Surface treatment method

Abstract

To provide a surface treatment method that efficiently removes a surface adhesion layer adhering to a surface of a steel structure to apply desired surface roughness onto a surface of a base material of the steel structure.SOLUTION: A surface treatment method for a steel structure comprises: an auxiliary removing step of removing a portion of a surface adhesion layer adhering to a surface of the steel structure; an adhesion layer removing step having a first laser irradiating step of irradiating the surface of the steel structure from which the portion of the surface adhesion layer is removed with a laser beam to remove the remaining surface adhesion layer; and a second laser irradiating step of removing an oxide coated layer formed on the surface of the steel structure by heat received in the adhesion layer removing step. The adhesion layer removing step comprises a film thickness measuring step of measuring a film thickness of the surface adhesion layer whose portion is removed in the auxiliary removing step. The first laser irradiating step is executed when the film thickness of the surface adhesion layer is 100 μm or less, to remove the surface adhesion layer to expose a base material of the steel structure and to provide predetermined surface roughness to a surface of the base material.SELECTED DRAWING: Figure 1

Description

Technical Field

[0005]

[0001] The present invention relates to a surface treatment method. In particular, it relates to a surface treatment method for removing a surface adhesion layer adhering to the surface of a steel structure while imparting a desired surface roughness to the base surface of the steel structure.

Background Art

[0002] Conventionally, various steel structures made of steel, such as bridges, towers, plants, tanks, and machine parts, have been known. For these steel structures, painting, spraying, etc. are performed on the surface of the steel structure for purposes such as corrosion prevention and reinforcement. When repairing such steel structures, after removing the paint layer or sprayed layer adhering to the surface of the steel structure, repair is performed, and new painting or spraying is performed again. However, due to the need to enhance the adhesion between the newly formed paint layer or sprayed layer and the base surface of the steel structure, it is required to impart a desired surface roughness to the base surface of the steel structure. Conventionally, in order to impart a desired surface roughness, blast treatment has generally been performed.

Summary of the Invention

Problems to be Solved by the Invention

[0003] However, in order to perform this blast treatment, due to considerations for the environment, it is necessary to apply curing to the steel structure so that the abrasive used for blasting does not scatter, and also, the equipment becomes large-scale, and there is a problem that it is difficult to apply it to small-scale repairs. <00,00017>

[0004] The present invention has been made to solve the above problems, and an object thereof is to provide a surface treatment method that can more simply and efficiently remove a surface adhesion layer adhering to the surface of a steel structure while imparting a desired surface roughness to the base surface of the steel structure.

Means for Solving the Problems

[0005] The object of the present invention is achieved by a surface treatment method for surface treatment of a steel structure, comprising: an auxiliary removal step of removing a portion of the surface adhesion layer adhering to the surface of the steel structure; a first laser irradiation step of irradiating the surface of the steel structure from which a portion of the surface adhesion layer has been removed with laser light to remove the remaining surface adhesion layer; and a second laser irradiation step of irradiating the surface of the steel structure with a laser output lower than the laser output irradiated in the first laser irradiation step to remove an oxide film layer formed on the surface of the steel structure by heat received in the first laser irradiation step, wherein the adhesion layer removal step comprises a film thickness measurement step of measuring the film thickness of the surface adhesion layer adhering to the surface of the steel structure after the auxiliary removal step, and the first laser irradiation step is performed when the film thickness of the surface adhesion layer measured in the film thickness measurement step is 100 μm or less, and is a step of removing the surface adhesion layer to expose the base material of the steel structure and imparting a predetermined surface roughness to the base material surface of the steel structure.

[0006] Furthermore, regarding the above surface treatment method, it is preferable that in the first laser irradiation step, the surface roughness Ra of the raw surface of the steel structure is adjusted to a range of 5 μm or more, and the surface roughness Rz is adjusted to a range of 30 μm or more.

[0007] Furthermore, the laser irradiation conditions in the first laser irradiation step are preferably such that the laser output is between 1500W and 2500W, the laser beam feed speed is between 800mm / sec and 1500mm / sec, the laser beam spot diameter is 1.0mm or less, and the laser beam focal length is between 200mm and 700mm.

