JP7563997B2 - Method for removing oxide scale and method for manufacturing stainless steel material - Google Patents

Description

The present invention relates to a method for removing oxide scale and a method for manufacturing stainless steel materials.

Stainless steel materials have various excellent properties such as corrosion resistance, and are therefore used in a wide range of applications, such as automobile parts, building parts, and kitchen utensils.
In the manufacturing process of stainless steel materials, oxide scales may be formed on the surface, and therefore, the oxide scales are removed by pickling. For example, a typical stainless steel sheet is manufactured by the following process. A hot-rolled sheet (thick sheet material) is obtained by continuously casting molten steel obtained by melting stainless steel raw materials, and hot rolling the slab. A cold-rolled sheet (thin sheet material) is obtained by cold rolling the hot-rolled sheet. The hot-rolled sheet and the cold-rolled sheet are wound into a coil as required. The hot-rolled sheet and the cold-rolled sheet are annealed as required. In the manufacturing of such stainless steel sheets, oxide scales are formed on the hot-rolled sheet after hot rolling and the cold-rolled sheet after cold rolling, and therefore, the oxide scales are removed by pickling.
In this specification, the term "stainless steel sheet" is a concept that includes both hot-rolled sheets and cold-rolled sheets. In addition, hereinafter, removing the oxide scale formed on the surface of a stainless steel material may be referred to as "descaling".

However, pickling may not be able to sufficiently remove oxide scale. For this reason, the pickling time is extended, or mechanical pretreatment such as using a scale breaker or shot blasting is performed to create cracks in the oxide scale before pickling. As a result, it takes too long to remove the oxide scale, which is a factor in reducing the production efficiency of stainless steel materials.

Therefore, various methods have been researched and implemented to efficiently remove the oxide scale from the surface of stainless steel materials. For example, Patent Document 1 proposes a method of removing the scale layer formed when annealing a stainless steel plate by irradiating the scale layer with laser light.

Japanese Unexamined Patent Publication No. 4-182020

When the method of removing oxide scale using a laser beam is actually applied to a manufacturing method of stainless steel material, it is necessary to irradiate the stainless steel material with a reciprocating scan by turning back the movement of the laser head halfway in the width direction of the stainless steel material, and to move the stainless steel material in the length direction of the stainless steel material. At this time, in the width direction of the stainless steel material, there are a turn-back portion and its vicinity where the laser beam of the forward scan and the laser beam of the return scan overlap during the reciprocating scan of the laser beam, and a steady main scanning portion where the laser beam does not overlap. Therefore, in the reciprocating scan of the laser beam, the amount of heat input to the turn-back portion and its vicinity is greater than that of the main scanning portion, so that the stainless steel material is likely to melt at the turn-back portion and its vicinity, forming excessively uneven portions. These excessively uneven portions cause surface defects. For example, if the oxide scale is removed using a laser after hot rolling and then cold rolling or other processes are performed in the subsequent steps, the excessively uneven parts will be crushed during the cold rolling, forming surface defects, significantly degrading the appearance of the stainless steel material, and there is also the risk that rolling oil and the like will penetrate and remain in the excessively uneven parts, reducing corrosion resistance.

The present invention has been made to solve the above-mentioned problems, and aims to provide a method for removing oxide scale that can suppress the formation of excessively uneven portions due to an increase in the amount of heat input at and near the turning points during the reciprocating scanning of laser light.
Another object of the present invention is to provide a method for producing stainless steel material with high production efficiency and good appearance.

As a result of intensive research, the inventors discovered that the above problems could be solved by using pulsed laser light as the laser light and controlling the fluence of the pulsed laser light at the turning point of the reciprocating scan and its vicinity to be smaller than the fluence of the pulsed laser light in the main scanning portion of the reciprocating scan, and thus completed the present invention.

