JPH11106985A - Electroplating method of hot-rolled steel sheet with scale - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a plated steel sheet excellent in adhesiveness using directly electric plating equipment by executing cathodic electrolytic pickling of a hot- rolled steel sheet having a scale of iron oxide of a specific thickness in a pickling tank in the electric plating equipment to remove the scale and continuously executing the electric plating successively after being washed with water. SOLUTION: A hot-rolled steel sheet having a scale consisting of iron oxide of 0.1-5 μm thickness per one side, is drawn, preferably to impart a stain by repeated bending, etc., and to provide the scale with cracks of 1-1000 pieces/mm and then, the electrolytic pickling is executed in a pickling tank in electric plating equipment with a steel sheet as a cathode. The electrolytic pickling uses a solution containing 10-300 g/l sulfuric acid and is preferably executed under the condition of 40-80 deg.C in temperature, 50-500 A/dm<2> in current density and 300-3,000 C/dm<2> in quantity of electricity. Alternatively, the cracks of 10-1,000 pieces/mm are provided in the scale and the hot-rolled steel may be pickled with a solution containing 50-500 g/l hydrochloric acid at a temp. of 50-80 deg.C for 2-10 seconds. The descaled steel sheet is washed with water and then, successively continuously, electric-plated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for electroplating a hot-rolled steel sheet, and a hot-rolled steel sheet having a hot-rolled scale having a thickness of 0.1 μm or more and 5 μm or less per surface on a conventional continuous electroplating equipment. The present invention relates to a method for obtaining electroplating having excellent adhesion.

[0002]

2. Description of the Related Art Conventionally, hot-rolled steel sheets are stripped of hot-rolled scale in a dedicated pickling facility, cold-rolled in a cold-rolling facility, annealed in an annealing facility, and further temper-rolled, and supplied to the market as steel sheets. It has been. Most of the electroplated steel sheets have been subjected to Zn-based plating, Sn plating, and Cr plating on the temper-rolled cold-rolled steel sheet for the purpose of improving rust prevention and coating properties. In recent years, with the advance of steelmaking technology and the advance of hot-rolling technology, a steel sheet as hot-rolled has been used for electroplating. Most of the hot-rolled steel sheets conventionally used have been stripped of hot-rolled scale by pickling equipment, but recently hot-rolled steel sheets with scales that have a thin and densely controlled hot-rolled scale have been used. It has become.

[0003] As described above, plating has also been performed on hot-rolled steel sheets. However, in this case, the hot-rolled steel sheet is removed by a conventional dedicated pickling equipment, and then the cold-rolled steel sheet is plated. Most were plated on equipment. This is because the hot-rolled scale is made of iron oxide, and even when plated over this iron oxide, the plating adhesion is extremely poor, plating hardly adheres by hot-dip plating, and plating adheres by electroplating, but rubs. This was because there was only enough adhesion to peel off.

However, in recent hot-rolling equipment, a hot-rolled steel sheet with a thin scale made in the latest hot-rolling equipment can be controlled to 5 μm or less, which has conventionally been more than 5 μm. A method of hot dip galvanizing with a zinc plating facility has been developed (Japanese Patent Application No. Hei 8-13787).
No. 4). This is to control the condition of the annealing furnace attached to the continuous hot-dip galvanizing equipment, to reduce the scale of 5 μm or less to Fe in the annealing furnace, and to perform continuous hot-dip galvanizing on it. By directly hot-dip galvanizing, it is possible to omit the use of the conventional equipment dedicated to pickling, thereby enabling a significant cost reduction.

On the other hand, even if an attempt is made to electroplate a hot-rolled steel sheet with a scale, the electroplating equipment does not have an annealing furnace.
As far as the above is concerned, satisfactory adhesion plating cannot be obtained. Equipment dedicated to pickling has a large pickling capacity and can very easily remove scales of 5 μm or less, whereas electroplating equipment is equipped with a pickling apparatus in addition to a plating apparatus. Due to the small washing ability, it could not be removed even on a scale of 5 μm or less. For this reason, when manufacturing products that perform electroplating on hot-rolled steel sheets with scales, it is necessary to always use dedicated pickling equipment and electroplating equipment. Costs could not be reduced.

[0006]

As described above, even if the thickness of the hot-rolled steel sheet with the scale is set to 5 μm or less, the electroplating equipment must be removed by a conventional dedicated pickling equipment unless the scale is removed. Can not be plated with good adhesion. The present invention provides a method of manufacturing a hot-rolled steel sheet having a scale thickness of 5 μm or less and using a direct electroplating facility to produce a coated steel sheet having excellent adhesion by solving the problems in the prior art. Things.

