Mg-comprising hot-dip galvanized steel sheet manufacturing method and manufacturing apparatus

Abstract

The present invention relates to an apparatus and a method of manufacturing a hot dipped galvanized steel sheet that is excellent in corrosion resistance and has no linear defects, thereby being available for automobile bodies, household appliances, construction materials, and the like which need aesthetic surfaces. The apparatus for manufacturing a hot dipped galvanized steel sheet includes: a plating pot filled with a galvanizing bath for coating of a steel sheet; a sink roll; a wiping device adjusting the thickness of the coating on the steel sheet; a top roll; an oxidation process chamber; and an air cooling device. According to the present invention, after an excess molten coating solution attached to the steel sheet is evenly removed, an oxide film is made to be 0.1 μm to 0.3 μm thick before a coating layer starts to solidify. Thus, linear defects can be prevented.

Claims

1. A method of manufacturing a hot dipped galvanized steel sheet containing Mg, the method comprising: adjusting a coating amount by an air knife after a steel sheet is immersed in and removed from a galvanizing bath of a plating pot to form a galvanized steel sheet by passing a sink roll; forming an oxide film by oxidizing the galvanized steel sheet air-cooling the galvanized steel sheet having the oxide film; and passing the galvanized steel sheet over a top roll after the air-cooling; wherein the forming of the oxide film is performed by using a corona discharge ozone generator.

2. The method of claim 1, wherein the forming of the oxide film is performed by using an ozone generator spraying an aqueous solution containing hydrogen peroxide.

3. The method of claim 2, wherein the aqueous solution contains 0.01% to 1% hydrogen peroxide.

4. The method of claim 2, wherein the forming of the oxide film is performed for 0.5 seconds to 1.5 seconds in a chamber, wherein the steel sheet is inserted in the chamber at a temperature from 385° C. to 410° C. and the steel sheet is taken out from the chamber at a temperature from 380° C. to 400° C., and wherein an ozone concentration in the chamber is 1 ppm to 100 ppm.

5. The method of claim 1, wherein the forming of the oxide film is performed for 0.5 seconds to 1.5 seconds in a chamber, wherein the steel sheet is inserted in the chamber at a temperature from 385° C. to 410° C. and the steel sheet is taken out from the chamber at a temperature from 380° C. to 400° C., and wherein an ozone concentration in the chamber is 1 ppm to 100 ppm.

6. The method of claim 1, wherein the oxide film is 0.1 μm to 0.3 μm thick.

7. The method of claim 1, wherein the forming of the oxide film is performed for 0.5 seconds to 1.5 seconds in a chamber, wherein the steel sheet is inserted in the chamber at a temperature from 385° C. to 410° C. and the steel sheet is taken out from the chamber at a temperature from 380° C. to 400° C., and wherein an ozone concentration in the chamber is 1 ppm to 100 ppm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features, and other advantages of the present invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a schematic view illustrating a conventional apparatus for manufacturing a hot dipped galvanized steel sheet according to the related art;

(3) FIG. 2 is a photograph illustrating wave-patterned defects generated on a surface of a hot dipped Zn—Al—Mg alloy coated steel sheet manufactured by conventional nitrogen wiping;

(4) FIG. 3 is a photograph illustrating an enlarged view of a surface of a conventional linear defect portion observed with an electron microscope;

(5) FIG. 4 is a photograph illustrating an enlarged view of a surface of a linear defect portion observed with an electron microscope according to the present invention;

(6) FIG. 5 is a photograph illustrating an enlarged view of a cross section of the linear defect portion observed with an electron microscope according to the present invention;

(7) FIG. 6 is a graph of an example of measurement of oxide thickness on a coating surface using a glow discharge mass spectrometer according to the present invention;

(8) FIG. 7 is a graph of an example of measurement of oxygen concentration in a cooling chamber depending on a value of high voltage according to the present invention;

(9) FIG. 8 is a schematic view illustrating an apparatus for manufacturing a hot dipped galvanized steel sheet according to the present invention; and

(10) FIG. 9 is a front view and a side view of an oxidation process chamber according to the present invention, respectively.