[0008] Furthermore, it is preferable to include an irradiation condition selection step for selecting laser irradiation conditions in the first laser irradiation step according to the thickness dimension of the surface adhesion layer measured in the thickness measurement step and the type of adhesion substance contained in the surface adhesion layer.

[0009] Furthermore, it is preferable that the laser output of the laser light irradiated in the second laser irradiation step is 0.1 times or less the laser output of the laser light irradiated in the first laser irradiation step.

[0010] Furthermore, it is preferable that the laser light irradiated in the first laser irradiation step is a CW laser, and the laser light irradiated in the second laser irradiation step is a pulsed laser with a single irradiation time of 10 microseconds or less.

[0011] Furthermore, the auxiliary removal step is preferably a step of removing at least a portion of the surface adhesion layer attached to the surface of the steel structure by using a release agent, by heating and melting the surface adhesion layer attached to the surface of the steel structure, or by irradiating it with laser light. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a surface treatment method that can more easily and efficiently remove surface adhesion layers adhering to the surface of a steel structure while simultaneously imparting a desired surface roughness to the raw surface of the steel structure. [Brief explanation of the drawing]

[0013] [Figure 1] This is a block diagram illustrating a surface treatment method according to one embodiment of the present invention. [Figure 2] This is an explanatory diagram illustrating the auxiliary removal step included in the surface treatment method according to the present invention. [Figure 3] This is a schematic diagram illustrating a laser processing apparatus that can be suitably used in the surface treatment method according to the present invention. [Figure 4] Figure 3 is a schematic diagram illustrating the laser irradiation head of the laser processing apparatus shown. [Figure 5] These images show the appearance of the steel plate after the completion of the first laser irradiation process S13, and the appearance of the steel plate after the completion of the second laser irradiation process S2. [Figure 6] This is a magnified image of the steel plate surface after the second laser irradiation process S2 has been completed. [Figure 7] Figure 6 shows a 3D processed image of the surface obtained from a magnified surface image, and a graph showing the surface irregularities (surface roughness) in an arbitrary cross-section. [Modes for carrying out the invention]

[0014] The surface treatment method according to one embodiment of the present invention will be described below with reference to the attached drawings. The surface treatment method according to the present invention is a method for performing surface treatment on a steel structure, and as shown in the block diagram of Figure 1, comprises an adhesion layer removal step S1 having a first laser irradiation step S13 and a second laser irradiation step S2. Here, the steel structure is not particularly limited, and examples include structures made of steel such as bridges, transmission towers, plants, tanks, and machine parts.

[0015] The adhesion layer removal step S1 comprises an auxiliary removal step S11, a film thickness measurement step S12 performed after the auxiliary removal step S11, and a first laser irradiation step S13 performed after the film thickness measurement step S12. The auxiliary removal step S11 is a step to remove a portion of the surface adhesion layer 2 adhering to the surface of the steel structure 1, as shown in Figure 2. Here, the surface adhesion layer 2 adhering to the surface of the steel structure 1 refers to layers other than steel material, such as a paint film layer applied to the surface of the steel structure 1, a thermal spray layer, mill scale, a rust layer formed by oxidation, a salt layer formed by salt adhesion, and an oil film layer.

[0016] In the auxiliary removal step S11, for example, a release agent can be used to remove the surface adhesion layer attached to the surface of the steel structure. The release agent is appropriately selected according to the type of surface adhesion layer and is not particularly limited, but for example, if the surface adhesion layer is a paint film layer, it can be an agent mainly composed of higher alcohols or an organic solvent. When performing the auxiliary removal step S11 using such a release agent, for example, the release agent is applied to the surface of the surface adhesion layer. When the release agent is applied, the paint film layer is dissolved or penetrates into the paint film, causing the existing paint film to swell and soften, thereby weakening the bonding force (adhesion) between the paint film layer and the steel structure, making it easier to remove the paint film layer. The method of applying the release agent is not particularly limited, but when the application area is large, it is preferable to apply it using an airless spray painter, and when the application area is small or the surface shape is complex, it is preferable to apply it using a brush or roller. Furthermore, after applying the paint film release agent, it is preferable to check whether the predetermined amount of release agent has been applied using a wet film thickness gauge or the like.

