JPH11293500A - Surface flaw removal method for hot rolled steel strip - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To remove the surface flaw of a hot-rolled steel strip without using acids, improve the yield, and increase the commercial value of the product. A method for removing surface flaws of a hot-rolled steel strip, in which after the hot-rolled steel strip is descaled, anodic electrolysis is performed in an electrolyte in which the solvent is water and the solute is mainly a chloride.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for removing surface defects of a hot-rolled steel strip by anodic electrolysis.

[0002]

2. Description of the Related Art Hot-rolled steel strips such as carbon steel are produced by hot-rolling slabs. Oxidized scale formed on the steel strip surface during hot rolling is pressed into the base metal by rolling rolls. , May cause surface flaws. Surface flaws
Since the steel strip after hot rolling is exposed by pickling the steel strip to remove oxide scale, the presence or absence of a flaw cannot be determined in a state where the oxide scale is attached.

Since pickling is performed for the purpose of removing (descaling) oxide scale on the surface of a steel strip, an inhibitor is usually added to the pickling solution so that only the scale is dissolved and the metal is not dissolved. An acid pickling solution is used. for that reason,
A steel strip having surface flaws generated by hot rolling has a flawed surface even after pickling, which significantly reduces the commercial value of the product. Also,
In particular, a steel strip having many surface flaws cannot be sold as a commercial product, and must be treated as scrap. As a result, manufacturing costs will increase.

[0004] As a general method of descaling a steel strip,
There is a method of immersion in about 10% hydrochloric acid, but recently, a descaling method by electrolytic treatment is also being studied.

For example, iron and steel edited by the Iron and Steel Institute of Japan,
S393 and S394 in Volume 67 (issued in 1981)
The page discloses a method for descaling steel wires electrolyzed in aqueous sodium chloride or sodium sulfate solutions.

[0006] Materials and Processes, also edited by the Iron and Steel Institute of Japan, Vol. 7 (published in 1994), page 1428, discloses a method for descaling hot rolled steel sheets electrolyzed in dilute hydrochloric acid aqueous solution. .

JP-A-57-161097 discloses a method of descaling a steel wire which is electrolyzed in an acidic chloride aqueous solution.

However, all of these methods aim at efficiently removing the scale on the surface of the steel material, and are not useful for removing the surface defects of the steel strip after the descaling.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to remove the surface flaws of a hot-rolled steel strip without using acids which are harmful to the human body and the environment and have high danger, thereby improving the yield (orthogonality rate). It is an object of the present invention to provide a method for removing surface flaws, which can increase the production cost while increasing the commercial value of a product.

[0010]

The gist of the present invention relating to a method for removing surface flaws of a hot-rolled steel strip is as follows.

(1) A method for removing surface flaws on a hot-rolled steel strip, which comprises descaling the hot-rolled steel strip, followed by anodic electrolysis in an electrolytic solution in which the solvent is water and the solute is mainly a chloride. .

(2) The hot-rolled steel strip according to claim 1, wherein anodic electrolysis is performed by an indirect energization method using an electrode made of nickel metal as a negative electrode and an electrode made of metal titanium coated with an oxide as a positive electrode. Surface flaw removal method.

The present inventors have conducted various studies on a method of removing surface defects of a hot-rolled steel strip after pickling. As a result, when anodic electrolysis is performed in an aqueous solution containing chloride as a main component, the metal of the hot-rolled steel strip is ingot. The present invention was completed based on the finding that is dissolved.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The surface flaw removing method of the present invention will be described below in detail.

The hot-rolled steel strip to which the method of the present invention is applied is preferably a carbon steel strip, and a low-alloy steel (including structural alloy steel) containing about 1% by mass or less of an alloying element such as chromium. It can also be applied to such steel strips.

The anodic electrolysis is applied to the hot-rolled steel strip in order to melt the surface of the steel strip to reduce or eliminate surface flaws. In the cathodic electrolysis, the steel strip surface cannot be melted, and is not suitable for removing surface flaws.

The descaling of the hot-rolled steel strip before the anodic electrolysis is performed because the ingot of the steel strip cannot be melted even if the anodic electrolysis is applied to the hot-rolled steel strip to which the scale is adhered. is there.

Next, anodic electrolysis will be described.

As the electrolyte, an aqueous solution containing chloride as a main component is used as a solute. As the chloride, sodium chloride is recommended, but is not particularly limited thereto, and may be potassium chloride, magnesium chloride, calcium chloride, or the like.