[0007]

According to the present invention, a hot-rolled steel sheet having a scale made of iron oxide having a thickness of 0.1 μm or more and 5 μm or less on one side is converted to a cathode by a pickling tank in an electroplating facility. The present invention provides an electroplating method for removing scale by electrolytic pickling, followed by continuous electroplating after washing with water. Here, the electrolytic pickling is performed in an amount of 10 to 300 g.
/ L sulfuric acid solution at a temperature of 40 to 80 ° C,
Electricity of 300 to 30 at current density of 50 to 500 A / dm ²
An electroplating method of electrolytic pickling of 00 c / dm ² is preferred. Further, the scale before the electrolytic pickling is 1 to 100.
An electroplating method for introducing cracks of 0 / mm is preferable. In addition, a hot-rolled steel sheet having a scale made of iron oxide having a thickness of 0.1 μm or more and 5 μm or less per side, cracks of 10-1000 / mm were put on the scale, and the pickling tank in an electroplating facility was used. Provided is an electroplating method in which a solution containing 50 to 500 g / L of hydrochloric acid is used, and the scale is removed by pickling without electrolysis at 50 to 80 ° C. for 2 to 10 seconds, followed by continuous electroplating after washing with water. Is done.

[0008] In the conventional electroplating equipment, the steel plate is firstly cleaned with a degreasing device to remove dirt and oil on the surface, then cleaned with an acid pickling device to remove rust on the surface, and then subjected to predetermined plating with an electroplating device. The pickling equipment in the electroplating equipment has a pickling capacity enough to remove rust on the steel sheet surface and an extremely thin iron oxide film having a thickness of less than 0.1 μm.
A method of immersing a steel sheet in a 00 g / L hydrochloric acid solution or sulfuric acid solution is often used. Some pickling equipment in the electroplating equipment performs alternating electrolytic pickling by supplying electricity to the steel sheet so that the steel sheet has a positive polarity or alternately between a positive polarity and a negative polarity, but the current density is at most 20 A / dm ² or less. The amount of electricity flowing is also 1
As small as 00 c / dm ² or less, a scale of 0.1 μm or more and 5 μm or less cannot be removed.

The inventor of the present invention has devised a method of removing a scale of 5 μm or less even in an acid pickling apparatus of an electroplating facility, and has reached the present invention. In order to remove the scale of the hot-rolled steel sheet having a thickness of 0.1 μm or more and 5 μm or less, electrolytic pickling is performed in a sulfuric acid aqueous solution using the steel sheet as a negative electrode. Normally, when a metal is dissolved by electrolysis in an acidic solution, electrolysis is performed using the metal as a positive electrode. However, when a hot-rolled scale is electrolyzed with a positive electrode, the dissolution rate is slow and it takes a long time to remove the metal. By doing so, it was found that even a small pickling apparatus in an electroplating facility can be removed in a short time. It is considered that a large amount of hydrogen gas is generated by setting the scale side to the negative electrode, and the scale is removed in a short time by the stirring effect and the cavitation effect by the gas generation. The electrolytic pickling of the present invention preferably uses a 10 g / L to 300 g / L sulfuric acid aqueous solution. Hydrochloric acid is not preferable because toxic chlorine gas is generated. If the sulfuric acid concentration is less than 10 g / L, it takes a long time to remove the scale, which is not preferable.
If it exceeds / L, the scale removal property is sufficient, but the corrosion of the pickling apparatus itself increases, which is not preferable. More preferably, the concentration is 50 g / L to 200 g / L. The temperature range of the sulfuric acid solution is preferably from 40C to 80C. If the temperature is lower than 40 ° C., it takes a long time to remove the scale.
If the temperature exceeds 0 ° C., the removability of the scale is sufficient, but the corrosion of the pickling apparatus itself increases, which is not preferable. A more preferred range is 50C to 70C. Electrolytic pickling is performed using a hot-rolled steel sheet with a scale as a negative electrode and a current density of 50 A / dm ^{2 to} 500 A.
/ Dm ² . Current density is 50A / dm
^{A value of} less than ² is not preferable because it takes a long time to remove the scale, and a value of more than 500 A / dm ^{2 is} not preferable because the scale removal property is sufficient, but the equipment for flowing the current becomes large and the equipment cost increases. A more preferred range is 75
It is an ^{A / dm 2 ~200A / dm 2} . The flow quantity of electricity is preferably in the range of ^{300c / dm 2 ~3000c / dm 2} . The quantity of electricity is less than 300c / dm ² is insufficient removal of the scale, the etching of the steel plate as well but descaling property is sufficient becomes electrolysis time and a long time is increased at 3000c / dm ² than Not preferred. A more preferred range is 300 c / dm ^{2 to} 1000 c.
/ Dm ² .

The inventors of the present application have also newly found that if cracks are formed in the scale before electrolytic pickling, the removability of the scale is improved. This is presumably because cracks in the scale promote the cavitation effect of hydrogen gas. Cracks to the scale may be put in a facility different from the electroplating facility, or may be put in a device provided in the electroplating facility. The method of cracking the scale is not particularly limited, but the crack can be cracked by giving a tensile strain to the steel sheet or a strain caused by repeated bending. An electroplating facility having a tension leveler or the like before the pickling apparatus is advantageous because cracks can be formed in the electroplating facility. Cracks put on the scale are 1 to 10 per 1 mm length
A range of 00 is convenient. When the number of cracks is less than 1 / mm, it is not preferable because the effect of improving the pickling property by the cracks is small. When the number of cracks exceeds 1000 / mm, a great deal of labor is required to form the cracks. More preferably, it is 10-500 lines / mm. The crack is measured by observing the surface of a hot-rolled steel sheet having a scale with an electron microscope, measuring the number of cracks intersecting an arbitrary virtual line of 1 mm, and calculating the average.

