JP2021087988A - LASER CUTTING METHOD FOR Zn PLATED STEEL SHEETS AND CUT PRODUCT - Google Patents

Abstract

To provide a laser cutting method for Zn plated steel sheets that can ensure the corrosion resistance of a cut surface, and a cut product of a Zn plated steel sheet that has a laser cut surface having excellent corrosion resistance.SOLUTION: A laser cutting method includes irradiating the surface of a Zn plated steel sheet with a laser beam and spraying an irradiation unit for the laser beam with an assistance gas for laser cutting. As the assistance gas, a mixed gas containing 5-20 vol.% oxygen is sprayed, to form a laser cut surface 1 having an oxide film 3 containing Fe3O4 with an average thickness 1.5 μm or less. The present disclosure also provides a cut product of a Zn plated steel sheet that has the laser cut surface 1 having the oxide film 3 containing Fe3O4 with an average thickness 1.5 μm or less.SELECTED DRAWING: Figure 1

Description

The present invention relates to a laser cutting method and a cut product of a Zn-based plated steel sheet.

Zn (zinc) -based plated steel sheets are used in automobile parts and home appliance parts in order to maintain corrosion resistance and durability. Further, when manufacturing a part having a complicated shape, a cutting processing method by laser cutting is often used. Laser cutting is a method in which the surface of a material to be cut is irradiated with a high-density laser beam, and the molten material to be cut is blown off with an assist gas to cut the material.

The surface of the Zn-based galvanized steel sheet is covered with a Zn-based plated metal having high corrosion resistance. However, since the base steel plate is exposed on the cut surface cut by the laser, red rust may occur and corrosion may proceed.

Therefore, it is necessary to perform repair painting at an early stage immediately after laser cutting. Conventionally, since the repair work takes time and labor, the workability of the cutting process is lowered, which has been a factor of increasing the cost. As described above, when the Zn-based plated steel sheet is laser-cut, there is a problem of ensuring the corrosion resistance and rust prevention of the cut surface. Therefore, there is a demand for a laser cutting technique capable of maintaining corrosion resistance on the cut surface after cutting.

The following prior arts have been reported regarding laser cutting methods for galvanized steel sheets. For example, Patent Document 1 describes a laser cutting method using a mixed gas containing 2 to 20% by volume of oxygen in nitrogen as an assist gas. By utilizing the heat of oxidation reaction of iron and oxygen, cutting is performed at a lower pressure than the conventional method of laser cutting under high pressure nitrogen gas, enabling cutting that reduces the amount of assist gas used. It is a thing.

In Patent Document 2, the plating layer-containing metal on the upper surface melted and / or evaporated by irradiation with laser light is flowed to the cut surface side of the plated steel plate by an assist gas or an auxiliary gas, and the plating layer-containing metal is applied to the cut surface. The laser cutting processing method (claim 1) for coating is described, and it is described that it is not necessary to perform the rust prevention treatment of the cut surface again after the laser cutting processing. Regarding the laser cutting conditions, the assist gas pressure should be adjusted in the range of 0.5 to 1.2 MPa (claim 3), and the laser cutting processing speed should be adjusted in the range of 1000 to 5000 mm / min (claim 4). It is described that a nitrogen gas or a mixed gas of 96% or more of nitrogen gas and 4% or less of oxygen gas is used as the assist gas (claim 6).

Patent Document 3 describes that in a laser cutting method for a Zn-based galvanized steel sheet, an oxide film is formed on the cut end face by using oxygen as an assist gas, and the film suppresses a decrease in rust prevention ability. (Paragraph 0007).

Japanese Unexamined Patent Publication No. 2001-335588 Japanese Patent No. 6238185 Japanese Unexamined Patent Publication No. 2014-237141

Patent Document 1 describes laser cutting using a mixed gas containing 2 to 20% by volume of oxygen in nitrogen. However, there is no description about the problem of deterioration of corrosion resistance on the cut surface of the Zn-plated steel sheet. Further, the examples of Patent Document 1 only disclose laser cutting performed at a cutting speed of 1.8 m / min or less.