DETAILED DESCRIPTION OF THE INVENTION

(11) Hereinbelow, an embodiment of the present invention will be described in detail with reference to accompanying drawings.

(12) An apparatus for manufacturing a hot dipped galvanized steel sheet according to the present invention is illustrated in FIG. 8. Referring to FIG. 8, the apparatus for manufacturing a hot dipped galvanized steel sheet includes: a plating pot 1 filled with a galvanizing bath 3 for coating of a steel sheet 8; a sink roll 2 turning the introduced steel sheet upward; a wiping device 4 adjusting the thickness of the coating on the steel sheet; an oxidation process chamber 5 in which the steel sheet passed the wiping device is oxidized; an air cooling device 6 cooling the oxidized steel sheet; and a top roll 7.

(13) FIG. 9 is a front view and a side view of the oxidation process chamber according to the present invention, respectively. Referring to FIG. 9, the oxidation process chamber 5 includes: a box-shaped chamber main body 9 through which the steel sheet passes a central portion of the main body 9; and a corona discharge ozone generator provided to face a front surface and a back surface of the steel sheet passing through the central portion of the chamber main body 9.

(14) Specifically, the corona discharge ozone generator includes: multiple tungsten wires 12 extending in the width direction of the steel sheet and facing the front surface and the back surface of the steel sheet passing through the central portion of the chamber main body 9; tungsten wire supporters 10 supporting opposite ends of the multiple tungsten wires 12; and a high voltage generator 11 applying a high voltage to the tungsten wires.

(15) In addition, multiple solution spraying nozzles 13 are provided in the width direction at a lower portion of the ozone generator in a manner facing the front surface and the back surface of the steel sheet passing through the central portion of the chamber main body 9.

(16) Here, the corona discharge ozone generator and the solution spraying nozzles may be provided together or may be separately provided and used individually.

(17) In general, Zn—Al—Mg alloy hot dip galvanizing is performed such that the steel sheet passes the plating pot at a temperature of 440° C. to 460° C. and is gas wiped to remove excess coating solution attached to the surface of the steel sheet and to adjust to a desired coating amount. Thereafter, the steel sheet is cooled to solidify the coating layer and kept being cooled so that the steel sheet is at a temperature of 300° C. or below when passing the top roll.

(18) Linear defects are not observed in the molten state of the coating layer, but appear after the end of solidification and generally visible. From this, linear defects are presumed to form when solidification proceeds. FIG. 4 is an example of a surface of a coating layer having linear defects observed with an electron microscope. The surface of the coating layer has wrinkles in a linear form, and when a cross section of the coating layer in which the line-shaped wrinkles are formed is observed with an electron microscope, the wrinkle height is as fine as about 0.2 μm as illustrated in FIG. 5. In other words, linear defects are defects in the surface of the coating layer which appear as lines in which wrinkles or curved lines are connected with each other due to solidification.

(19) The present inventors have found that no line defect occurs when a surface oxide film on the coating layer is 0.1 μm to 0.3 μm thick with respect to the molten metal containing Mg. Considering that the height difference of the wrinkles of linear defects is 0.2 μm, it is possible to prevent the occurrence of linear defects when the thickness of the surface oxide film proposed in the present invention is 50% to 150% of the wrinkle height formed by linear defects.

(20) The reason why no linear defect occurs when the surface oxide film is 0.1 μm to 0.3 μm thick by the surface oxidation is unclear, but it is presumed as follows.