That is, the present invention provides a method for removing oxide scale formed on a surface of a stainless steel material after hot rolling or cold rolling by irradiating the material with a pulsed laser beam, comprising the steps of:
an irradiation step of irradiating the stainless steel material with the pulsed laser light while scanning it back and forth in an X direction and moving the stainless steel material in a Y direction perpendicular to the X direction;
and a pickling treatment step of performing pickling treatment after the irradiation step,
The pulse width of the pulsed laser light is 10 to 1000 ns,
a fluence of the pulsed laser light in a return portion of the reciprocating scan and in the vicinity thereof is made smaller than a fluence of the pulsed laser light in a main scanning portion of the reciprocating scan;
The irradiation step includes a first irradiation step of irradiating a portion A of the surface of the stainless steel material with the pulsed laser light, and a second irradiation step of irradiating a portion B of the surface of the stainless steel material with the pulsed laser light after the first irradiation step,
In the method for removing oxide scale, an overlap of an irradiation area of the pulsed laser light in the X direction between the part A and the part B is 1 to 50% of a scanning width in the X direction of each of the part A and the part B.

The present invention also relates to a method for producing stainless steel material, which includes the method for removing oxide scale.

According to the present invention, it is possible to provide a method for removing oxide scale that can suppress the formation of excessively large unevenness caused by an increase in the amount of heat input to turning portions and their vicinity during reciprocating scanning of laser light.
Furthermore, according to the present invention, a method for producing stainless steel material having high production efficiency and good appearance can be provided.

FIG. 1 is a flow diagram showing an example of a method for producing a stainless steel sheet. 1 is a schematic diagram of a laser light irradiation device used for descaling by irradiating pulsed laser light. 1 is a schematic diagram showing the trajectory of the center of a pulsed laser beam scanning back and forth across the surface of a stainless steel plate. FIG. 1 is an optical microscope photograph of the surface of a stainless steel plate on which surface defects have been formed. 1 is a schematic diagram showing the trajectory of the center of a pulsed laser beam scanning back and forth across the surface of a stainless steel plate. FIG. FIG. 2 is a diagram showing optical microscope photographs and cross-sectional curve results in an example.

The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements to the following embodiments, which are made based on the ordinary knowledge of those skilled in the art, fall within the scope of the present invention, without departing from the spirit of the present invention.
In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

The method for removing oxide scale according to the embodiment of the present invention is carried out by irradiating the surface of a stainless steel material with a pulsed laser beam to remove the oxide scale formed thereon. Also, the method for producing a stainless steel material according to the embodiment of the present invention includes the method for removing oxide scale.
The stainless steel material is not particularly limited, but is preferably a stainless steel sheet. In this specification, the term "steel sheet" is a concept including a steel strip. The stainless steel sheet may be a hot-rolled sheet after hot rolling or a cold-rolled sheet after cold rolling, or may be an annealed sheet after annealing them.
Here, a method for manufacturing a stainless steel sheet will be taken as an example of a method for manufacturing a stainless steel material and will be described with reference to the flow chart of FIG.

(Method of manufacturing stainless steel sheet)
Fig. 1 shows an example of a manufacturing method for a hot-rolled annealed stainless steel sheet. As shown in Fig. 1, this manufacturing method includes a steelmaking step (S1), a continuous casting step (S2), a hot rolling step (S3), a continuous annealing step (S4), and an oxide scale removal step (S5).
The type of stainless steel used as the material for the stainless steel plate is not particularly limited. For example, various stainless steels can be used as the material, including pickling-resistant steel containing 2.5 to 5.0% Si, 18 to 20% Cr, and 12 to 20% Ni, and general-purpose steels such as SUS304, SUS430, and SUS316.

First, in the steelmaking process (S1), scrap mainly composed of Fe, Cr, Ni, etc. is melted in an electric furnace, and then the composition is adjusted in a refining furnace to produce molten steel.
Next, in the continuous casting process (S2), the molten steel produced in the refining furnace is poured into a ladle, and then continuously poured from the ladle into a mold for cooling. The cooled and solidified stainless steel is then cut into a predetermined length to produce a number of slabs.