It is not necessary to strictly control the chloride concentration in the aqueous solution, but if it is less than 1% by mass, the dissolution rate of the steel strip decreases, and at the same time, the electric resistance increases and the power loss accompanying electrolysis increases. Therefore, 1% or more is desirable. The higher the concentration, the higher the dissolution rate of the steel strip and the lower the electrical resistance. However, if the concentration is higher than the saturation concentration, supersaturated chlorides precipitate and the dissolution of the steel strip surface tends to be uneven, so the upper limit is 30 mass%. %. In addition, 30 mass% of chloride means 30 mass% of chloride and 70 mass% of water,
The same applies to the concentration of the electrolyte. Also, the solvent of the electrolytic solution,
All solutes are% by mass.

In addition to chlorides, salts such as sulfates and nitrates may coexist in an amount of less than 1%, but in order to increase the chloride concentration, an aqueous solution containing only chloride is desirable.

The pH of the aqueous chloride solution is less than 1 and 10
Above, the electrode is liable to be melted or deteriorated.
It is good to be in the range of. If the pH is 3 or more, iron dissolved by electrolysis may precipitate as hydroxide and adhere to the surface of the steel strip. Therefore, it is desirable to dissolve the precipitate with dilute hydrochloric acid or dilute sulfuric acid after electrolysis and then wash with water.

When the temperature of the aqueous chloride solution is lower than 10 ° C., the dissolution rate is low, and the precipitation of chloride is likely to occur. The dissolution rate is higher as the temperature is higher, but if it exceeds 90 ° C., the lining of the electrolytic cell is liable to deteriorate.

FIG. 1 is a diagram schematically showing an anodic electrolysis method by direct current supply. A method is recommended in which anodic electrolysis is performed between the counter electrode (negative electrode) 3 in the aqueous chloride solution 6 in the electrolytic cell 5 while supplying positive electricity to the steel strip 4 from the DC power supply 1 via the energizing roll 2. You.

FIG. 2 is a diagram schematically showing an anodic electrolysis method by indirect current supply. The anodic electrolysis may be a method in which electrolysis is performed by an indirect energization method in an aqueous chloride solution 6 in an electrolysis tank 5 without using an energizing roll. However, in this case, the anodic electrolysis is performed only when the steel strip passes near the negative electrode 3 to dissolve the steel strip surface, but the steel strip surface passing near the positive electrode 7 or between the positive electrode and the negative electrode does not dissolve. Therefore, it is necessary to lengthen the electrolytic cell accordingly.

The current density during electrolysis has a great influence on the dissolution rate on the steel strip surface, and the higher the current density, the higher the dissolution rate. When the value is less than 20 A / m ² , the dissolution rate is low, so that it takes a relatively long time to remove flaws, and
If it exceeds A / m ² , the generation of chlorine gas becomes remarkable.
A current density of about ²⁰ to 10000 A / m ² is recommended, but is not necessarily limited to this.

As the negative electrode used in the aqueous chloride solution, an electrode made of metallic nickel is recommended. This is because metallic nickel has almost no melting loss due to cathodic electrolysis in a chloride aqueous solution and is hardly corroded even when electrolysis is not performed.

Although stainless steel and titanium metal can be used, in the former case, pitting may occur during non-electrolysis, so that a negative current of about several A / m ² is passed to prevent the occurrence of pitting. It is desirable. In addition, when metal titanium is used, if the pH of the aqueous chloride solution is 3 or less, hydride of titanium is generated and becomes brittle, and the electrode may be damaged. .

On the other hand, as the positive electrode, an electrode made of metallic titanium coated with an oxide such as iridium is recommended. As the oxide to be coated, a mixture of an oxide of a noble metal such as platinum or osmium or an oxide of titanium other than iridium can be used. The oxide has an effect of preventing an increase in electric resistance due to anodic oxidation of the titanium surface.

As a method of inspecting the degree of surface flaws of the steel strip after pickling, visual judgment can be applied when the steel strip passing speed is relatively low. For this purpose, it is desirable to use a flaw inspection device to which a CCD camera or an image processing device is applied. In order to reduce the electrolytic treatment cost, it is preferable to inspect the degree of surface flaws of the steel strip after pickling, and to control the electrolysis conditions according to the degree. The controllable electrolysis conditions include the concentration, temperature, and immersion time of the electrolytic solution, and the dissolution rate can be easily adjusted by these.

