remaining amount bright 50 △ △ △ Comparative example 4 0.20 0.057 0.009 Remaining amount bright 50 △ × △ Comparative example 5 0.20 0.063 0.018 Remaining amount bright 50 × × × Comparative example 6 0.20 0.048 0.028 Remaining amount bright 50 × × × Comparative example 7 0.20 0.032 0.036 Remaining amount bright 50 × × × Comparative example 8 0.20 0.035 0.065 Remaining amount bright 50 × △ × Comparative example 9 0.018 0.068 Remaining amount bright 50 M △ × Comparative example 10 0.009 0.071 Remaining amount bright 50 ◎ △ △ Comparative example 11 0.004 0.071 Remaining amount bright 50 ◎ △ △ Comparative example 12 0.004 0.075 Remaining amount bright 50 ◎ △ △ Comparative example 13 0.004 0.108 Remaining amount bright 50 ◎ × × Comparative example

No. Temper Rolling Plating bath composition (%) Plating characteristic evaluation Remarks Roughness Rolling force PB Sb Zn Flow Pattern Index Over time adhesion Plating layer hardness Fitting defect One BRT 50 0.004 0.075 Remaining amount △ ◎ 50 × Comparative example 2 BRT 100 0.004 0.075 Remaining amount △ ◎ 48 × Comparative example 3 BRT 150 0.004 0.075 Remaining amount △ ◎ 47 △ Comparative example 4 BRT 200 0.004 0.075 Remaining amount M ◎ 52 M Comparative example 5 BRT 250 0.004 0.075 Remaining amount O ◎ 66 M Comparative example 6 SBT 50 0.004 0.075 Remaining amount △ ◎ 55 △ Comparative example 7 SBT 100 0.004 0.075 Remaining amount M ◎ 54 O Comparative example 8 SBT 150 0.004 0.075 Remaining amount O ◎ 60 ◎ Inventive Example 9 SBT 250 0.004 0.075 Remaining amount ◎ ◎ 61 ◎ Inventive Example 10 SBT 300 0.004 0.075 Remaining amount ◎ ◎ 64 ◎ Inventive Example

As described above, according to the present invention, the surface appearance is excellent by preventing plating over time by adjusting the plating bath components, applying a work roll roughness to temper rolling, and removing flow patterns and fitting defects by optimizing the rolling force. It has the effect that it is possible to manufacture hot-dip galvanized hot-rolled steel sheet,

In addition, according to the present invention, it is possible to economically produce HGI materials having excellent surface quality without surface defects such as plating peeling, flow pattern, pitting, etc. by over time adjustment of plating bath components and optimization of temper rolling conditions without additional equipment supplement or modification. It is possible.

In addition, there is an additional advantage of preventing black spots and black stools that become black due to damage of the plating layer during transportation or processing due to the increase in the hardness of the plating layer due to the addition of the plating bath Sb and the increase in the temper pressure.

Claims (1)

Plating the pickled hot rolled steel sheet by dipping in an plating bath composed of Al 0.18-0.25%, Pb 0.01% or less, Sb 0.05-0.075%, and the balance of Zn and unavoidable impurities; Preparing a SM material by spraying with ammonium hydrogen phosphate solution immediately before the molten zinc solidifies; A method of manufacturing hot-dip galvanized hot-rolled steel sheet having excellent surface quality, comprising the steps of: roughly rolling the surface of the plating layer using a shot blasting roll with a reduction force of 150 tons or more.