KR100290567B1 - Method for producing electrolytic galvanized steel sheets having superior surface quality and processability - Google Patents

Abstract

PURPOSE: A method for producing electrolytic galvanized steel sheets having superior surface quality and processability by further heating electrolytic galvanized steel sheet after plating at a temperature range of 250 to 450deg.C for 5-30 seconds. CONSTITUTION: The method for producing electrolytic galvanized steel sheets having superior surface quality and processability includes (i) performing electrolytic galvanizing on both sides of a steel sheet in carosel type zinc bath; (ii) heat treating the electrolytic galvanized steel sheet at a temperature range of 250 to 450deg.C for 5-30 seconds in order to slightly alloy surface texture of the electrolytic galvanized steel sheet.

Description

Manufacturing method of electrogalvanized steel sheet with excellent workability and surface quality

The present invention relates to a method for plating zinc in a carousel (Carosel) type electroplating bath, and more particularly, after the plating is completed by heating the steel sheet at a temperature of 250-450 ℃ for 5-30 seconds and excellent workability It is about the manufacturing method of the galvanized steel plate which obtains the plating film with a beautiful surface appearance.

Carousel-type electroplating bath is 2060mm long and 2440mm in diameter conductive roll is half immersed in the plating solution, and the steel plate is rolled around the conductive roll, when the electricity is applied, the plating is made in the steel plate part immersed in the solution. The conductive roll is formed of a metal conductor (band) so that only the middle 700 mm portion is energized, and the remaining portion is formed of synthetic rubber. Since the temperature of the plating solution is usually 60 ° C, the surface of the conductive roll is raised to this temperature, and as the work proceeds, heat is generated by the flow of current, causing the synthetic rubber to swell. If the rubber is excessively swelled, the rubber height becomes higher than the band, which is the conduction part of the conductive roll, and the contact between the steel plate and the band becomes uneven, and the current concentrates on the uneven contact part to generate Joule heat (Q) as shown in Equation 1 below. This causes a phenomenon in which the steel sheet is fused to the band of the conductive roll. This fusion not only causes a problem on the surface quality of the steel sheet but also causes a great damage to the conductive rolls, thereby reducing the productivity by reducing the input current or reducing the line speed.

Q = I ² Rt ......................... (1)

(Where Q: heat generated (joule, watts × sec), I: current (A), R: resistance (Ω), t: time of current flow (sec))

Since heat (Q) generated as in Equation (1) is proportional to the square of current (I) and is proportional to resistance (R) and time (t), concentration of current rather than resistance or time has the greatest effect on heat generation. Crazy Therefore, in order to suppress the temperature rise of the conductive roll, cooling water is flowed into the conductive roll, and pure water or plating solution is sprinkled so as not to dry between the steel sheet and the band. Also, considering that the rubber has a larger coefficient of thermal expansion than the metal conductor band, the band is about 0.7mm higher than the rubber at room temperature. It is composed. And the steel sheet is given a tension of 6-11 kg / mm ² according to the material and thickness so that the steel sheet and the conductive roll is in complete contact.

The standard electrode potential of zinc is -0.76V, and the base metal iron is -0.44V, so zinc is electrochemically more inferior to iron. Therefore, when iron and zinc form a galvanic battery, zinc is preferentially corroded than iron, and when zinc is plated on a steel sheet, zinc is first corroded to act as a sacrificial method. . However, since the crystal structure of zinc is composed of hexagonal dense lattice and the melting temperature is low at 460 ° C., the hardness of the metal is considerably lower than that of iron or zinc iron alloy. Therefore, it has a feature that is easily damaged even by small external stresses. This can be easily seen by looking at the marks (band marks) in which the plating layer is generated at the edge of the conductive roll band during the electroplating described above. The band of the conductive roll is required to have a very high melting temperature as well as wear resistance and acid resistance.