Patent Document 2 describes a laser cutting method in which a plated metal is allowed to flow into a cut surface to improve end face rust prevention, and an oxide layer is formed on the cut surface by an assist gas containing oxygen (paragraphs 0125 to 0126). Is described. However, the composition of the oxide layer formed on the cut surface and the relationship between the composition and the corrosion resistance are not described.

Since the laser cutting method of Patent Document 3 uses an assist gas of 100% oxygen, the oxide film formed on the cut surface is relatively thick and brittle, so that peeling or cracking occurs and the cut surface is sufficiently prepared. Cannot be coated on.

Therefore, an object of the present invention is to provide a laser cutting method capable of ensuring the corrosion resistance of the cut surface in the laser cutting method for Zn-based galvanized steel sheets. Another object of the present invention is to provide a cut processed product of a Zn-based plated steel sheet having a laser cut surface having good corrosion resistance.

In the present invention, in order to solve the above problems, in a method of laser cutting a Zn-based plated steel plate, an oxide film having good corrosion resistance is formed on the laser cut surface by performing laser cutting based on specific cutting conditions. This has led to the completion of the present invention. Specifically, the present invention provides the following.

(1) The present invention is a laser cutting processing method of irradiating the surface of a Zn-based plated steel plate with a laser beam and injecting an assist gas at the irradiated portion of the laser beam to perform laser cutting. This is a laser cutting processing method in which a mixed gas containing ~ 20% by volume of oxygen is injected to form a laser cutting surface having an oxide film containing _{Fe 3} O _{4 having an average thickness of 1.5 μm or less.}

(2) The present invention is the laser cutting processing method according to (1), wherein the laser cutting is performed at a cutting speed of 2.0 to 8.0 m / min.

(3) The present invention is the laser cutting processing method according to (1) or (2), wherein the oxide film further contains FeO.

(4) The present invention is the laser cutting method according to any one of (1) to (3), wherein the Zn-based plated steel sheet is a Zn-Al-Mg-based plated steel sheet.

(5) The present invention is a cut product of a Zn-based plated steel sheet provided with a laser cut surface, and the laser cut surface has an oxide film containing _{Fe 3} O _{4 having an average thickness of 1.5 μm or less.} It is a cut processed product.

(6) In the present invention, the oxide film is the cut processed product according to (5), further containing FeO.

(7) The present invention is the cut processed product according to (6) _{, wherein the oxide film has an outer layer containing Fe 3} O _{4 and an inner layer containing Fe O.}

(8) In the present invention, the Zn-based galvanized steel sheet is a Zn-Al-Mg-based galvanized steel sheet, which is a cut processed product according to any one of (5) to (7).

According to the present invention, _{an oxide film containing Fe 3} O ₄ having a thickness of 1.5 μm or less is formed on the laser cut surface, so that the corrosion resistance and rust resistance of the laser cut surface are maintained at the initial stage after cutting. Can be done. Therefore, unlike the conventional method, it is not necessary to perform early repair coating after laser cutting, which is effective in reducing the work after cutting and reducing the manufacturing cost.

It is a figure for demonstrating the structure of the 1st pattern concerning the oxide film of a cut surface. It is a figure for demonstrating the structure of the 2nd pattern concerning the oxide film of a cut surface. It is a figure for demonstrating the thickness of the oxide film of a cut surface. It is a figure for demonstrating the thickness of the oxide film of a cut surface. It is a figure for demonstrating the test about the outdoor exposure test of an Example, (a) is a figure which shows the outline of the test, and (b) is a figure which shows the main part of the test apparatus.

Hereinafter, embodiments according to the present invention will be described. The present invention is not limited to the following description.

This embodiment is a laser cutting method in which the surface of a Zn-based galvanized steel sheet is irradiated with a laser beam and an assist gas is injected onto the irradiated portion of the laser beam to perform laser cutting. The surface of the Zn-based galvanized steel sheet is irradiated with the laser beam while the laser beam is moved to melt the substrate of the Zn-based galvanized steel sheet, thereby cutting and finishing into an arbitrary shape.

In this specification, "corrosion resistance" and "rust prevention" are used interchangeably, and the corrosion resistance of the cut surface may be referred to as "end face corrosion resistance". Further, the "laser cut surface" means a laser-cut cut surface, and may be simply referred to as a "cut surface".