Next, in the hot rolling process (S3), the slab produced in the continuous casting process (S2) is heated to a predetermined temperature, and the slab that has reached the predetermined temperature is hot rolled in a hot rolling mill. Specifically, the slab is hot rolled to a predetermined thickness by performing multiple stages of rough rolling and finish rolling on the slab to produce a hot-rolled sheet.

Next, in the continuous annealing step (S4), the hot-rolled sheet produced in the hot rolling step (S3) is continuously annealed in an annealing furnace to produce a hot-rolled annealed sheet. By continuously annealing the hot-rolled sheet, the hot-rolled sheet, which has been hardened and has reduced ductility, is softened and the ductility of the hot-rolled sheet is restored.
Next, in the oxide scale removal step (S5), the oxide scale formed on the surface of the hot-rolled annealed steel sheet is descaled by irradiating it with a pulsed laser beam (hereinafter referred to as "laser descaling"). This laser descaling is a technique that utilizes laser ablation. Laser ablation can evaporate a substance to be irradiated by irradiation with laser beam. In addition, in the oxide scale removal step (S5), after the laser descaling, descaling by pickling treatment may be performed as necessary.

The above manufacturing method is merely an example, and is not limited to the content and order of each process described above, or the equipment used. In addition, if the stainless steel sheet is a cold-rolled annealed sheet, a cold rolling process and a continuous annealing process may be performed between the continuous annealing process (S4) and the oxide scale removal process (S5), or the cold rolling process and the continuous annealing process may be performed in sequence after the oxide scale removal process (S5), and then the oxide scale removal process (S5) may be performed.

Next, the oxide scale removal step (S5) will be described in detail.
Laser descaling includes an irradiation step of irradiating the stainless steel plate with a pulsed laser beam while scanning back and forth in the X direction, and moving the stainless steel plate in the Y direction perpendicular to the X direction. This irradiation step can be performed using a laser beam irradiation device 10 as shown in FIG. 2. The laser beam irradiation device 10 includes a laser oscillator 11, a laser head 12, a drive unit (not shown), and a control unit (not shown). In this laser beam irradiation device 10, a pulsed laser beam 13 oscillated from the laser oscillator 11 is irradiated from the laser head 12 while scanning back and forth in the X direction (the width direction of the stainless steel plate 1 in FIG. 2), and the stainless steel plate 1 is moved in the Y direction perpendicular to the X direction (the conveying direction of the stainless steel plate 1 in FIG. 2). This irradiation step allows the pulsed laser beam 13 to be continuously irradiated to the oxide scale, so that the oxide scale can be efficiently evaporated.

There is no particular limitation on the type of the laser oscillator 11 as long as it is capable of oscillating a pulsed laser beam. From the viewpoint of shortening the descaling time, the laser oscillator 11 is preferably a solid-state laser having a large laser output per unit area.
The laser head 12 focuses the pulsed laser light 13 oscillated from the laser oscillator 11 and irradiates the oxide scale formed on the surface of the stainless steel plate 1 with the light.

The drive unit is connected to the laser head 12 and drives the laser head 12 to move the irradiation position of the pulsed laser light 13. The control unit is connected to the laser oscillator 11 and the laser head 12 and controls the irradiation conditions of the pulsed laser light 13. Examples of the irradiation conditions of the pulsed laser light 13 include the fluence, pulse width, oscillation period, laser output, irradiation speed, irradiation width, and beam diameter.

The oscillation period of the pulsed laser light 13 is not particularly limited, but is preferably 60 to 120 kHz. By controlling the oscillation period of the pulsed laser light 13 within such a range, the pulsed laser light 13 with an optimized fluence can be irradiated to the oxide scale formed on the surface of the stainless steel plate 1. If the oscillation period of the pulsed laser light 13 is smaller than 60 kHz, the number of irradiations per unit time to the stainless steel plate 1 will be too small. Therefore, there is a risk that the pulsed laser light 13 with a sufficient fluence for descaling cannot be irradiated. Furthermore, if the oscillation period of the pulsed laser light 13 is greater than 120 kHz, the fluence per pulse of the pulsed laser light 13 will be small. Therefore, there is a risk that the pulsed laser light 13 with a sufficient fluence for descaling cannot be irradiated.