As a method of dissolving the surface of the steel strip, for example, a method of immersing the steel strip in an aqueous hydrochloric acid solution can be considered. However, anodic electrolysis in an aqueous chloride solution according to the method of the present invention has a greater effect of removing flaws. The reason for this is not clear, but it is presumed that in the case of electrolysis, a large amount of current flows through the convex portions of the irregularities on the surface of the steel strip, so that the dissolution proceeds and the surface is smoothened promptly. Further, since the dissolution rate itself is considerably faster than the immersion treatment in the hydrochloric acid aqueous solution, there is an advantage that the treatment time can be shortened and the length of the treatment tank can be shortened. Since the chloride used in the method of the present invention is a neutral salt, not only is it less harmful or dangerous to the human body or the environment than acids such as hydrochloric acid, the amount of alkali required for waste liquid treatment is small,
There is an advantage that the amount of generated sludge is small.

[0032]

EXAMPLES Example 1 A hot rolled steel strip (thickness 3-5 mm) of two types of structural carbon steel and two types of low-alloy structural steel having the chemical compositions shown in Table 1 is 100 mm long and 150 mm wide. A test piece having a size of These contain 10% by mass of an inhibitor (trade name: IBIT) manufactured by Asahi Chemical Industry Co., Ltd.
The sample was immersed in a mass% hydrochloric acid (temperature: 80 ° C.) for 2 to 5 minutes to completely descaling. At this stage, the degree of the surface flaw of the test piece was visually evaluated in five steps. In the evaluation, 1 is the one with the most surface flaws, 5 is that no surface flaws are visually observed, 2 to 4 are the degree of surface flaws between 1 and 5, and the larger the numerical value, the higher the surface flaw. Indicates that there are few flaws.

After the flaws were evaluated, anodic electrolysis was performed in an aqueous solution containing a chloride such as sodium chloride, using the metal nickel plate as a counter electrode (negative electrode).

[0034]

[Table 1]

FIG. 3 is a schematic diagram of the experimental electrolysis apparatus used. The test piece having the above dimensions connected to the DC power supply 1 was immersed in the electrolytic solution 6, and electrolysis was performed with the metal nickel plate 8 facing the negative electrode on both sides of the test piece.

Eight types of electrolytes shown in Table 2 were used. Table 3 shows the electrolysis conditions.

[0037]

[Table 2]

[0038]

[Table 3]

As shown in Tables 2 and 3, as a comparative example, cathodic electrolysis in an aqueous solution containing chloride, electrolysis in an aqueous solution of sulfate and nitrate, and 10% hydrochloric acid at 80 ° C. containing no inhibitor. The degree of surface flaws when immersed was also examined.

After the electrolysis, the test pieces were washed with water and dried, and the degree of surface flaws was visually evaluated according to the same evaluation criteria as above in five steps.

Table 2 summarizes the dissolution loss due to electrolysis and the degree of surface flaws.

As is clear from Table 2, examples of the present invention in which anodic electrolysis was carried out in an electrolyte containing chloride as a main component (Test Nos. 1 to 4)
In No. 14), no surface flaw was observed after electrolysis. On the other hand, among the comparative examples, those subjected to cathodic electrolysis in a chloride electrolyte (Test Nos. 15 and 16: a negative current density indicates that cathodic electrolysis was performed) almost completely dissolves the steel strip surface. The degree of surface flaws was not different from that after descaling (ie, no surface flaws were removed).

In the comparative examples, when the electrolyte solution did not contain chloride (Test Nos. 17 to 20), the surface was hardly dissolved, and the surface flaw was not removed at all. In addition, after descaling, no inhibitor
When immersed in 0% and 10% hydrochloric acid for 60 seconds (Test No. 2
In Nos. 1 to 24), the surface was melted to some extent and the surface flaws were reduced, but were not as good as those of the present invention.

(Example 2) A steel strip having a width of 300 mm was cut out from the steel strips A and D shown in Table 1, and the descaling treatment and electrolysis were continuously performed using a continuous pickling-electrolysis apparatus for testing. I did it.

FIG. 4 is a side view schematically showing the four types of apparatuses used.

FIG. 4 (a) shows an electrolytic cell 13 of the type shown in FIG. 1 in which an aqueous solution of a chloride aqueous solution is placed after a hydrochloric acid immersion cell 12 for descaling (a negative electrode made of a current-carrying roll and metallic nickel in the electrolytic solution). ) Is disposed.

FIG. 4 (b) shows an electrolytic cell 14 of the type shown in FIG. 2 in which an electrolytic solution of a chloride aqueous solution is placed after a hydrochloric acid immersion tank 12 for descaling (a positive electrode made of iridium oxide-coated metal titanium in the electrolytic solution). And a metal nickel negative electrode).