[Assist gas]
In the laser cutting processing method according to the present embodiment, the assist gas is injected from the periphery of the cutting nozzle that irradiates the laser beam onto the surface of the Zn-based galvanized steel sheet. It is preferable to inject a mixed gas containing 5 to 20% by volume of oxygen as the assist gas. In the laser beam irradiation portion, laser cutting proceeds in an oxygen-containing atmosphere. Therefore, in the cut portion of the Zn-based galvanized steel sheet, the base steel sheet is exposed by cutting, while Fe of the base steel sheet and oxygen in the atmosphere chemically react with each other to form Fe ₃ O _{4 having corrosion resistance on the surface of the cut portion.} A containing oxide film is formed. Since the cut surface of the Zn-based plated steel sheet is imparted with corrosion resistance due to the oxide film, the effect of suppressing a decrease in end face corrosion resistance can be obtained.

When the mixing ratio of oxygen in the mixed gas is less than 5% by volume, the _{oxide film containing Fe 3} O ₄ is not formed on the entire cut surface, so that the deterioration of the end face corrosion resistance cannot be sufficiently suppressed. Therefore, it is preferable to use a mixed gas containing a predetermined ratio of oxygen as the assist gas of the present embodiment. The mixing ratio of the oxygen is preferably 5% by volume or more, and may be 8% by volume or more.

On the other hand, when the mixing ratio of oxygen in the assist gas exceeds 20% by volume, the thickness of the oxide film formed on the laser cut surface becomes excessive, and the oxide film tends to crack or peel off. Therefore, the coating state of the oxide film deteriorates, and an exposed portion of the base steel sheet is generated, which causes a decrease in end face corrosion resistance. Therefore, the mixing ratio of the oxygen is preferably 20% by volume or less, and may be 15% by volume or less.

As the assist gas, a mixed gas in which a predetermined amount of oxygen is mixed with nitrogen or an inert gas can be used. Air may be mixed as oxygen. Ar may be used as the inert gas. The flow rate and pressure of the assist gas can be appropriately set according to cutting conditions such as the thickness of the Zn-based galvanized steel sheet and the moving speed of the laser beam. Further, the assist gas also has an effect of blowing off the residue melted by laser light irradiation to facilitate cutting, and an effect of generating heat due to an oxidation reaction at the cutting portion to facilitate melting. For example, the pressure for injecting the assist gas can be 0.5 to 3.0 MPa.

[Cut speed]
The laser cutting method according to the present embodiment is preferably performed at a cutting speed of 2.0 to 8.0 m / min. The formation state of the oxide film on the cut surface differs depending on the degree of cutting speed. Since cutting is performed while moving the laser beam, the cut surface is cooled immediately after cutting. It is considered that the condition where the cutting speed is slow corresponds to the state where the cooling speed is slow by that amount, and the condition where the cutting speed is high corresponds to the state where the cooling speed is high by that amount.

When the cutting rate is slow, the cooling rate is slow, the oxidation reaction time between oxygen and the base steel sheet at the time of cutting becomes long, and the oxide film of FeO tends to grow thick. Further, since the cooling time is long, an inner layer containing FeO is formed between the inner layer and the base steel sheet, and an _{outer layer containing Fe 3} O ₄ tends to be formed on the surface side of the inner layer. The _{outer layer containing Fe 3} O ₄ has good corrosion resistance and is coated in close contact with the surface of the base steel sheet, so that the outer layer containing the Fe 3 O 4 is provided with the corrosion resistance required at the initial stage of the laser cut surface.

When the cutting speed is high, the cooling speed is high, the oxidation reaction time between oxygen and the base steel sheet is short, and the oxide film of FeO is difficult to grow. Further, since the cooling time is also shortened, the oxide film on the cut surface tends to have a structure in which _{Fe 3} O ₄ is precipitated inside the FeO phase and Fe O _{and Fe 3 O 4 are mixed.} Since the Fe ₃ O ₄ has good corrosion resistance, the mixed oxide film imparts the necessary corrosion resistance to the initial stage of the laser cutting surface.