Fig. 3 is a schematic diagram showing the trajectory of the centre of the pulsed laser beam 13 scanning back and forth across the surface of the stainless steel sheet 1. When the pulsed laser beam 13 is irradiated while scanning back and forth in the X direction (e.g. the width direction of the stainless steel sheet 1) and the stainless steel sheet 1 is moved in the Y direction perpendicular to the X direction (e.g. the conveyance direction of the stainless steel sheet 1), the centre of the pulsed laser beam 13 follows a trajectory 15 as shown in Fig. 3.
The energy of the pulsed laser beam 13 is concentrated in the turn-back portion 21 and its vicinity 22 compared to the main scanning portion 20, so that the amount of heat input to the portion increases. Therefore, if the pulsed laser beam 13 is irradiated to the turn-back portion 21 and its vicinity 22 in the same manner while maintaining the fluence of the pulsed laser beam 13 required for descaling in the main scanning portion 20, the surface of the stainless steel sheet 1 melts and excessively uneven portions are formed. These excessively uneven portions cause surface defects. For example, when oxide scale is removed using a laser beam after hot rolling and then cold rolling or the like is performed in a subsequent process, the excessively uneven portions are crushed during cold rolling and surface defects are formed. An optical microscope photograph of the surface of the stainless steel sheet 1 on which surface defects have been formed is shown in FIG. 4. In FIG. 4, (a) is an optical microscope photograph, and (b) is an enlarged photograph of the inside of the rectangular frame in (a). When surface defects are formed in this manner, the appearance of the stainless steel sheet 1 is significantly deteriorated, and there is also the risk that rolling oil and the like will penetrate and remain in the excessively uneven parts, thereby reducing corrosion resistance.
Excessively large unevenness that can cause surface defects has an arithmetic mean height Ra exceeding 5.0 μm and a maximum height Rz exceeding 20.0 μm. In this specification, the arithmetic mean height Ra and the maximum height Rz are those measured in accordance with JIS B0601:2013.

Therefore, in the oxide scale removal method according to an embodiment of the present invention, the fluence of the pulsed laser light 13 at the turning point 21 and its vicinity 22 of the reciprocating scan is made smaller than the fluence of the pulsed laser light 13 at the main scanning portion 20 of the reciprocating scan. By controlling in this manner, it is possible to prevent an excessive increase in the amount of heat input at the turning point 21 and its vicinity 22, where the energy of the pulsed laser light 13 is likely to concentrate. As a result, it is possible to prevent the formation of excessively large unevenness at the turning point 21 and its vicinity 22 while maintaining the descaling effect.

After laser descaling, the surface of the folded portion 21 and its vicinity 22 preferably has an arithmetic mean height Ra of 5.0 μm or less and a maximum height Rz of 20.0 μm or less. By controlling the surface roughness to this level, surface defects are unlikely to occur even if cold rolling or other processes are performed in later processes.

The method for controlling the fluence of the pulsed laser light 13 is not particularly limited, and any method known in the art can be used. For example, the fluence can be adjusted by electronically controlling the laser oscillator 11. Alternatively, the fluence can be adjusted by controlling the pulse width by disposing an Fθ lens at the tip of the laser head 12.
In addition, the fluence of the pulsed laser light 13 at the return portion 21 of the reciprocating scan and its vicinity 22 may be controlled so as to be gradually reduced to a predetermined value, or may be controlled so as to be suddenly reduced to a predetermined value.

The area of the turn-back portion 21 and its vicinity 22 may be set appropriately according to the pulse width, but is preferably an area of 1% or less of the scanning width in the X direction from each end of the scanning width, and more preferably an area of 0.5 to 1%. For example, if the scanning width in the width direction (X direction) of the stainless steel plate 1 is 200 mm, the area of the turn-back portion 21 and its vicinity 22 can be an area of 1 to 2 mm from each end of the scanning width. With an area in this range, excessive increases in the amount of heat input in the turn-back portion 21 and its vicinity 22 can be stably suppressed.

