JP4919747B2 - Manufacturing method of hot-dip galvanized steel sheet with suppressed spangle pattern - Google Patents

Description

  The present invention relates to a method for producing a hot dip galvanized steel sheet having a beautiful and smooth surface.

  In the continuous hot dip galvanizing line, the plating base plate after reduction annealing is introduced into the plating pot, and after adjusting the adhesion amount of the hot dip metal adhering to the surface of the steel strip pulled up from the plating pot, air cooling and chemical spraying The hot dip plating layer is formed by solidifying with the same method. Crystal growth of metallic zinc occurs in the process of solidification from the molten state, and the spangle pattern formed at the grain boundary tends to appear in the hot dip galvanized layer. Since the spangle pattern restricts the use of the hot-dip galvanized steel sheet, a minimized spangle or zero spangle that is suppressed as much as possible is desired.

  Conventionally, a method of spraying a chemical solution such as dibasic ammonium phosphate or sodium dihydrogen phosphate on the hot-dip galvanized layer immediately before solidification has been adopted for forming the minimized spangle. In this method, if the timing of spraying the chemical solution varies, the remaining of spangles cannot be avoided, and processing unevenness tends to occur. In addition, since a highly corrosive chemical solution is used, the peripheral equipment is corroded and the working environment is deteriorated. If the chemical solution adhering to the peripheral equipment is separated and adheres to the hot dip galvanized surface, the quality deteriorates. Furthermore, a large burden is imposed on the waste liquid treatment.

If it becomes possible to suppress spangle patterns without using chemicals, it is expected that hot-dip galvanized steel sheets with a stable surface quality in a clean environment will be obtained. I came. For example, there is known a method of minimizing spangles by attaching scratches to a plating plate and refining metal zinc crystals crystallized from a liquid phase (Patent Document 1).
JP 59-197553

In Patent Document 1, the scratch wrinkle is attached by rubbing the surface of the plating original plate with a brush roll, a scotch roll, a belt sander or the like containing abrasive grains. It becomes. A linear wrinkle has a problem in uniform refinement of a spangle pattern, and generally, the effect of suppressing the spangle pattern in a direction orthogonal to the wrinkle is not sufficient.
It is presumed that the scratch wrinkle exhibits anisotropy with respect to the suppression of the spangle pattern as follows. A scratch wrinkle is a wrinkle generated along the rubbing direction of a brush roll, a scotch roll or the like, and normally extends in the longitudinal direction of the plating original plate. When molten zinc comes into contact with the surface plating plate with scratches and solidifies, the uneven distribution of the scratches, which is the starting point for the formation of zinc crystals, differs in the extending direction of the scratches.

In the extending direction of the scratches, the uneven distribution is dense and a large number of crystal nuclei are generated. However, in the direction perpendicular to the scratches, the frequency of generation of crystal nuclei decreases. As a result, although the spangle pattern is suppressed as a whole, the appearance tends to be different between the extending direction of the scratch ridge and the orthogonal direction.
The present inventors have studied various methods for replacing rubbing using brush rolls, scotch rolls, etc., in order to eliminate defects caused by directional scratch flaws. As a result, when roughening the surface of the plating plate with non-directional pickling or shot blasting, etc., it is possible to obtain a hot-dip galvanized steel sheet that suppresses spangle patterns uniformly and exhibits a surface state without anisotropy. I found it.

  The present invention is based on such knowledge, and by applying fine irregularities uniformly on the surface of the plating original plate by pickling, blasting, etc., the hot dip galvanized layer formed on the surface of the steel strip pulled up from the hot dipping bath An object of the present invention is to obtain a hot dip galvanized steel sheet that suppresses spangle patterns that tend to occur and has a beautiful and smooth surface.

In order to achieve the object, the present invention provides isotropic unevenness in which a blast treatment is performed and 30 or more peaks with a height from the center line of 3 μm or more are measured in a visual field of measurement of surface roughness of 1280 μm × 960 μm. The attached plating original plate is introduced into a hot dip galvanizing bath, pulled up from the hot dip galvanizing bath, and then cooled by air or mist at a cooling rate of 0.5 ° C./second or more.
After pickling treatment for 30 to 60 seconds with an acid solution having a concentration of 10 to 20% by mass, the number of peaks having a height from the center line of 3 μm or more is 37 or more in a measurement field of surface roughness of 1280 μm × 960 μm, etc. After introducing the plating original plate with anisotropic irregularities into the hot dip galvanizing bath and pulling it up from the hot dip galvanizing bath, it may be cooled by air or mist at a cooling rate of 0.5 ° C./second or more.