FIG. 4C shows an apparatus in which a hydrochloric acid immersion tank 15 for removing surface flaws is arranged after a hydrochloric acid immersion tank 12 for descaling.

FIG. 4D shows an apparatus in which a treatment tank for removing surface flaws is not disposed after the hydrochloric acid immersion tank 12 for descaling.

The device shown in FIGS. 4 (a) to 4 (c)
Spray 16 for washing after descaling and after electrolysis
Is provided. In each device, the coil 18 is unwound, and the steel strip 11 continuously passes through each processing tank, and is wound up to form the coil 19.

Table 4 summarizes the descaling conditions, the electrolysis conditions after descaling, and the degree of surface flaws after the treatment. The evaluation criteria for surface flaws are the same as those in Table 2.

[0052]

[Table 4]

Tests Nos. 1 and 2 were conducted using an apparatus of the type shown in FIG. 4 (a). The length of the electrolytic cell for flaw removal was about 3 m, and the net length at which electrolysis was performed was 1.5 m. It is. Therefore, when the passing speed is 4.5 m / min, 20
In the case of 1.5 m / min, anodic electrolysis is performed for 60 seconds. Since the total current of the electrolysis is 900 A and the electrolysis area is 0.45 m ² , the current density is 2000 A / m ² .

Tests Nos. 3 and 4 were conducted using an apparatus of the type shown in FIG. 4B. The length of the electrolytic cell for flaw removal was about 5 m, and the net length for anodic electrolysis was 1. 5 m, so that the passing speed is 4. as in Test Nos. 1 and 2.
Anode electrolysis for 20 seconds at 5 m / min and 60 seconds at 1.5 m / min (current density: 2000 A /
m ² ) is performed.

Tests Nos. 5 and 6 were conducted using an apparatus of the type shown in FIG. 4C. The length of the immersion tank for removing flaws was about 5 m and the net immersion length was 4.5 m. Plate speed is 4.
The immersion treatment is performed for 60 seconds at 5 m / min and 180 seconds at 1.5 m / min.

Tests No. 7 and No. 8 used the apparatus of the type shown in FIG. 4D, and did not carry out the processing for removing flaws. The length of the immersion tank for descaling is shown in FIG.
(A) to (d) are 10 m and the net immersion length is 9 m. Therefore, when the passing speed is 4.5 m / min, the immersion time is 120 seconds, and when the passing speed is 1.5 m / min, the immersion time is 360 seconds. Processing is performed. As is clear from the experimental results in Table 4, in Comparative Examples of Test Nos. 5 to 8, the degree of surface flaw was 1 to 3
On the other hand, in the examples of the present invention in Test Nos. 1 to 4, the degree of surface flaws was all 5, and the surface flaws could be completely eliminated.

[0057]

According to the method of the present invention, it is possible to easily remove the surface flaw of the steel strip generated by hot rolling without using acids which are harmful to the human body and the environment and which are highly dangerous. Not only can the commercial value of the product be increased, but there is also a great industrial benefit, such as the yield of steel strip production (direct rate) can be increased and the production cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a steel strip anodic electrolysis apparatus using a current-carrying roll and a negative electrode in an electrolyte solution.

FIG. 2 is a schematic diagram of a steel strip electrolysis apparatus using a negative electrode and a positive electrode for indirect energization in an electrolytic solution.

FIG. 3 is a schematic view of an electrolysis experiment apparatus in which a cut plate test piece is directly energized.

FIG. 4 is a schematic diagram showing a configuration of a continuous descaling-surface flaw removal experiment apparatus.

[Brief description of reference numerals]

DESCRIPTION OF SYMBOLS 1 DC power supply 12 Hydrochloric acid tank for descaling 2 Electricity roll 13, 14 Electrolytic tank 3 Negative electrode 15 Hydrochloric acid tank 4, 11 Steel strip 16 Spray washing tank 5 Electrolytic tank 6 Electrolytic solution 7 Positive electrode 9 Test piece

Claims (2)

[Claims] 1. A method for removing surface flaws in a hot-rolled steel strip, comprising the step of descaling the hot-rolled steel strip, followed by anodic electrolysis in an electrolytic solution in which the solvent is water and the main component of the solute is chloride. 2. The hot-rolled steel strip according to claim 1, wherein the anode is made of nickel metal as the negative electrode and the anode is made of metal oxide-coated titanium as the positive electrode by anodic electrolysis by an indirect current applying method. Surface flaw removal method.