Although the fluence of the pulsed laser beam 13 in the main scanning unit 20 is not particularly limited, it is preferable to determine an appropriate range according to the pulse width. Specifically, when the fluence of the pulsed laser beam 13 in the main scanning unit 20 is F [J/ ^cm2 ] and the pulse width is τ [ns], it is preferable to satisfy the following formula (1).
1×(τ/100) ^1/2 ≦F≦15×(τ/100) ^1/2 ...(1)
The descaling effect can be ensured by setting the fluence of the pulsed laser beam 13 in the main scanning unit 20 within the range of formula (1). The lower limit of formula (1) is preferably 2×(τ/100) ^1/2 , and more preferably 3×(τ/100) ^1/2 . The upper limit of formula (1) is preferably 13×(τ/100) ^1/2 , and more preferably 10×(τ/100) ^1/2 .
For example, when the pulse width is 100 ns, the fluence of the pulsed laser light 13 in the main scanning unit 20 is preferably 1 to 15 J/cm ² , more preferably 2 to 13 J/cm ² , and even more preferably 3 to 10 J/cm ^2. The pulse width can be set in the range of 10 to 1000 ns. The pulse width is preferably set in the range of 30 to 500 ns, and more preferably 50 to 300 ns.

The pulse width is the time per pulse that the pulsed laser light 13 exerts a thermal effect on the irradiated object. The fluence is the energy density per unit area of the pulsed laser light 13. In this specification, the fluence is the value obtained by dividing the energy per pulse of the pulsed laser light 13 by the irradiated area of the pulsed laser light 13. Therefore, if other conditions are the same, the longer the pulse width of the pulsed laser light 13, the greater the fluence of the pulsed laser light 13.

The fluence of the pulsed laser light 13 at the turning portion 21 and its vicinity 22 is not particularly limited as long as it is smaller than the fluence of the pulsed laser light 13 at the main scanning portion 20, but is preferably 20 to 90%, and more preferably 20 to 60%, of the fluence of the pulsed laser light 13 at the main scanning portion 20. By controlling it to such a range, it is possible to stably suppress an excessive increase in the amount of heat input at the turning portion 21 and its vicinity 22.

The fluence Fm [J/cm ² ] of the pulsed laser beam 13 in the turning portion 21 and its vicinity 22 can be expressed by formula (2) under the condition that it is smaller than the fluence of the pulsed laser beam 13 in the main scanning portion 20 .
0.5×(τ/100) ^1/2 ≦Fm≦10×(τ/100) ^1/2 ...(2)
In the formula, τ is the pulse width [ns].
By setting the fluence Fm to 10 × (τ/100) ^1/2 or less, even if surface defects occur, they can be removed by pickling, etc. Also, by setting the fluence Fm to 0.5 × (τ/100) ^1/2 or more, the descaling effect can be sufficiently obtained.
The lower limit of formula (2) is preferably 1 × (τ/100) ^1/2 , more preferably 2 × (τ/100) ^1/2 , and the upper limit of formula (2) is preferably 8 × (τ/100) ^1/2 , more preferably 7 × (τ/100) ^1/2 .
For example, when the pulse width is 100 ns, the fluence of the pulsed laser light 13 at the turning portion 21 and its vicinity 22 is preferably 0.5 to 10 J/cm ² , more preferably 1 to 8 J/cm ² , and further preferably 2 to 7 J/cm ² .