(21) The surface oxide film starts to be formed from the wiping step, and when the thickness of the oxide film reaches a predetermined thickness, the oxide film blocks the contact between the molten metal and the air. After the oxide film is formed, solidification of the coating solution under the oxide film proceeds. The solidification of the coating layer progresses by the growth of metal phases varying depending on temperature interval. As solidification proceeds, the metal phases grow in the coating layer in the molten state so that a flow of fine molten metal occurs, and the oxide film formed on the coating surface moves by this flow whereby it is presumed that line defects having a height difference of about 0.2 μm occur.

(22) Considering that no linear defect occurs in air wiping and a thicker oxide layer is formed in air wiping compared with nitrogen wiping, it can be assumed that the thicker the oxide film, the less the effect of the flow and the tendency of linear defects occurring decreases.

(23) According to an experiment, no linear defect occurred in the case when the oxide film was at least 0.1 μm, and there was negligible additional effect at an increased thickness. However, in the case that the oxide film is thicker than 0.3 μm, when a post treatment such as coating with a chromate film or a coating with a Cr-free film is performed on the coating layer, there is a possibility that the characteristics of the film obtained by the post treatment may be changed, which is not preferable.

(24) When nitrogen wiping is performed at the time of hot dip coating, the molten coating solution is uniformly removed and an extremely thin oxide film being 0.1 μm thick or less is formed. In addition, once the oxide film is formed, the thickness of the oxide film does not significantly increase even after a lapse of time according to a conventional hot-dip coating method.

(25) Therefore, the present invention proposes a method that prevents the occurrence of linear stripe defects. Specifically, according to the method, after performing nitrogen wiping to remove the excess molten coating solution attached to the surface of the steel sheet, the oxide film is made to be 0.1 μm to 0.3 μm thick before the coating layer starts to solidify.

(26) According to the hot dip galvanizing method of the present invention in which a steel sheet immersed in a conventional Zn—Al—Mg-based galvanizing bath containing 1 wt % to 5 wt % of Mg and 1 wt % to 17 wt % of Al is removed from the plating pot, the coating amount is adjusted by nitrogen wiping to prevent wave patterned defects, and the steel sheet is cooled to a temperature of 300° C. or less while passing the top roll, oxidizing of the coating surface is required to be started at a steel sheet temperature of 385° C. or higher and terminated at a temperature of 380° C. or higher in order to perform the oxidation before the coating layer is solidified after nitrogen wiping.

(27) When the coating layer is solidified by cooling after wiping, although varying depending on composition of the coating layer, two or three or more phases among Zn single phase, Zn—Al binary eutectic, MgZn.sub.2 single phase, Zn—MgZn.sub.2 binary eutectic phase, and Zn—MgZn.sub.2—Al ternary eutectic phase may be formed in a mixed manner. The solidification starts at a minimum of 380° C., and the solidification is terminated when Zn—MgZn.sub.2—Al ternary eutectic phase is formed at around 340° C. In particular, Mg in the coating layer exists in the form of intermetallic compounds of MgZn.sub.2 or Mg.sub.2Zn.sub.11 and starts to be formed mainly at 380° C.

(28) According to an experiment carried out by the present inventors, it was confirmed that the surface oxidation of the coating layer is effective when the surface oxidation starts immediately after the nitrogen wiping and terminates before the coating layer starts to solidify. More precisely, the surface oxidation of the coating layer is required to be terminated before the intermetallic compound of Mg is formed. When the oxidation starts at 385° C. or below, a primary solidification phase may in progress and thus the effect of the oxidation is not sufficient. MgZn.sub.2 or Mg.sub.2Zn.sub.11 starts to be formed when the oxidation is carried out to a temperature of 380° C. or below. Therefore, oxidation of these intermetallic compound particles may occur, and thus black spots may appear.

(29) Generally, in the molten coating layer containing Al and Mg, the solidification initiation temperature varies depending on the composition. Therefore, it is safer to start the oxidation at a steel sheet temperature of about 410° C. after nitrogen wiping.

(30) In order to form the coating layer to be at least 0.1 μm to 0.3 μm thick, the present invention proposes a method in which the steel sheet passes through a chamber where ozone concentration is controlled.