Therefore, since the hardness is also very high, it does not matter if it is in uniform contact with the galvanized layer, but since the steel sheet is wound and turned with high tension, it forms a pressed mark at the edge of the band. Carousel-type electroplating bath is composed of one conductive plate and several conductive rolls. If one band mark is formed each time one conductive roll passes, 10 band marks are formed through 10 conductive rolls. Since the formed band mark affects the normal plating layer color and glossiness and damages the appearance of the plating layer surface and is transferred as it is after coating, the quality of the product is degraded, which causes a lot of complaints from the demanders.

In addition, as described above, the zinc plated layer has a low hardness and is composed of a dense hexagonal lattice, and when the ambient temperature is lowered to below freezing, the galvanized layer is vulnerable, causing cracking and increasing friction during machining. It may result in the adsorbed zinc powder, which reduces the workability.

The band mark of the galvanized layer is largely removed in two ways. It is considered to reduce the tension of the steel sheet so that the band mark does not occur in the plating bath and to lower the band height of the conductive roll than the rubber. It causes more serious problems because it causes an arc spot that is fused to the band. Therefore, at present, the band mark is removed by pressing the steel sheet between rolls at a constant rolling reduction rate by a skin pass mill, but this method requires one process to be performed in a separate space. There are problems such as cost increase due to roughness and imbalance of the work process, and damage to the plating layer during the next process.

Therefore, the present invention is made to solve the problems of the prior art described above, after the plating is completed in the carousel-type electroplating bath, the steel sheet is heated at a constant temperature to slightly alloy the surface structure of the galvanized layer, the surface is beautiful An object of the present invention is to provide a method for producing an electrogalvanized steel sheet excellent in workability of the plating layer.

1 is a graph showing the change in the coefficient of friction of the surface of the steel sheet when the heating while changing the temperature while maintaining a constant heating time of 20 seconds for the galvanized steel sheet.

2 is a graph showing a change in the friction coefficient of the surface of the steel sheet after heating while changing the heating time while maintaining the heating temperature constant at 350 ° C with respect to the galvanized steel sheet.

Figure 3 (a) is a photograph of the surface texture of the pure galvanized steel sheet without heating,

Figure 3 (b) is a photograph of the surface structure of the electro-galvanized steel sheet heated in accordance with the present invention.

In order to achieve the above object, in the present invention, after the plating is completed in the carousel type electroplating bath, the steel sheet is heated at a temperature of 250-450 ° C. for 5-30 seconds to slightly alloy the surface structure of the galvanized layer, thereby making the surface beautiful and the plating layer. It also provides a method of manufacturing an electrogalvanized steel sheet excellent in workability.

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

The plating bath used in the present invention is a radial carousel type, and the technical gist of the alloying of the surface of the steel sheet is weakly alloyed by heating the obtained steel plate for 5-30 seconds in the temperature range of 250-450 ° C. The heating temperature of the plated steel sheet is limited to 250-450 ° C. because at 250 ° C. or less, alloying of the plated layer is not easy, and there is no change in the surface, so that the band mark cannot be removed and thus the workability does not improve. This is because the band mark is removed when heated to more than the temperature, but the plating layer is discolored and plating adhesion deteriorates due to excessive alloying.

The heating time is limited to 5-30 seconds because the alloying does not occur because it is too short when it is 5 seconds or less, so there is no effect on the removal of the band mark and workability, and when heated for 30 seconds or more, discoloration occurs due to excessive alloying and poor plating adhesion. Rather, it is counterproductive.

Hereinafter, the present invention will be described with reference to Examples.

Example 1

1 and 2 show the change of the surface friction coefficient of the galvanized steel sheet according to the heating temperature and time. The galvanized steel sheet used here was double-sided plated, and the coating amount of 20 g / m ² was used. The specimen size was cut to 400 mm in length and 40 mm in width to change the temperature and time in the infrared furnace, and after applying rust-proof oil, the coefficient of friction was obtained through the following formula (2) Was measured.