When the reciprocating scanning width of the pulsed laser beam 13 is smaller than the length of the stainless steel plate 1 in the X direction (e.g., the width direction), it is preferable to divide the region in the X direction into multiple regions and repeatedly perform reciprocating scanning of the pulsed laser beam 13 in each of the multiple regions.
As an example, FIG. 5 shows a schematic diagram illustrating the trajectory of the center of the pulsed laser beam 13 when the region in the X direction is divided into two and reciprocating scanning of the pulsed laser beam 13 is repeated in each of the two regions.
As shown in FIG. 5, the pulsed laser beam 13 irradiation step includes a first irradiation step in which the pulsed laser beam 13 is irradiated to a portion A of the surface of the stainless steel plate 1, and a second irradiation step in which the pulsed laser beam 13 is irradiated to a portion B of the surface of the stainless steel plate 1 after the first irradiation step. In the first irradiation step, the center of the pulsed laser beam 13 follows a locus 16, and in the second irradiation step, the center of the pulsed laser beam 13 follows a locus 17. At this time, the overlap D of the irradiation area of the pulsed laser beam 13 in the X direction between the portion A and the portion B is preferably 1 to 50%, more preferably 2 to 30%, of the scanning width in the X direction of each of the portion A and the portion B. By setting the overlap D of the irradiation area to 1% or more, the effect of descaling can be enhanced. Furthermore, by setting the overlap D of the irradiation area to 50% or less, efficient descaling can be achieved even when the reciprocating scanning width of the pulsed laser beam 13 is smaller than the length of the stainless steel plate 1 in the X direction (for example, the width direction).

After the irradiation step (laser descaling), a pickling step may be further included as necessary. By carrying out the pickling step, oxide scale can be stably removed.
The method of pickling is not particularly limited, but may be such _that the stainless steel sheet 1 is immersed in a pickling bath containing, for example, _HNO3 (nitric acid), _H2SO4 (sulfuric acid), _FeCl3 (ferric chloride solution), or a mixture of _HNO3 and HF (hydrofluoric acid).

As a pretreatment for the pickling process, a neutral salt electrolysis treatment may be performed on the surface of the stainless steel sheet 1 between the irradiation process and the pickling process. The neutral salt electrolysis treatment may be performed by immersing the stainless steel sheet 1 in a neutral salt aqueous solution containing a neutral salt such as _{Na2SO4 , K2SO4 ,} or _NaNO3 as a main component. By performing the neutral salt electrolysis treatment, the stainless steel sheet 1 after the irradiation process is electrolyzed in the neutral salt aqueous solution. Therefore, even if the oxide scale is an oxide having a dense structure and chemically stable structure such as a Cr-based oxide, the oxide scale can be reliably removed by the pickling process after the neutral salt electrolysis treatment.

In addition, when cold rolling is performed, the surface of the stainless steel sheet 1 may be subjected to a molten salt immersion (salt bath) treatment as a pretreatment, for example, between the irradiation step (S51) and the pickling treatment step. The salt bath treatment is performed by immersing the stainless steel sheet 1 in a mixed alkaline molten salt bath maintained at 400 to 500°C with caustic soda (NaOH) as the main agent.

In the oxide scale removal step (S5), it is not essential to subject the surface of the stainless steel sheet 1 after the laser light irradiation step to pickling. Although it depends on the type of steel of the stainless steel sheet 1 to be treated and the conditions of the laser irradiation, if the oxide scale can be sufficiently removed by the laser light irradiation step alone, the pickling step may be omitted. In other words, in the oxide scale removal step (S5), it is sufficient that at least the laser light irradiation step is performed.
In addition, although the above description has been specifically given using stainless steel plate 1 as an example, it should be noted that the method for removing oxide scale according to an embodiment of the present invention can also be applied to stainless steel materials other than stainless steel plate 1.

The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these.
By carrying out the steelmaking step (S1), the continuous casting step (S2), the hot rolling step (S3), and the continuous annealing step (S4) described above, a hot-rolled annealed sheet (stainless steel sheet) of SUS316 steel type was produced.
Next, the hot-rolled annealed sheet was subjected to laser descaling as an oxide scale removal step (S5). The laser descaling was performed using a commercially available device (LaserClear50A manufactured by IHI Inspection and Measurement Co., Ltd.). The hot-rolled annealed sheet was placed on the movable stage of this device, and the hot-rolled annealed sheet was irradiated with a pulsed laser beam while scanning back and forth at a constant speed in the sheet width direction (X direction) from above the hot-rolled annealed sheet, and moved at 0.5 m/min along the rolling direction (Y direction). The reciprocating scanning width was 25 mm, and the irradiation conditions of the pulsed laser beam were as follows.