(31) Ozone is contained in the atmosphere at about 0.4 ppm and is known as a strong oxidizing agent.

(32) When ozone concentration was less than 1 ppm, there is no oxidation effect by ozone, so an oxide film having a thickness of less than 0.1 μm was formed. In this case, linear defects occur. At or above 100 ppm, there is no effect on the product quality, but there is a risk that the ozone concentration around the equipment increases and the work environment deteriorates due to high concentration of ozone. In addition, the oxide film becomes 30 μm thick or more, which may change the post treatment characteristics.

(33) Therefore, the ozone concentration is preferably 1 ppm to 100 ppm.

(34) As a method of controlling ozone in the air cooling the steel sheet within the range of 1 ppm to 100 ppm, using a corona discharge ozone generator is most convenient way. For plate-formed steel sheets, wire-type corona discharge electrodes are preferably used to obtain uniform ozone concentration in the width direction. Particularly in this case, ozone generated by a corona discharge is moved to the steel sheet by electric force so that the surface of the coating layer can be more uniformly and effectively oxidized. A value of the high voltage for generating a corona discharge is determined by the thickness of wires and the fine surface roughness of wire surfaces. In the case that a large number of tungsten wires having thicknesses of about 0.2 μm to 0.3 μm are installed in the width direction of the steel sheet, a high voltage of −10 kV or higher is required to generate a corona discharge, and it is possible to control the ozone concentration within the range proposed in the present invention by adjusting the intensity of the high voltage.

(35) With respect to the generation of ozone, oxygen may be supplied in addition to air to increase the oxygen concentration in the cooling air, which may be more effective in generating ozone.

(36) In addition, it is possible to cool the steel sheet faster by installing nozzles on the rear side of the tungsten wires that draws air cooling the steel sheet, and by allowing the air sprayed from the nozzle to pass across the tungsten wires.

(37) In addition, as a method of oxidizing a coating surface, spraying an aqueous solution containing 0.01% to 1% of hydrogen peroxide toward the steel sheet starts when the steel sheet temperature is 385° C. or higher and terminates at a temperature of 380° C. or higher such that it is possible to prevent the occurrence of linear defects. When the sprayed aqueous solution comes into contact with the surface of the steel sheet, hydrogen peroxide in the solution acts as an oxidizing agent and promotes oxidation of the surface of the coating layer. When the concentration of hydrogen peroxide is less than 0.01%, the concentration is too low and the effect of preventing linear defects is insufficient. When the concentration of hydrogen peroxide is more than 1%, the oxidation of the surface of the coating layer occurs to a great extent whereby the thickness of the oxide film is excessively increased and thus the post treatment characteristics may be changed.

(38) It is also possible to oxidize the coating layer by spraying both the ozone-containing cooling air and the hydrogen peroxide-containing aqueous solution proposed in the present invention to the steel sheet.

(39) In addition, when the oxidation is performed for 0.5 seconds to 1.5 seconds, the oxide film having the thickness proposed in the present invention can be obtained. Even when the oxidation is performed for 1.5 seconds or more, the thickness of the oxide film is generally constant without being increased. It is preferable to perform the oxidation for one second. In a galvanizing plant continuously manufacturing galvanized steel sheets, the length of an oxidation treatment tank is required to be long to increase the oxidation time, which is costly to install. Therefore, when an oxidation tank having a length corresponding to about 1 second of the oxidation time at the maximum speed is provided, it is possible to manufacture galvanized steel sheets without linear defects.

(40) Hereinbelow, the present invention will be described in detail with reference to embodiments.

Embodiment 1

(41) A steel sheet with a thickness of 0.7 mm was immersed in a plating pot filled with a galvanizing bath containing Mg, Al, and the balance of Zn at a temperature of 450° C. The steel sheet was removed from the plating pot. Total coating amount attached to surfaces of the steel sheet was adjusted to 120 g/m.sup.2 by nitrogen wiping, and then an oxide film was formed.