[0017] Furthermore, if the surface adhesion layer is a rust layer, acidic rust removers mainly composed of inorganic acids such as phosphoric acid or hydrochloric acid, or organic acids such as malic acid or oxalic acid, or neutral rust removers mainly composed of thioglycolate, can be used as stripping agents. Even when using a rust remover as a stripping agent in this way, the stripping agent (rust remover) can be applied to the surface adhesion layer in the same way as when stripping the paint film layer described above. In addition, the rust layer can also be removed using power tools such as grinders and chippers, or manual tools such as scrapers.

[0018] In the auxiliary removal step S11, instead of using a release agent, for example, at least a part of the surface adhesion layer adhering to the surface of the steel structure may be removed by heating and melting the surface adhesion layer. When heating and melting the surface adhesion layer in this way, it is preferable to perform high-frequency heating. By performing high-frequency heating, the base surface of the steel structure can be intensively and rapidly heated (for example, 180°C to 240°C) by induction heating, whereby the coating film layer softens and the adhesive force is weakened, and the coating film layer can be removed. When performing high-frequency heating, a dielectric heating device is used.

[0019] In the auxiliary removal step S11, instead of using a release agent or high-frequency heating, at least a part of the surface adhesion layer may be removed by irradiating laser light. When irradiating laser light to remove a part of the surface adhesion layer in this way, a laser processing device 10 is used. As shown in the schematic diagram of FIG. 3, this laser processing device 10 includes a laser irradiation head 11, a head support 12, a laser transmitter 13, an operation control device 14, a purge gas supply device 15, and the like.

[0020] The laser irradiation head 11 is connected to the laser transmitter 13 via a laser transmission cable made of an optical fiber cable or the like. The laser light emitted from the laser transmitter 13 is sent to the laser irradiation head 11 and is configured to be irradiated from the tip of the laser irradiation head 11 toward the steel structure having the surface adhesion layer as the irradiated object. The beam spot, which is the location where the steel structure as the irradiation object is irradiated with laser light, may be arranged to substantially coincide with the focal point, or may be arranged at a predetermined distance from the focal point (defocused). Also, as shown in the schematic diagram of FIG. 4, the laser irradiation head 11 is configured to have a collimating lens 16, a focusing lens 17, a protective glass 18, and a nozzle 19 arranged in order from the optical fiber cable. As the laser transmitter 13, it is preferable to use, for example, a CW laser transmitter that oscillates a high-output laser output with an output of 1500 W or more and 2500 W or less.

[0021] The collimating lens 16 is an optical element that collimates the laser light emitted through the laser transmission cable into parallel light. The condensing lens 17 is an optical element (condensing optical system) that condenses the laser light incident as parallel light from the collimating lens 16 so as to converge at a predetermined focal point. These collimating lens 16 and condensing lens 17 are fixed inside the housing of the laser irradiation head 11.

[0022] The protective glass 18 is a member that prevents foreign matters such as dust scattered from the steel structure side from adhering to the condensing lens 17 and the like. The protective glass 18 is preferably formed, for example, substantially in a flat plate shape. Further, the protective glass 18 has a function of partitioning the space portion in which the condensing lens 17 and the like are accommodated in a substantially airtight state from the region on the inner diameter side of the nozzle 19. The protective glass 18 is preferably configured to be detachable from the main body portion of the laser irradiation head 11 so that it can be easily replaced when contaminated or burned out.

[0023] The nozzle 19 is a cylindrical member provided at the end portion on the steel structure side of the laser irradiation head 11, protrudes from the protective glass 18 side toward the steel structure side, and the laser light is emitted through the inner diameter side of the nozzle 19. In FIG. 4, the nozzle 19 is formed in a tapered cylindrical shape with the diameter of the tip end portion reduced.