Effects and embodiments of the invention

  The spangle pattern, which is the solidification structure of zinc crystals, is generated and grown in the following process. In the solidification process after hot dip galvanizing, first, spangle nuclei are generated, and a primary dendrite arm is generated from the spangle nuclei, a secondary dendrite arm is generated from the primary dendrite arm, and a secondary dendrite arm is generated from the secondary dendrite arm. Will occur. The spangles stop growing when adjacent primary spangle primary dendrite arms collide with each other.

Assuming the spangle generation / growth process, a hot-dip galvanized steel sheet with a smooth and beautiful surface that suppresses the spangle pattern is generated by increasing the number of spangles by generating many spangle nuclei and increasing the number of spangles. can get. In this regard, in the present invention, the surface of the plating original plate is roughened with isotropic unevenness, whereby the surface state is modified to be suitable for promoting the generation of spangle nuclei.
The effect of roughening on spangle refinement is presumed as follows.

  For example, a fine dimple d is formed on the surface of the blasted plating original plate b (FIG. 2), and the surface area of the original plating plate b increases in the portion where the dimple d is present, and the adhered molten zinc m solidifies. Further, since the interfacial energy per unit area between the surface of the plating original plate b and the molten zinc m becomes small, the solid phase becomes stable and solidification nuclei are easily generated. That is, as the number of the depressions d increases, the number of spangle nuclei increases, so that the number of spangles per unit area increases, and a hot-dip galvanized layer in which spangles are refined and spangle patterns are suppressed is formed.

  Moreover, since the dimples dipped by pickling and blasting are uniformly distributed (isotropic) without being biased in either the rolling direction or the sheet width direction, the effect of spangle refinement by the dimples works evenly and rolling. The surface state where the spangle pattern is suppressed in both the direction and the plate width direction. In contrast, a directional scratch rod such as a brush roll cannot uniformly generate a large number of solidified nuclei on the surface of the plating original plate, so that a hot-dip galvanized layer with suppressed spangle patterns cannot be obtained.

  The present inventors quantitatively investigated and examined the influence of the depression d on the expression of the spangle pattern from such a viewpoint, and the height from the center line is 3 μm or more in the measurement field of surface roughness 1280 μm × 960 μm. It has been found that roughening with 30 or more peaks is effective. Hereinafter, the number of peaks is referred to as the number of peaks on the surface of the plating original plate or simply the number of peaks. By roughening the number of peaks: 30 or more, the spangle size is suppressed to 0.5 mm or less, and a surface state comparable to the minimized spangle obtained by conventional chemical spraying can be obtained, as will be seen in the examples described later. .

  The number of peaks is indicated by a value measured with a laser microscope (OLS 1200, manufactured by Olympus). Also, the surface of the hot-dip galvanized layer was observed with a microscope at a magnification of 50 times, three spangles in the 3 mm × 3.5 mm field of view were selected from the larger one, the primary dendrite arm length was measured, and the average value of the three fields of view As spangle size.

  The appropriate roughening can be adjusted by processing conditions such as blasting and pickling, but the cooling rate after being pulled up from the hot dip galvanizing bath also has a great influence on the spangle pattern. That is, coarse zinc crystals are generated as the cooling is slowly performed, and the dents of the crystal grain boundaries become larger, and a large spangle pattern appears. The faster the cooling rate, the finer the zinc crystals and the smaller the spangle pattern, but from the investigation and research results by the present inventors, the cooling rate is set to 0.5 ° C./second or more (preferably 3 ° C./second or more). As a result, the spangle size becomes 0.5 mm or less, and a hot-dip galvanized layer in a surface state in which a spangle pattern cannot be visually observed. A cooling rate of 0.5 ° C./second or more can be sufficiently achieved by air cooling, mist cooling, etc., and in combination with appropriate surface roughening, a hot and smooth surface hot dip galvanized layer is produced.

  The plating base plate is not particularly restricted by the steel type, but general low carbon Al killed steel or high strength steel added with Si, Mn, etc. can be used. The hot-rolled steel sheet descaled by hydrochloric acid pickling or sulfuric acid pickling has a roughened surface and is suitable as a plating base plate of the present invention. On the other hand, since a cold-rolled steel sheet obtained by cold rolling of a hot-rolled steel sheet exhibits a flat surface, it needs to be roughened by pickling, blasting or the like before the reduction treatment.