μ = frictional load / pressure = (F _D -R _D ) / π F _P ‥‥‥‥‥‥ (2)

(F _D : drawing load when using a fixed bead, R _D : drawing load when using a roller bead, F _P : pressing force when using a fixed bead)

1 is a result of measuring the friction coefficient while keeping the heating time constant at 20 seconds and changing the temperature only, and FIG. 2 is a result obtained by measuring the friction coefficient while maintaining the heating temperature constant at 350 ° C and changing the heating time. It is shown. As can be seen in FIG. 1, the unheated steel sheet has a very high coefficient of friction of 0.4 and gradually decreases with increasing heating temperature, showing a low coefficient of friction of 0.25 from a heating temperature of 250 ° C. Moreover, as can be seen from FIG. 2, it can be seen that the friction coefficient rapidly decreases when the heating time elapses for 5 seconds, thereby being excellent in workability. The reason for this phenomenon is that as the plating layer is heated, alloying of zinc-iron on the surface is slightly progressed, thereby increasing the surface hardness.

Example 2

3 shows the surface structure of the plated steel sheet heated at 350 ° C. of Example 1 for 20 seconds and the pure zinc plated steel sheet not heated at all. Figure 3 (a) is a picture of the surface texture of the pure galvanized steel sheet, Figure 3 (b) is a picture of the surface texture of the hot galvanized steel sheet, as the surface is alloyed by heating, the original pure zinc structure is seen to change Can be. Such change of the structure and alloying increases the hardness of the surface of the plating layer and further lowers the coefficient of friction, thereby improving workability. However, heating above 450 ° C. causes excessive alloying to discolor the surface and degrade plating adhesion.

Example 3

The presence or absence of a band mark, the peeling degree of the plated layer, and the surface friction coefficient were measured while changing the heating temperature and the heating time of the electrogalvanized steel sheet, and are shown in Table 1 below.

* Plating layer peeling: large amount ← × △ □ ◎ → None

In Table 1, the evaluation of plating adhesion was performed by cutting the steel sheet into 100 mm × 25 mm, cooling it for 10 minutes in an atmosphere of -5 ° C, and taking it out. The amount of the plated film which came out was visually observed and the most severe case was represented by x, and the most excellent case was represented by?. In addition, to determine the presence or absence of band marks, a galvanized steel sheet having a plating layer of 20 g / m ² was used on both sides of a thickness of 0.6 mm produced by an electroplating plant, and after the test was completed by cutting the band marks. The presence or absence of a band mark was visually determined. Surface friction coefficient was measured in the same manner as in Example 1.

In the case of the conventional example without heating the steel sheet, the band mark is clear, the peeling of the plating layer occurs a lot, and the surface friction coefficient is 0.4, which is very high, but as can be seen in Comparative Examples 1 to 6, the steel sheet is heated once. It can be seen that the surface friction coefficient decreases or the peeling amount of the plating layer decreases, and the band mark also disappears when the heating time is long regardless of whether the heating temperature is high or low. However, none of the comparative examples satisfies all three items.

All three items cannot be satisfied beyond a range of heating temperature and heating time as can be seen in the invention. In particular, even if the heating temperature is more than 450 ℃ or even if the heating time exceeds 30 seconds, the color of the surface of the coating layer is darkened by oxidation, resulting in a sudden poor plating adhesion.

Therefore, according to the manufacturing method of the electro-galvanized steel sheet of the present invention described above, the surface of the plating layer by the weak alloying on the surface of the steel sheet by heating the galvanized steel sheet for 5-30 seconds at the temperature of 250-450 ℃ range after completion of plating By increasing the hardness, workability is improved, and at the same time, the band mark generated in the plating bath is removed to obtain an electrogalvanized steel sheet with a beautiful surface appearance.

Claims (1)

Performing electro zinc plating on both sides of the steel plate in a carousel type electroplating bath; Process for producing an electro-zinc plated steel sheet having excellent workability and surface quality, characterized in that the galvanized plated steel sheet is immediately heated at a temperature of 250 to 450 ° C. for 5 to 30 seconds to slightly alloy the surface structure of the galvanized steel sheet. .