(42) Table 1 shows an example of surface oxidation performed by passing the steel sheet through a chamber in which ozone concentration was controlled. To change the ozone concentration, tungsten wires with a diameter of 0.3 mm thick were disposed in parallel to each other in the width direction to face the steel sheet. A high voltage generated from a high voltage generator connected to the tungsten wires was applied to the tungsten wires to cause a corona discharge, and thus ozone was generated. At this time, the generated ozone was moved to the steel sheet by electric force and oxidized the surfaces of the coating layer in the molten state attached to the steel sheet. The ozone concentration was adjusted by controlling a value of applied high voltage. The ozone concentration was measured with an ozone meter generally used. FIG. 7 is a graph of an example of measuring the ozone concentration in the chamber depending on the value of high voltage. At −10 kV, there was no ozone generation because there was no corona discharge. However, when the high voltage was increased to a level of higher than −10 kV, the ozone concentration gradually increased. Then, the ozone concentration was rapidly increased at −20 kV or more, and 120 ppm of ozone was generated at −26 kV. In this embodiment, oxidizing of the coating surface was performed by varying the high voltage value according to the example of FIG. 7 and adjusting the ozone concentration.

(43) The surface of the solidified coating layer after oxidation was observed to evaluate occurrence of linear defects. “∘” is a state in which no linear defect is present, “Δ” is a state in which fine linear defects are present, and “X” is a state in which linear defects are observed clearly. A pyrometer was used to measure the temperature of the steel sheet in oxidation.

(44) In order to measure the oxide film thickness of the coating layer, oxygen concentration in the depth direction of the coating layer was analyzed by a glow discharge mass spectrometer. FIG. 6 is a graph of an example in which the coating thickness was converted from the obtained value of oxygen concentration in the depth direction of the coating layer. That is, as illustrated in FIG. 6, two trend lines were drawn based on an inflection point of an oxygen concentration measurement curve, and points where the trend lines meet were defined as oxide film thickness on the coating surface.

(45) In order to confirm the occurrence of black spots on the surface of the coating surface, galvanized specimens were stored for seven days under a condition of humidity of 85% and temperature of 85° C., and then it was determined whether black spots were formed on the surface. “∘” is a state in which no black spot was present, and “X” is a state in which black spots were present.

(46) TABLE-US-00001 TABLE 1 Composition of Oxidation Oxide galvanizing bath temperature Ozone film Black (wt %) (° C.) concentration thickness Linear surface Category Mg Al Zn Start Finish (ppm) (μm) defects spots on Comparative 1.5 1.5 Balance 410 385 0.4 0.07 X ◯ Example 1 Comparative 1.5 1.5 Balance 410 385 0.5 0.09 Δ ◯ Example 2 Comparative 1.5 1.5 Balance 383 380 100 0.06 X ◯ Example 3 Comparative 4 17 Balance 410 370 100 0.35 ◯ X Example 4 Comparative 3 3 Balance 380 365 50 0.06 X X Example 5 Example 1 1.5 1.5 Balance 410 385 1 0.12 ◯ ◯ Example 2 1.5 1.5 Balance 385 380 100 0.3 ◯ ◯ Example 3 4 17 Balance 410 400 40 0.2 ◯ ◯ Example 4 3.0 3.0 Balance 405 390 50 0.25 ◯ ◯

(47) The results will be described with reference to Table 1.

(48) Comparative Example 1 was not applied with high voltage. The ozone concentration in the chamber was as low as 0.4 ppm, the oxide film was as thin as 0.07 μm thick, and linear defects were observed.

(49) Comparative Example 2 was cooled within the temperature range proposed in the present invention. The ozone concentration was as low as 0.5 ppm, the oxide film was 0.09 μm, and linear defects were faintly visible.