When the cutting speed exceeds 8.0 m / min, the cooling rate becomes high and the distribution ratio of _{Fe 3} O _{4, which is rich in corrosion resistance, decreases on the surface side of the oxide film.} Further, since the oxide film is liable to crack or peel due to quenching, sufficient end face corrosion resistance cannot be ensured by the oxide film. Therefore, the cutting speed according to the present embodiment is preferably 8.0 m / min or less, more preferably 7.0 m / min or less. On the other hand, when the cutting speed is excessively slow, the oxide film on the laser cut surface grows thick, and the entire surface is almost occupied by FeO, making it difficult to form an outer layer containing _{Fe 3} O _4. In addition, the cutting time becomes long, which is disadvantageous in terms of work efficiency and economy. Therefore, the cutting speed according to the present embodiment is preferably 2.0 m / min or more, and more preferably 4.0 m / min or more.

[Oxide film]
In the laser cutting processing method according to the present embodiment, an oxide film containing _{Fe 3} O ₄ (magnetite) having corrosion resistance is formed on the surface of the laser cutting surface. Further, the oxide film may further contain FeO (wustite). In that case, there are the following two patterns as the structure of the oxide film containing _{Fe 3} O _{4 and Fe O.}

As shown in FIG. 1, the first pattern (hereinafter referred to as “first pattern”) has a structure in which the oxide film 3 formed on the base steel plate 2 has a mixed structure of _{Fe 3} O _{4 and Fe O.} Is. In the laser cut surface 1, a FeO film is formed on the base steel plate 2 after cutting, and then Fe ₃ O ₄ is precipitated in the _{FeO in the cooling process (during quenching), so that Fe 3} O ₄ and Fe O are formed. It is considered that a mixed structure of iron oxide was formed.

In the second pattern (hereinafter referred to as "second pattern"), as shown in FIG. 2, the oxide film 3 formed on the base steel plate 2 forms an outer layer 4 containing _{Fe 3} O _{4 and Fe O.} It is a structure having an inner layer 5 including the inner layer 5. After cutting, the laser cut surface 1 has an FeO film formed on the base steel sheet 2, reaches a temperature region below the transformation point in the cooling process (during slow cooling), and Fe ₃ O on the side of the FeO film that comes into contact with the outside air. _It is considered that four phases were formed.

In the laser cutting method according to the present embodiment, it is preferable to form an oxide film containing _{Fe 3} O ₄ having an average thickness of 1.5 μm or less from the viewpoint of adhesion to a steel substrate. Therefore, _{the average thickness of the oxide film containing Fe 3} O ₄ is preferably 0.5 μm or more, more preferably 0.7 μm or more, from the viewpoint of imparting end face corrosion resistance. On the other hand _{, if the average thickness of the oxide film containing Fe 3} O ₄ is excessive, the oxide film is cracked or peeled off, so that the cut surface cannot be sufficiently covered with the oxide film, and the end face is not covered. Corrosion resistance is reduced. Therefore, the average thickness of the oxide film is preferably 1.5 μm or less, more preferably 1.0 μm or less.

The average thickness of the oxide film according to the present embodiment is specified by a numerical value measured using an enlarged photograph of an SEM (scanning electron microscope) and an analysis result of EBSD (electron backscatter diffraction). Specifically, as shown in FIG. 3, a Zn-based plated steel sheet 6 having a length of 100 mm was laser-cut from substantially the center in the width direction toward the longitudinal direction. A cutting nozzle 12 for performing laser light irradiation and assist gas injection was attached to the tip of the cutting head 11 of the laser cutting processing apparatus, and the laser light 13 was irradiated from the center of the cutting nozzle 12. Assist gas was injected from around the laser beam. As shown in FIG. 4, the central portion of the cut surface 1 of the cut test material 7 after cutting, which has a length of 12 mm and is within a range of ± 6 mm from the central point in the longitudinal direction, was selected as the measurement region 14. In the measurement area 14, a sample for analysis was cut out, the center position in the plate thickness direction was observed by SEM, and the thickness of the oxide film was measured at 5 points. The average value was calculated from the maximum value and the minimum value among the obtained measured values, and the average thickness of the oxide film according to the present embodiment was specified by this average value. In addition, EBSD (Electron Backscatter Diffraction) analysis was performed on the same site.