In addition, the inventors of the present application have found that if cracks are formed in the scale, the scale can be removed even by immersion and pickling in a hydrochloric acid solution under specific conditions. In this case, the range of the number of cracks is 10 / mm to 1000 / mm. If the number of cracks is less than 10 / mm, it is not preferable because the effect of improving the pickling property by the cracks is small.
If the number exceeds 0 / mm, a large amount of labor is required to form a crack, which is not preferable. After cracking, the scale can be suitably removed by pickling with hydrochloric acid. At this time, the reaction is performed at a hydrochloric acid concentration of 50 g / L to 500 g / L. When the concentration of hydrochloric acid is less than 50 g / L, it takes a long time to remove the scale, which is not preferable. When the concentration exceeds 500 g / L, the scale removal property is sufficient, but the corrosion of the pickling apparatus itself is increased, which is not preferable. A more preferred range is from 100 g / L.
It is 300 g / L. Hydrochloric acid immersion temperature is 50 ℃ ～ 80 ℃
Perform within the range. If the temperature is lower than 50 ° C., it takes a long time to remove the scale, which is not preferable. The time for pickling with hydrochloric acid is 2 seconds to 1 hour.
A range of 0 seconds is preferred. If the pickling time is less than 2 seconds, the removal of the scale is incomplete, and if it exceeds 10 seconds, the etching of the steel sheet itself becomes larger than the removal of the scale, which is not preferable.

The hot-rolled steel sheet with scale used in the present invention may be a plate of a fixed length or a coiled hot-rolled steel strip. If necessary, the shape may be a shape other than a plate such as a bar. The electroplating step performed by the method of the present invention includes:
The present invention can be applied to any electroplating usually performed on a steel sheet surface, and is not limited. For example, Zn plating, Z
Zn alloy plating such as nNi, Ni plating, Cr plating,
The plating may be Sn plating, Fe plating, alloy plating thereof, or plating in which alloying treatment is performed after electroplating, and may be single-layer or multilayer plating.

[0013]

An embodiment of the present invention will be described. Hot-rolled steel sheets having various thickness scales were pickled under the conditions shown in Table 2, washed with water, and subsequently electroplated as shown in Table 1. After the plating, the steel sheet is bent according to JIS G3313 electrogalvanized steel sheet and the strip bending test of the plating specified in the steel strip (so that a test piece with a tape attached in advance to the compression side is adhered), and is bent by 0T. -The amount of plating adhered to the tape at the bent portion after bending and stretching was measured by fluorescent X-rays for the Zn count number for Zn-based plating, the Sn count number for Sn plating, C
In the case of r plating, it was obtained as a Cr count number and used as an index of plating adhesion.

The cracks were introduced into the scale by applying elongational strain to the scaled hot-rolled steel sheet using a tensile tester. The number of cracks was observed with a scanning electron microscope, and the number of cracks per 1 mm was measured.
The average value of the measurements of 0 observations was used.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

The pickling conditions and plating adhesion are shown in Table 2.
According to the method of the present invention, 0.1 μm or more per side,
It can be seen that a hot-rolled steel sheet having a scale made of iron oxide having a thickness of 5 μm or less can be electroplated with good adhesion. According to the method of the present invention, a hot-rolled steel sheet with a scale can be directly manufactured into an electroplated product by using a conventional electroplating facility, and therefore has high industrial utility.

Claims (4)

[Claims] 1. A hot-rolled steel sheet having a scale made of iron oxide having a thickness of 0.1 μm or more and 5 μm or less per one side is electrolytically pickled in a pickling tank in an electroplating facility using the steel sheet as a cathode to remove scale. A method for electroplating a hot-rolled steel sheet with a scale, characterized by continuously electroplating after washing with water. 2. The method according to claim 1, wherein the electrolytic pickling is carried out at a sulfuric acid concentration of 10 to 300 g / L.
Using a solution containing acid, temperature 40-80 ° C, current density
50-500A / dm^Two300-3000 c /
dm ^TwoThe scale according to claim 1, which is electrolytic pickling.
Electroplating method for hot rolled steel sheet. 3. The scale before the electrolytic pickling is 1 to 1000
The method for electroplating a hot-rolled steel sheet with a scale according to claim 1 or 2, wherein a crack of (mm / mm) is formed. 4. A hot-rolled steel sheet having a scale made of iron oxide having a thickness of 0.1 μm or more and 5 μm or less per one side is subjected to cracking of 10-1000 / mm in the scale and pickling in an electroplating facility. Using a solution containing 50 to 500 g / L hydrochloric acid in a bath, pickling at 50 to 80 ° C. for 2 to 10 seconds to remove the scale, and continuously electroplating after washing with water; Method of electroplating hot rolled steel sheet