(50) Comparative Example 3 was a case where the oxidation started at a temperature of 383° C. lower than the temperature proposed in the present invention. Although the ozone concentration was as high as 100 ppm, the oxide film was as thin as 0.06 thick μm and had a bad surface quality where linear defects were clearly visible.

(51) Comparative Example 4 was a case where the oxidation started at a temperature of 410° C. satisfying the temperature proposed in the present invention but the oxidation terminated at a temperature as low as 370° C. The oxide film was 0.35 μm thick, and no linear defect was present. However, a large number of black spots were observed in a humidity test. It is presumed that the black spots were generated by oxidation of Mg intermetallic compounds.

(52) Comparative Example 5 was a case where the oxidation started at a temperature as low as 380° C. and terminated at a temperature as low as 365° C. The oxide film was 0.06 μm thick, linear defects were clearly visible, and black spots were observed in a humidity test.

(53) Example 1 was a case where the cooling was started at a temperature of 410° C. and terminated at a temperature of 385° C. under a condition of the ozone concentration of 1 ppm. The oxide film was 0.12 μm thick. There was no linear defect and no black spot so that the surface quality was excellent.

(54) Example 2 was a case where the cooling was started at a temperature of 385° C. and terminated at a temperature of 380° C. under a condition of the ozone concentration of 100 ppm. The oxide film was 0.3 μm thick. There was no linear defect and no black spot so that the surface quality was excellent.

(55) Example 3 was a case where the cooling was started at a temperature of 410° C. and terminated at a temperature of 400° C. under a condition of the ozone concentration of 40 ppm. The oxide film was 0.2 μm thick. There was no linear defect and no black spot so that the surface quality was excellent.

(56) Example 4 was a case where the cooling was started at a temperature of 405° C. and terminated at a temperature of 390° C. under a condition of the ozone concentration of 50 ppm. The oxide film was 0.25 μm thick. There was no linear defect and no black spot so that the surface quality was excellent.

(57) It was confirmed from the results in Table 1 that when the coating surface oxidation, in which the steel sheet passes through the chamber in which the ozone concentration was controlled to be 1 ppm or higher and 100 ppm or below, started at a temperature of 385° C. or higher and terminated at a temperature of 380° C. or higher, an oxide film having a thickness of 0.1 μm to 0.3 μm proposed in the present invention was formed on the surface. Thus, the coating with an aesthetic appearance was obtained without linear defect, and it was confirmed that the thickness of the oxide film is mainly affected by the ozone concentration.

Embodiment 2

(58) A steel sheet with a thickness of 0.7 mm was immersed in a plating pot filled with a galvanizing bath containing Mg, Al, and the balance of Zn at a temperature of 450° C. The steel sheet was removed from the plating pot. Total coating amount attached to surfaces of the steel sheet was adjusted to 120 g/m.sup.2 by nitrogen wiping, and then an oxide film was formed. Then, surface oxidization of a coating layer was performed in which an aqueous solution containing hydrogen peroxide was sprayed on the surfaces of the steel sheet removed from the plating pot, and the steel sheet was cooled. Table 2 shows how much linear defects were present through observation of oxide film thickness and the surface of the solidified steel sheet were observed. A two-fluid spray nozzle spraying air and the solution together was used as a solution spraying manner. A spray pressure was 3 kg/cm.sup.2 for the air and 2 kg/cm.sup.2 for the solution.