  The blasting includes air blasting in which an abrasive is injected with compressed air and shot blasting in which an abrasive is injected with an impeller. Steel-based abrasives, ceramic-based abrasives, and the like are used as the abrasives. As long as the roughened surface defined in the present invention can be obtained, the blasting method and the type of abrasive are not limited, but an abrasive having an average diameter of 0.1 to 1.0 mm is suitable. . In the polishing material whose diameter does not reach 0.1 mm, the unevenness given to the surface of the plating original plate is too small, and the roughened state necessary for spangle refinement is not obtained. On the other hand, in the abrasive having a large particle diameter exceeding 1.0 mm, the unevenness formed on the surface of the plating original plate is too large, and on the contrary, the appearance of the hot dip galvanized layer is impaired.

  In the pickling treatment, hydrochloric acid, sulfuric acid, nitric acid, etc., which are used for descaling hot-rolled steel sheets at low cost and easy to handle, are used for the pickling solution. The pickling treatment conditions are preferably determined under the conditions shown in Table 1 for each pickling solution used. If the conditions shown in Table 1 are used, the surface roughening can be performed in combination with descaling when the hot-rolled steel sheet is used as the plating base plate, and the required roughening state can be obtained even when the cold-rolled steel plate is used as the plating base plate. Can be achieved.

However, if the concentration of the pickling solution, pickling solution temperature, and pickling time do not reach the conditions shown in Table 1, the unevenness imparted to the surface of the plating original plate is small and the number of peaks is less than 30, and the spangle cannot be refined. On the contrary, the effect of roughening is saturated at the pickling solution concentration, pickling solution temperature, and pickling time exceeding the conditions shown in Table 1. Pickling can be performed on the pickling line, but if a pickling facility is installed on the inlet side of the hot dipping line, both descaling and surface roughening can be performed on hot-rolled steel sheets. It is suitable in terms of cost.
In hot dip galvanizing, the line speed, reduction annealing conditions, and the like are not particularly limited, and are appropriately determined according to the type, thickness, etc. of the plating original plate.

  The hot dip galvanizing bath suppresses the formation of a Fe—Zn alloy layer at the interface between the hot dip galvanized plating layer and the original plating plate and improves plating adhesion, so Al: 0.1 to 0.25% by mass It is preferable to add. Further, impurity elements such as Pb and Sb for growing spangles are preferably regulated to 0.01 mass% or less. The plating bath temperature and the original plate temperature during hot dip galvanizing are not particularly limited, and normal conditions are employed. Adjustment of the amount of plating can also be performed by a general gas wiping method.

  The cooling rate after hot dip galvanization is adjusted to 0.5 ° C./second or more. When the cooling rate is less than 0.5 ° C./second, spangles cannot be refined even when a plating original plate that satisfies the roughening condition is used. Cooling rate: As long as 0.5 ° C / second or more can be achieved, there are no particular restrictions on the cooling means, but air cooling with an air jet cooler, water mist cooling using a mist spraying device, etc. are adopted as suitable cooling methods it can.

Plated: Low-carbon Al-killed steel hot-rolled steel sheet with a thickness of 3 mm and a width of 1000 mm, which was descaled and roughened by pickling, and cold-rolled steel sheet cold-rolled to a thickness of 1 mm after pickling The original plate and two types of plating original plates were passed through a hot dipping line (FIG. 3).
The original plating plate 1 unrolled from the reel is degreased with an electrolytic degreasing bath 2, roughened with a pickling device 3 or a blasting device 4 attached to the inlet side of a hot dipping line, and then hot-dip zinc through a reduction annealing furnace 5 Introduced into plating bath 6. The hot dip galvanized steel sheet pulled up from the hot dip galvanizing bath 6 was cooled with an air jet cooler 7 and a mist spraying device 8. The operation specifications at this time are described below.

  Test pieces were collected from hot-dip galvanized steel sheets produced under each condition, and spangles were visually observed. Moreover, the surface of the plating layer was photographed with an optical microscope at a magnification of 50 times, and the spangle size was measured. When hot-rolled steel sheet roughened by pickling in Table 3 is used for the plating original sheet, cold-rolled steel sheet roughened by blasting is used in Table 4, and roughened by pickling in Table 5 The result at the time of using the made cold-rolled steel plate for a plating original plate is shown.

  In the present invention examples (test Nos. 1 to 24) of the hot-rolled steel sheet (Table 3), the number of peaks on the surface of the plating original plate is 30 or more, and the surface is modified to a surface suitable for refinement of spangles. Therefore, after hot-dip galvanizing, the hot-dip galvanized layer is solidified at a cooling rate of 0.5 ° C./second or more to form a spangle size of 0.5 mm or less. A plated steel sheet was obtained. On the other hand, as seen in Comparative Examples No. 25 to 36, when the pickling is insufficient, the number of peaks on the surface of the plating original plate is less than 30 and is not sufficiently roughened, and when the cooling rate is less than 0.5 ° C./sec, In the process, it was inevitable that spangles grew to 0.5 mm or more, and all became hot-dip galvanized steel sheets with spangle patterns.