[0024] The purge gas supply device 15 introduces a purge gas that prevents foreign matters such as dust from entering the inside of the nozzle 19 during laser irradiation into the nozzle 19 through a pipe. As the purge gas, for example, various gases such as clean air or inert gas from which foreign matters have been removed by a filter or the like can be used.

[0025] The head support 12 includes an arm portion 121 to which the laser irradiation head 11 is attached and which holds the laser irradiation head 11, and a base portion 122 to which the base end of the arm portion 121 is attached. The connection structure between the tip of the arm portion 121 and the laser irradiation head 11 is not particularly limited, but it is preferable that it be configured so that the direction in which the laser irradiation head 11 faces can be changed as appropriate.

[0026] The base portion 122 on which the arm portion 121 is installed is preferably equipped with a lifting device for changing the height position of the laser irradiation head 11, and is also preferably equipped with a horizontal moving device for moving the laser irradiation head 11 in the left-right direction when the steel structure to which the laser is irradiated is viewed from the front, and for adjusting the distance between the laser irradiation head 11 and the surface of the steel structure to which the laser is irradiated. When the base portion 122 is configured to be equipped with a lifting device and a horizontal moving device, the operation of these devices can be controlled by the operation control device 14.

[0027] Furthermore, the arm portion 121 may be configured as a multi-joint robot arm. Preferably, the multi-joint robot arm is configured to hold the laser irradiation head 11 and to allow the laser irradiation head 11 to move relative to the steel structure in the up, down, left, and right directions according to a predetermined processing path. When the arm portion 121 is configured as a multi-joint robot arm, its operation can be controlled by the motion control device 14.

[0028] When using such a laser processing device 10 to remove a portion of the surface adhesion layer attached to the surface of a steel structure, laser light is irradiated onto the surface adhesion layer to be removed. As a result, the high-energy-density region in the central part of the beam spot on the surface adhesion layer melts, vaporizes, and is thermally destroyed, thereby removing the surface adhesion layer.

[0029] The film thickness measurement step S12 is a step in which the film thickness of the surface adhesion layer remaining and adhering to the surface of the steel structure after the auxiliary removal step S11 described above is measured. In this film thickness measurement step S12, the film thickness of the surface adhesion layer remaining and adhering to the surface of the steel structure is measured using a film thickness gauge. The film thickness gauge used is not particularly limited, but for example, an electromagnetic film thickness gauge that measures the film thickness by measuring the change in the amount of current flowing through an electromagnet based on a change in magnetic flux density can be preferably used. Alternatively, an infrared film thickness gauge that measures the film thickness based on a spectrum obtained by irradiating the object to be measured with infrared light and spectrally analyzing the transmitted or reflected light can also be used.

[0030] The first laser irradiation step S13 is a step in which a high-power laser beam is irradiated onto the surface of the steel structure from which a portion of the surface adhesive layer has been removed by the auxiliary removal step S11 to remove the remaining surface adhesive layer. This step is performed when the thickness of the surface adhesive layer measured by the thickness measurement step S12 described above is 100 μm or less. If the remaining thickness of the surface adhesive layer exceeds 100 μm, the auxiliary removal step S11 is performed again. This first laser irradiation step S13 is a step in which the remaining surface adhesive layer is removed to expose the base material of the steel structure, and a predetermined surface roughness is also imparted to the base material surface of the steel structure.

[0031] This first laser irradiation step S13 can be carried out using the laser processing device 10 that can be used in the auxiliary removal step S11 described above. Furthermore, the laser irradiation conditions irradiated in the first laser irradiation step S13 are preferably in the range of a laser output of 1500W to 2500W, a laser beam feed speed of 800mm / sec to 1500mm / sec, a laser beam spot diameter of 1.0mm or less, and a laser beam focal length of 200mm to 700mm. By using such irradiation conditions, it becomes possible to form a substrate surface with a surface roughness Ra (arithmetic mean roughness) of 5μm or more and a surface roughness Rz (maximum height) of 30μm or more, which improves the adhesion strength with the new paint layer or thermal spray layer when the substrate surface of the steel structure is repainted or thermal sprayed. Here, the surface roughness Ra and Rz can be calculated from the surface roughness profile using, for example, a non-contact laser surface roughness meter and calculation software, based on JIS standard B0601 (2013).