The oxide film according to this embodiment preferably has a structure of a first pattern having an outer layer containing _{Fe 3} O _{4 and an inner layer containing Fe O.}

In the laser cutting processing method according to the present embodiment, the type of laser light to be irradiated is not limited, and a CO ₂ laser, a YAG laser, a fiber laser, or the like can be used.

As the conditions of the laser beam at the time of cutting, the laser spot diameter, the laser output, the pressure and the flow rate of the assist gas on the surface of the Zn-based galvanized steel sheet can be appropriately set depending on the thickness of the Zn-based galvanized steel sheet to be cut.

As the Zn-based plated steel sheet, one in which the Zn-based plated layer on the base steel sheet has a composition of Zn-Fe, Zn-Al, Zn-Al-Mg, Zn-Al-Mg-Si, or the like can be used. For example, a steel sheet having a Zn-based plating layer of Zn-6% Al-3% Mg in mass% can be used.

The Zn-based plated steel sheet is not limited. Those having a plate thickness of 0.5 to 6.0 mm can be used.

[Cut processed product]
The cut product according to the present embodiment is a Zn-based plated steel sheet provided with a laser cut surface that has been cut. The laser cut surface preferably has an oxide film containing _{Fe 3} O ₄ having an average thickness of 1.5 μm or less. _{Since it has an oxide film containing Fe 3} O ₄ having an average thickness of 1.5 μm or less, the base steel plate is not exposed on the laser cut surface, and rust prevention can be ensured for a certain period of time. Therefore, since it is not necessary to perform early repair painting after laser cutting, it is effective in reducing the work load and manufacturing cost after cutting.

The oxide film of the cut product is the same as the description of the oxide film in the above laser cutting method.

The cut processed product according to the present embodiment preferably contains _{Fe 3} O _{4 and Fe O in the oxide film.} Further, the oxide film preferably has a structure of the first pattern having an outer layer containing _{Fe 3} O _{4 and an inner layer containing Fe O.} Further, the Zn-based plated steel sheet is preferably a Zn-Al-Mg-based plated steel sheet.

Hereinafter, examples according to the present invention will be described. The present invention is not limited to the following description.

As the Zn-based plated steel sheet, a plated steel sheet having a total three-point average minimum adhesion amount of 140 g / m ² on both sides and a plating layer of Zn-6% Al-3% Mg (mass%) was used. The dimensions of the plated steel sheet before cutting were 40 mm in width × 100 mm in length, and two types of plate thicknesses of 1.6 mm and 3.2 mm were used. As shown in FIG. 3, the Zn-based plated steel sheet 6 was laser-cut from substantially the center in the width direction toward the longitudinal direction. As shown in FIG. 4, the test material 7 was obtained by cutting into dimensions of 20 mm in width × 100 mm in length under various cutting conditions. The test material 7 was subjected to an evaluation test described later.

As the laser cutting machine, a fiber laser device (manufactured by IPG) having a beam spot diameter of φ0.2 mm and a maximum output of 10 kW was used. A cutting nozzle having an inner diameter of φ2 mm was attached to the tip of the cutting head to irradiate the laser beam and inject an assist gas in the coaxial direction with the laser beam. The focal position of the laser was set on the surface of the steel sheet to be cut. The distance (standoff) from the tip of the cutting nozzle to the steel plate was set to 0.5 mm.

The following three cutting speeds were used. The numerical value in parentheses indicates the laser output at that time. 5m / min (output 1.25kW), 7.5m / min (output 2.0kW), 10m / min (output 2.5kW)

As the assist gas, four types of gas, a mixed gas of nitrogen gas and oxygen gas (N ₂ + O ₂ ), oxygen gas (O ₂ ), nitrogen gas (N ₂ ), and argon gas (Ar) were used. As the oxygen mixing ratio (volume%) of the mixed gas, 5%, 10%, 20%, 40%, 60%, and 80% were selected.

[Surface analysis of cut surface]
The film formed on the cut surface was analyzed using the test material after cutting. The analysis position corresponds to the measurement area 14 described above. Using a scanning electron microscope (SEM) and an attached energy dispersive X-ray analyzer (SEM-EDX), the film on the cut surface was observed and surface analyzed to confirm the formation of an oxide film of iron (Fe), and oxidation was performed. The average thickness of the film was measured. For the test material in which the formation of the oxide film was confirmed, EBSD analysis (crystal orientation analysis) was subsequently performed to analyze the structure of the oxide film.