  Even when the cold-rolled steel sheet is roughened by blasting (Table 4), the spangle size: 0.5 mm or less is sufficiently refined by setting the number of peaks to 30 or more as seen in Test Nos. 37 to 42. Thus, a hot dip galvanized steel sheet having a beautiful surface without spangle pattern was obtained. However, even if the number of peaks on the surface of the plating plate is 30 or more, in test Nos. 43 and 44, which were slowly cooled to 0.1 ° C./second after hot dip galvanizing, the spangle size exceeded 0.5 mm, and the spangle pattern was A hot-dip galvanized layer having a surface that was visually observed was formed. Furthermore, even in Test Nos. 45 and 46 where the number of peaks on the surface of the plating original plate was less than 30 and the roughening treatment was insufficient, spangles grew to a size exceeding 0.5 mm, and were hot-dip galvanized steel sheets with spangle patterns. .

  When cold-rolled steel sheets are roughened by pickling (Table 5), the number of peaks on the surface of the plating plate: 30 or more, and in test Nos. 47 to 70 modified to a surface suitable for spangle refinement, hot dip galvanization Thereafter, the hot dip galvanized layer was solidified at a cooling rate of 0.5 ° C./second or more to obtain a spangle size of 0.5 mm or less, and a hot dip galvanized steel sheet having a beautiful surface without a spangle pattern was obtained. Also, from the SEM image (Fig. 4) of the surface of the cold-rolled steel sheet (test No. 53) roughened by pickling, isotropic unevenness is formed on the surface of the roughened plating sheet. Can be confirmed. On the other hand, as seen in Comparative Examples Nos. 71-82, due to insufficient pickling, the number of peaks is less than 30 and insufficient surface roughening, or a slow cooling rate below 0.5 ° C./sec after hot dip galvanization. When it was cooled, the spangle grew to a size of 0.5 mm or more, and a hot-dip galvanized layer in which the spangle pattern was visually observed was formed.

  As described above, the surface of the plating base plate roughened by pickling, blasting, etc., has a fine function that functions as a nucleation site for zinc crystals when molten zinc solidifies to form a molten galvanized layer. Unevenness is evenly dispersed. Therefore, the galvanized steel sheet which suppresses the coarse growth of zinc crystals, and thus the spangle pattern, and exhibits a beautiful and smooth surface state can be obtained. And since it is not the method by chemical spraying, the deterioration of work environment, the burden of waste liquid processing, the fall of plating quality, etc. are eliminated, and a high quality hot dip galvanized steel sheet can be provided inexpensively with high productivity.

Optical micrograph explaining spangle of hot dip galvanized layer, spangle core, primary dendrite arm, secondary dendrite arm Schematic diagram explaining that spangle patterns are suppressed when a hot-dip galvanized layer is formed on the surface of a plating original plate that has been recessed by blasting. Schematic of hot dip galvanizing line embodying the present invention SEM image showing that the irregularities formed on the plating plate surface are isotropic

Explanation of symbols

b: Plated original plate m: Molten zinc d: Dimple
1: Plated base plate 2: Electrolytic degreasing bath 3: Pickling device 4: Blasting device 5: Reduction annealing furnace 6: Hot dip galvanizing bath 7: Air jet cooler 8: Mist spraying device

Claims (2)

Introducing a plating original plate height from the center line gave a isotropic irregularities that peak is 3μm or more is 30 or more in the measurement visual field 1280μm × 960μm on the surface roughness blasted in molten zinc plating bath Then, the steel sheet is pulled out from the hot dip galvanizing bath, and then cooled at a cooling rate of 0.5 ° C./second or more, and is cooled by air or mist. A method for producing a hot dip galvanized steel sheet with suppressed spangle patterns. After pickling treatment for 30 to 60 seconds with an acid solution having a concentration of 10 to 20% by mass, the number of peaks having a height from the center line of 3 μm or more is 37 or more in a measurement field of surface roughness of 1280 μm × 960 μm, etc. A spangle pattern characterized by introducing a plating original plate with anisotropic irregularities into a hot dip galvanizing bath, pulling up from the hot dip galvanizing bath, and then cooling with air or mist at a cooling rate of 0.5 ° C./second or more. The manufacturing method of the hot dip galvanized steel plate with which the suppression was carried out .