[0032] Furthermore, the surface roughness Ra of the raw material surface of the steel structure is more preferably in the range of 8 μm or more, and the surface roughness Rz is more preferably in the range of 50 μm or more.

[0033] The second laser irradiation step S2 is a step in which the surface of the exposed steel structure is irradiated with a low-power laser, lower than the laser power irradiated in the first laser irradiation step S13, to remove the oxide film layer formed on the surface of the steel structure due to heat received in the first laser irradiation step S13. In this second laser irradiation step S2, a laser processing apparatus with the same configuration as the laser processing apparatus 10 described above can be used, but it is preferable to use a laser processing apparatus capable of irradiating a pulsed laser with an irradiation time of 10 microseconds or less per pulse. A pulsed laser can effectively suppress the formation of a new oxide film because the temperature of the laser-irradiated surface does not rise easily. In addition, because the amount of energy per irradiation point is large, the removal capacity is high and the oxide film layer can be removed efficiently.

[0034] Furthermore, it is preferable that the laser output of the laser light irradiated in the second laser irradiation step S2 described above is 0.1 times or less the laser output of the laser light irradiated in the first laser irradiation step S13. By adopting such output conditions, it becomes possible to effectively remove the oxide film layer and effectively suppress the formation of a new oxide film.

[0035] According to the surface treatment method of the present invention described above, it is possible to efficiently remove surface adhering layers such as coating layers, thermal spray layers, and mill scale to expose the bare surface of the steel structure, and to impart a desired surface roughness to the bare surface of the steel structure, thereby improving the adhesion strength with the coating layer or thermal spray layer that is again formed on the bare surface of the steel structure. Furthermore, since a predetermined surface roughness can be formed without excessively thinning the bare surface of the steel structure, it is possible to effectively suppress a decrease in the strength required of the steel structure.

[0036] In particular, in this invention, the remaining thickness of the surface adhesion layer is controlled to be 100 μm or less, and then the first laser irradiation step S13 is performed to remove the remaining surface adhesion layer while imparting a predetermined surface roughness to the base surface of the steel structure. Therefore, it is possible to efficiently remove the remaining surface adhesion layer and impart a predetermined surface roughness to the base surface without applying excessive laser energy to the base surface. Furthermore, the distribution of minute irregularities formed on the base surface of the steel structure can be made uniform, and the occurrence of locally excessively large surface roughness on the base surface can be effectively suppressed. Moreover, the risk of excessive thinning of the base surface of the steel structure and the increase in the thickness of the oxide film layer formed by laser irradiation can be suppressed very effectively.

[0037] Furthermore, the surface treatment method according to the present invention does not require the steel structure to be protected to prevent the abrasive material used in blasting from scattering, as is the case when surface roughness is imparted by conventional blasting. Moreover, the apparatus configuration can be made smaller, making it even easier to efficiently remove the surface adhesion layer adhering to the surface of the steel structure while simultaneously imparting the desired surface roughness to the raw surface of the steel structure.

[0038] Although a surface treatment method according to one embodiment of the present invention has been described above, its specific configuration is not limited to the above embodiment. For example, in the above embodiment, the CW laser irradiated in the first laser irradiation step S13 may be configured to irradiate the steel structure as a pseudo-pulsed laser. As a method for converting the CW laser into a pseudo-pulsed laser, for example, a rotating disc having slits formed radially from the center at equal angular intervals is placed between the focusing lens 17 and the protective glass 18, and the CW laser is irradiated while the rotating disc is rotated, thereby forming laser light passing through the slits and laser light blocked in the portions between the slits. This allows the CW laser to be converted into a pseudo-pulsed laser light and irradiated onto the steel structure. By configuring the CW laser as a pseudo-pulsed laser in this way and scanning the surface of the steel structure with the pseudo-pulsed laser, it becomes possible to form even more uniform surface irregularities (surface roughness) on the raw surface of the steel structure.