[Evaluation test of corrosion resistance]
The corrosion resistance of the cut surface of the test material after cutting was evaluated. For the evaluation of corrosion resistance, an outdoor exposure test was conducted in accordance with JIS Z2381, and the period until the occurrence of predetermined red rust was investigated. In order to evaluate the corrosion resistance of the cut surface with a length of 50 mm that is not affected by disturbances such as the influence of water pools by the fixing jig, the evaluation surface was set to a range of ± 25 mm from the center point in the longitudinal direction of the test material. As shown in FIG. 5A, a test stand 15 having a surface inclined at an angle of 35 ° with the ground was installed outdoors, and the test material 7 was placed on the inclined surface. The cut surface 1 of the test material 7 was placed so as to be parallel to the inclined surface of the test stand 15. Specifically, as shown in FIG. 5B, the test material 7 was placed on the plate material having an inclined surface in the test stand 15 in which the plate materials were combined. The evaluation surface of the corrosion resistance of the cut surface was kept constant, and then the evaluation surface of the cut surface 1 was photographed every 5 days. When red rust occurred on the evaluation surface, the area of the red rust was measured by an image processing device, and the ratio of the red rust area to the entire evaluation surface was calculated. When the area ratio of red rust was less than 10%, it was judged to be good (◯), and when the area ratio of red rust was 10% or more, it was judged to be unsuitable (x). The test material judged to be unsuitable did not continue the subsequent measurement.

(Example 1)
Using a Zn-based galvanized steel sheet having a plate thickness of 1.6 mm, laser cutting was performed by changing the oxygen mixing ratio of the assist gas at a cutting speed of 7.5 m / min and an assist gas pressure of 1.0 MPa. The obtained test material was subjected to surface analysis of the cut surface and evaluation test of corrosion resistance according to the above procedure. The results are shown in Table 1. “○” and “×” in the outdoor exposure test indicate the results based on the criteria described above, and “-” indicates that the measurement was not performed.

[Result of surface analysis]
Test material No. 1-No. No. 7 uses oxygen as the assist gas. According to the results of surface analysis, it was confirmed that each element of iron (Fe) and oxygen (O) was detected on the surface of the cut surface, and that an iron oxide film was formed. Test material No. 8, No. In No. 9, nitrogen and argon were used as the assist gas, and according to the surface analysis, oxygen (O) was not detected on the surface of the cut surface, and an iron oxide film was not formed.

According to the SEM enlarged photograph and the EBSD analysis result, as shown in Table 1, the test material No. 1-No. The oxide film of No. 3 has an average thickness of 1.5 μm or less, and the test material No. 3 has an average thickness of 1.5 μm or less. 4 to No. The average thickness of the oxide film of No. 7 exceeded 1.5 μm. The larger the oxygen mixing ratio of the assist gas, the larger the average thickness of the oxide film formed on the laser cut surface tended to be.

In addition, the test material No. 1-No. Each of the oxide films of _{No. 7 had an outer layer containing Fe 3} O ₄ on the surface layer side thereof and an inner layer containing Fe O on the lower side thereof, and had a film structure of a second pattern.

[Results of corrosion resistance evaluation test]
Test material No. included in the scope of the present invention. 1-No. In No. 3, laser cutting was performed using a mixed gas having an oxygen mixing ratio of 5 to 20% by volume as an assist gas, so that an oxide film having an average thickness of 1.5 μm or less containing _{Fe 3} O _{4 having excellent corrosion resistance was laser cut.} It was formed in close contact with the surface. Therefore, it was confirmed that the rust preventive property of the cut surface was maintained from the start of the outdoor exposure test to 10 days, and the corrosion resistance at the initial stage after cutting was good.