(59) TABLE-US-00002 TABLE 2 Composition of Oxide galvanizing bath Oxidation H.sub.2O.sub.2 film Black (wt %) temperature (° C.) concentration thickness Linear spots on Category Mg Al Zn Start Finish (ppm) (μm) defects surface Comparative 1.5 1.5 Balance 410 385 0 0.07 X ◯ Example 6 Comparative 1.5 1.5 Balance 410 385 0.05 0.09 Δ ◯ Example 7 Comparative 1.5 1.5 Balance 383 380 0.1 0.08 Δ ◯ Example 8 Comparative 4 17 Balance 410 370 0.1 0.3 ◯ X Example 9 Comparative 3 3 Balance 410 385 1.2 0.4 ◯ ◯ Example 10 Note 1) Example 5 1.5 1.5 Balance 410 385 0.1 0.12 ◯ ◯ Example 6 1.5 1.5 Balance 385 380 1 0.3 ◯ ◯ Example 7 4 17 Balance 410 400 0.3 0.18 ◯ ◯ Example 8 3.0 3.0 Balance 405 390 0.6 0.24 ◯ ◯

(60) Note 1) When the galvanized steel sheet was chromated, the wettability between a chrome solution and the coating surface was poor, and thus the Cr film was in poor condition.

(61) The results will be described with reference to Table 2.

(62) Comparative Example 6 was cooled within the temperature range proposed in the present invention by blowing air. The hydrogen peroxide concentration was 0%, which means hydrogen peroxide was not added. The oxide film was as thin as 0.07 μm thick, and linear defects were visible.

(63) Comparative Example 7 was cooled within the temperature range proposed in the present invention. The hydrogen peroxide concentration was as low as 0.05 ppm, the oxide film was 0.09 μm thick, and linear defects were faintly visible.

(64) Comparative Example 8 was a case where the oxidation started using a solution containing 0.1% hydrogen peroxide at a temperature of 383° C. lower than the temperature proposed in the present invention. The oxide film was 0.08 μm thick and had surface quality where linear defects were faintly visible.

(65) Comparative Example 9 was a case where the oxidation started using a solution containing 1% hydrogen peroxide at a temperature of 410° C. and terminated at a temperature as low as 370° C. The oxide film was 0.3 mm thick, and no linear defect was present. However, a large number of black spots were observed in a humidity test. It is presumed that the black spots were generated by oxidation of Mg intermetallic compounds.

(66) Comparative Example 10 was a case where the oxidation started using a solution containing 1.2% hydrogen peroxide at a temperature of 410° C. and terminated at a temperature as low as 385° C. The oxide film was 0.4 mm thick. Although there was no linear defect and no black spot, the wettability between the chrome solution and the surface of the coating layer was poor when the steel sheet was chromated. Thus, a problem was observed in which the Cr film was unevenly formed.

(67) Example 5 was a case where the cooling was started using a solution containing 0.1% hydrogen peroxide at a temperature of 410° C. and terminated at a temperature of 385° C. The oxide film was 0.12 μm thick. There was no linear defect and no black spot so that the surface quality was excellent.

(68) Example 6 was a case where the cooling was started using a solution containing 1% hydrogen peroxide at a temperature of 385° C. and terminated at a temperature of 380° C. The oxide film was 0.3 μm thick. There was no linear defect and no black spot so that the surface quality was excellent.

(69) Example 7 was a case where the cooling was started using a solution containing 0.3% hydrogen peroxide at a temperature of 410° C. and terminated at a temperature of 400° C. The oxide film was 0.18 μm thick. There was no linear defect and no black spot so that the surface quality was excellent.

(70) Example 8 was a case where the cooling was started using a solution containing 0.6% hydrogen peroxide at a temperature of 405° C. and terminated at a temperature of 390° C. The oxide film was 0.24 μm thick. There was no linear defect and no black spot so that the surface quality was excellent.

(71) It was confirmed from the results of Table 2 that when spraying of the solution containing hydrogen peroxide concentration of 0.1% to 1% started at a temperature of 385° C. or higher and terminated at a temperature of 380° C. or higher, an oxide film having a thickness of 0.1 μm to 0.3 μm proposed in the present invention was formed on the surface. Thus, the coating with an aesthetic appearance was obtained without linear defect, and it was confirmed that the thickness of the oxide film is mainly affected by the hydrogen peroxide concentration.