[0039] Furthermore, in the above embodiment, the configuration of the laser processing apparatus 1 is exemplified as a stationary type laser processing apparatus in which the laser irradiation head 11 is attached to the head support 12. However, the configuration is not limited to this, and a handheld, compact laser processing apparatus may be used, and the first laser irradiation process, the second laser irradiation process, etc., may be performed while holding it by hand.

[0040] Furthermore, in the above embodiment, the system may be configured to include an irradiation condition selection step in which the laser irradiation conditions in the first laser irradiation step S13 are selected according to the thickness dimension of the surface adhesion layer measured in the thickness measurement step S12 and the type of adhering substance contained in the surface adhesion layer. By performing laser irradiation with the irradiation conditions set by this irradiation condition selection step, it becomes possible to remove the surface adhesion layer remaining on the surface of the steel structure, exposing the base material, while controlling the surface roughness of the base material surface to a desired roughness.

[0041] In this irradiation condition selection process, a chart is prepared in advance showing the relationship between the laser irradiation conditions and the surface roughness Ra and Rz of the substrate surface obtained by laser irradiation, when the remaining thickness of the surface adhesion layer on the surface of the steel structure and the type of adhesion material are changed in various ways. Based on this chart, the laser irradiation conditions to be irradiated in the first laser irradiation process S13 can be set.

[0042] Furthermore, in the above embodiment, the first laser irradiation step S13 is configured using a CW laser, but it is also possible to use a pulsed laser, and the second laser irradiation step S2 is configured using a pulsed laser, but it is also possible to use a CW laser.

[0043] Next, the inventors of the present invention conducted tests using the surface treatment method according to the present invention to remove the surface adhesion layer of a steel structure while simultaneously imparting a desired surface roughness to the raw surface of the steel structure. These tests will be described below.

[0044] In the test, a rectangular steel plate (steel structure) with dimensions of 120 mm in length, 120 mm in width, and 3.2 mm in thickness was prepared, and the first laser irradiation process S13 and the second laser irradiation process S2 were performed on the steel plate. The surface of the steel plate had a mill scale (surface adhesion layer) attached, and prior to the test, an auxiliary removal process was performed to adjust the thickness of the mill scale (surface adhesion layer) to 100 μm. For the first laser irradiation process S13, a handheld laser processing device, model number Laser Keren LZK-2000, manufactured by Furusato Kogyo Co., Ltd., was used, and for the second laser irradiation process S2, a handheld laser processing device, model number QF-100, manufactured by P-laser, Inc., was used. In both the first and second laser irradiation processes S13 and S2, the laser processing devices were mounted on an arm that could move in both the left-right and up-down directions for the test.

[0045] The conditions for the first laser irradiation process S13 were as follows: a CW laser was used as the laser light source, with a laser output of 2000W, a laser light feed speed of 1100mm / second in the left-right direction relative to the steel plate, a laser light feed speed of 1mm / second in the up-down direction relative to the steel plate, a laser light spot diameter of 0.8mm, and a laser light focal length of 210mm. The conditions for the second laser irradiation process S2 were as follows: a pulsed laser was used as the laser light source, with a laser output of 100W, a laser light feed speed of 100KHz in the left-right direction relative to the steel plate, a laser light feed speed of 2mm / second in the up-down direction relative to the steel plate, a laser light spot diameter of 0.5mm, and a laser light focal length of 250mm.

[0046] Table 1 shows the measurement results of the surface roughness of the steel sheet after the completion of the first laser irradiation process S13, and after the completion of the second laser irradiation process S2. Surface roughness Ra and Rz were measured at 10 arbitrary locations using a Mitutoyo SJ-210 laser.

[0047] [Table 1]

[0048] Figure 5 also shows images of the appearance of the steel plate after the completion of the first laser irradiation process S13 and after the completion of the second laser irradiation process S2. The upper half of Figure 5 shows the appearance of the steel plate after the completion of the first laser irradiation process S13, and the lower half shows the appearance of the steel plate after the completion of the second laser irradiation process S2, which is performed following the first laser irradiation process S13. From Figure 5, it can be seen that the oxide film layer formed on the surface of the steel plate (steel structure) due to heat absorption in the first laser irradiation process S13 is cleanly removed by the execution of the second laser irradiation process S2.