On the other hand, the test material No. 4 to No. In the comparative example using the mixed gas of No. 6, laser cutting was performed using a mixed gas having an oxygen mixing ratio of more than 20% by volume as an assist gas, so that the oxide film formed on the cut surface grew too much, and the oxide film was grown too much. The average thickness exceeded 1.5 μm. Test material No. Similarly, in the comparative example using oxygen gas of No. 7, the average thickness of the oxide film exceeded 1.5 μm. Therefore, a gap is formed between the oxide film and the base steel plate to reduce the adhesion, and cracks and peeling occur in a part of the oxide film. As a result, the rust resistance of the cut surface could not be maintained from the start of the outdoor exposure test to 5 days, and the corrosion resistance at the initial stage after cutting was insufficient.

Test material No. 8. Test material No. Reference numeral 9 denotes an example in which laser cutting is performed using an assist gas containing no oxygen. Since the base steel plate was exposed on each of the cut surfaces, the rust preventive property of the cut surface could not be maintained from the start of the outdoor exposure test to 5 days.

(Example 2)
Next, the effect of cutting speed was investigated. A test material obtained by laser cutting according to the same procedure as in Example 1 except that nitrogen gas having an oxygen mixture ratio of 20% by volume was used as the assist gas and various cutting speeds were selected. The surface analysis of the cut surface and the evaluation test of corrosion resistance were carried out. The results are shown in Table 2.

[Result of surface analysis]
As shown in Table 2, the test material Nos. were performed at cutting speeds of 2.5 m / min, 5.0 m / min, and 7.5 m / min. 11-No. The average thickness of the oxide film of No. 13 was 1.5 μm or less. On the other hand, the test material No. The average thickness of the oxide film of No. 14 exceeded 1.5 μm, and when the cutting speed was increased to 10.0 m / min or more, an oxide film having a large average thickness was formed.

Further, the test material No. which was cut at a cutting speed of 2.5 m / min and 7.5 m / min. 11 and test material No. Regarding No. 13, the oxide film formed on the surface thereof _{has an outer layer containing Fe 3} O ₄ on the surface layer side thereof and an inner layer containing Fe O on the lower side of the outer layer, and has a film structure of a second pattern. It was. Test material No. cut at a cutting speed of 5.0 m / min and 10.0 m / min. With respect to No. 12, the oxide film formed on the surface thereof was a structure in which Fe ₃ O ₄ and Fe O were mixed, and had a film structure of the first pattern. In addition, the test material No. Test material No. 14 rather than Twelve tended to have a higher proportion _{of Fe 3} O ₄ in the FeO layer.

[Results of corrosion resistance evaluation test]
As shown in Table 2, the test material Nos. Included in the scope of the present invention. 11-No. 13, together with the oxygen mixed ratio using 20% by volume of the mixed gas as an assist gas, so were laser cut by the cutting speed in the range of 2 to 8 m / min, the average thickness including the Fe ₃ O ₄ rich in corrosion resistance of An oxide film of 1.5 μm or less was formed in close contact with the laser cut surface. Therefore, it was confirmed that the rust preventive property of the cut surface was maintained from the start of the outdoor exposure test to 10 to 25 days, and the corrosion resistance at the initial stage after cutting was good.

Test material No. 1 having a cutting speed of 2.5 m / min. No. 11 was able to maintain the rust preventive property from the start of the outdoor exposure test to the 15th day. Test material No. 1 having a cutting speed of 7.5 m / min. No. 13 is the test body No. 13 of Example 1. Similar to No. 3, the assist gas was cut at an oxygen mixture ratio of 20% by volume, and the result of the outdoor exposure test was 10 days as shown in Table 2. Test material No. No. 11 is the test material No. Compared with 13, the corrosion resistance in the initial stage after cutting was improved. This is the test material No. Since the cutting speed of No. 11 was slow and the cooling rate was slow, it _{is presumed that the outermost layer, Fe 3} O _4, was formed thickly to improve the corrosion resistance.

Test material No. 1 having a cutting speed of 5.0 m / min. No. 12 was able to maintain the rust preventive property from the start of the outdoor exposure test to the 25th. It is presumed that the oxide film on the cut surface has improved corrosion resistance due to the formation of a mixed structure of _{Fe 3} O _{4 and Fe O.}

On the other hand, the test material No. In 14 comparative examples, the rust resistance of the cut surface could be maintained only from the start of the outdoor exposure test to 5 days, and the corrosion resistance in the initial stage after cutting was insufficient. This was cut under the condition that the cutting speed exceeded 8 m / min, and the average thickness of the oxide film formed on the cut surface exceeded 1.5 μm, so that a close contact oxide film was formed. It is probable that it was not.