[0049] Furthermore, Figure 6 shows a magnified image of the steel plate surface after the second laser irradiation process S2 is completed. Figure 7 shows a 3D processed image of the steel plate surface obtained from the magnified surface image in Figure 6, as well as an image of a graph showing the surface irregularities (surface roughness shape) in an arbitrary cross-section. From Figures 6 and 7 and Table 1 above, it can be seen that a substrate surface with good surface roughness can be formed on the substrate surface of the steel plate (steel structure) that will have improved adhesion strength with the new paint layer or thermal spray layer when the substrate surface of the steel structure is repainted or thermal sprayed. [Explanation of Symbols]

[0050] S1 Adhesion layer removal process S11 Auxiliary removal process S12 Film Thickness Measurement Process S13 First laser irradiation process S2 Second laser irradiation process 10 Laser processing equipment 11 Laser irradiation head 12 Head support 13. Laser Transmitter 14. Operation control device 15. Purge gas supply device 16 Collimating lenses 17. Focusing lens 18 Screen Protector 19 nozzles

Claims

1. A surface treatment method for performing surface treatment on steel structures, An adhesion layer removal step comprising: an auxiliary removal step of removing a portion of the surface adhesion layer adhering to the surface of the steel structure using a release agent or by heating and melting the surface adhesion layer adhering to the surface of the steel structure; and a first laser irradiation step of removing the remaining surface adhesion layer by irradiating the surface of the steel structure from which a portion of the surface adhesion layer has been removed with laser light. The system includes a second laser irradiation step, in which the surface of the steel structure is irradiated with a laser output lower than that of the first laser irradiation step, thereby removing the oxide film layer formed on the surface of the steel structure due to heat received in the first laser irradiation step. The aforementioned adhesive layer removal step includes a film thickness measurement step for measuring the film thickness of the surface adhesive layer adhering to the surface of the steel structure after the auxiliary removal step, If the thickness of the surface-adhered layer measured by the aforementioned thickness measurement step exceeds 100 μm, the auxiliary removal step is performed again. The surface treatment method is characterized in that the first laser irradiation step is performed when the thickness of the surface adhesion layer measured by the thickness measurement step is 100 μm or less, and the first laser irradiation step is performed to remove the surface adhesion layer to expose the base material of the steel structure, and to adjust the surface roughness Ra of the base material surface of the steel structure to a range of 5 μm or more and the surface roughness Rz to a range of 30 μm or more.

2. The surface treatment method according to claim 1, characterized in that the laser irradiation conditions irradiated in the first laser irradiation step are in the range of a laser output of 1500 W or more and 2500 W or less, a laser beam feed speed of 800 mm / sec or more and 1500 mm / sec or less, a laser beam spot diameter of 1.0 mm or less, and a laser beam focal length of 200 mm or more and 700 mm or less.

3. The surface treatment method according to claim 1 or 2, further comprising an irradiation condition selection step for selecting laser irradiation conditions in the first laser irradiation step according to the thickness dimension of the surface adhesion layer measured in the thickness measurement step and the type of adhesion substance contained in the surface adhesion layer.

4. The surface treatment method according to claim 1 or 2, characterized in that the laser output of the laser light irradiated in the second laser irradiation step is 0.1 times or less the laser output of the laser light irradiated in the first laser irradiation step.

5. The surface treatment method according to claim 1 or 2, characterized in that the laser light irradiated in the first laser irradiation step is a CW laser, and the laser light irradiated in the second laser irradiation step is a pulsed laser with a single irradiation time of 10 microseconds or less.

6. The surface treatment method according to claim 1 or 2, characterized in that, in the auxiliary removal step, if the surface adhering layer is a coating layer, a release agent mainly composed of a higher alcohol or mainly composed of an organic solvent is used, and if the surface adhering layer is a rust layer, an acidic rust remover or a neutral rust remover is used as the release agent.

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