(Example 3)
Using a Zn-based galvanized steel sheet having a plate thickness of 3.2 mm, laser cutting was performed under the cutting conditions shown in Table 3, and the obtained test material was subjected to surface analysis of the cut surface and evaluation test of corrosion resistance.

Test material No. 21 corresponds to the example of the present invention. Laser cutting was performed under the conditions of a gas pressure of 1.5 MPa, a cutting speed of 5 m / min, and a laser output of 3.0 kW. Test material No. Reference numeral 22 denotes a comparative example to which the cutting conditions described in the examples of Patent Document 1 of the prior art are applied. Laser cutting was performed under the conditions of a gas pressure of 1.2 MPa, a cutting speed of 1.8 m / min, and a laser output of 1.5 kW. As the assist gas, a mixed gas (N ₂ + 5% O ₂ ) _{containing 5% by volume O 2} was used for both of the test materials. Other cutting conditions were the same as in Example 1. Table 3 shows the surface analysis results of the cut surface and the evaluation test results of corrosion resistance.

Test material No. Oxide film 21 had a coating structure of a first pattern comprising a tissue mixed with Fe ₃ O ₄ phase and FeO on the base steel sheet. According to the results of the corrosion resistance evaluation test, the occurrence of red rust was good until the 25th. It is presumed that this is because the average thickness of the oxide film is as thin as 1.5 μm or less, and an oxide film having good corrosion resistance and adhesion is formed. From this result, it was confirmed that even if the assist gas composition is the same, the formation of the oxide film on the cut surface is different due to the difference in the cutting conditions, and the corrosion resistance of the cut surface is different.

Test material No. The oxide film of No. 22 had a second pattern film structure in which an inner layer containing FeO was formed on a base steel sheet _{and an outer layer containing Fe 3} O _{4 was formed on the inner layer.} According to the results of the corrosion resistance evaluation test, the occurrence of red rust was poor in 15 days. Test material No. No. 22 is the test material No. Since cutting was performed under conditions where the cutting speed was slower and the amount of heat input was higher than that of 21, the oxidation time at high temperature was long, and the cooling rate after cutting was slow. It is presumed that the average thickness of the oxide film increased, the adhesion to the base steel sheet decreased, and cracks and peeling occurred, resulting in a decrease in corrosion resistance.

According to the above test results, it was confirmed that the laser cut surface according to the present invention is superior in corrosion resistance to the laser cut surface obtained in the examples of Patent Document 1.

1 Laser cut surface 2 Base steel plate 3 Oxide film 4 Outer layer 5 Inner layer 6 Zn-based galvanized steel plate 7 Test material 11 Cutting head 12 Cutting nozzle 13 Laser beam 14 Measurement area 15 Test stand

Claims (8)

A laser cutting method in which the surface of a Zn-based galvanized steel sheet is irradiated with a laser beam and an assist gas is injected onto the irradiated portion of the laser beam to perform laser cutting.
A laser cutting method in which a mixed gas containing 5 to 20% by volume of oxygen is injected as the assist gas to form a laser cutting surface having an oxide film containing _{Fe 3} O _{4 having an average thickness of 1.5 μm or less.} The laser cutting processing method according to claim 1, wherein the laser cutting is performed at a cutting speed of 2.0 to 8.0 m / min. The laser cutting processing method according to claim 1 or 2, wherein the oxide film further contains FeO. The laser cutting method according to any one of claims 1 to 3, wherein the Zn-based plated steel sheet is a Zn-Al-Mg-based plated steel sheet. It is a cut processed product of Zn-based plated steel sheet equipped with a laser cut surface.
The laser cut surface is a cut processed product having an oxide film containing _{Fe 3} O ₄ having an average thickness of 1.5 μm or less. The cut processed product according to claim 5, wherein the oxide film further contains FeO. The cut processed product according to claim 6, wherein the oxide film has an _{outer layer containing Fe 3} O _{4 and an inner layer containing Fe O.} The cut processed product according to any one of claims 5 to 7, wherein the Zn-based plated steel sheet is a Zn-Al-Mg-based plated steel sheet.

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