(Irradiation Condition A)
Wavelength: 1085 nm
Pulse width: 100ns
Oscillation frequency: 120kHz
Irradiation speed: 100Hz
Laser beam diameter: 90 μm
Fluence: 6.55 J/ ^cm2

(Irradiation condition B)
Wavelength: 1085 nm
Pulse width: 100ns
Oscillation frequency: 120kHz
Irradiation speed: 100Hz
Laser beam diameter: 90 μm
Fluence: 3.93 J/ ^cm2

The surface of the hot-rolled and annealed sheet after laser descaling was observed using an optical microscope, and the results (optical microscope photograph at 150 magnifications) are shown in FIG.
For the observed optical microscope photographs, the areas where oxide scale remains and the areas where oxide scale has been removed were binarized using the difference in brightness, and the area of the areas where oxide scale has been removed was divided by the area of the observation range to calculate the scale removal rate. As a result, when laser descaling was performed under irradiation condition A, the scale removal rate of the turn-back portion and its vicinity was 75%, and the scale removal rate of the main scanning portion was 72%. When laser descaling was performed under irradiation condition B, the scale removal rate of the turn-back portion and its vicinity was 55%, and the scale removal rate of the main scanning portion was 52%. Note that if the scale removal rate is 50% or more, the remaining scale can be easily removed by pickling or the like, and therefore it can be determined that the effect of scale removal is good.

In addition, the cross-sectional curve was measured in the main scanning portion (P1) of the reciprocating scan and near the turning portion (P2) in the optical microscope photograph, and the arithmetic mean height Ra and maximum height Rz were calculated. P1 was located 700 μm from the end, and P2 was located 100 μm from the end. The cross-sectional curve was measured using a laser microscope (OLS4100, manufactured by Olympus Corporation). The results of the measured cross-sectional curve are shown in Figure 6.

As shown in Fig. 6, when laser descaling was performed under irradiation condition A (upper part of Fig. 6), the formation of excessively uneven parts was confirmed in the folded part and its vicinity. The excessively uneven parts had an arithmetic mean height Ra of 4.7 µm and a maximum height Rz of 30.0 µm. Therefore, the excessively uneven parts cause surface defects. On the other hand, no excessively uneven parts were confirmed in the main scanning part, and the arithmetic mean height Ra was 1.1 µm and the maximum height Rz was 7.4 µm.
In addition, when laser descaling was performed under irradiation condition B (lower part of FIG. 6), small irregularities were confirmed in the folded portion and its vicinity. The arithmetic mean height Ra of this small irregularity was 2.1 μm, and the maximum height Rz was 14.9 μm. Therefore, this small irregularity is unlikely to cause surface defects. Furthermore, no small irregularities were confirmed in the main scanning portion, and the arithmetic mean height Ra was 0.9 μm, and the maximum height Rz was 6.5 μm.
Therefore, when irradiating the pulsed laser light while scanning back and forth, it is presumed that by irradiating the pulsed laser light in the main scanning portion under irradiation condition A and irradiating the turning portion and its vicinity under irradiation condition B, descaling can be performed without forming excessively uneven portions in the turning portion and its vicinity.

As can be seen from the above results, the present invention can provide a method for removing oxide scale that can suppress the formation of excessively uneven portions due to an increase in the amount of heat input at and near the turning points during the reciprocating scanning of the laser light. Furthermore, the present invention can provide a method for manufacturing stainless steel material that has high production efficiency and a good appearance.

REFERENCE SIGNS LIST 1 Stainless steel plate 10 Laser light irradiation device 11 Laser oscillator 12 Laser head 13 Pulsed laser light 15, 16, 17 Trajectory of central part of pulsed laser light 20 Main scanning section 21 Turning section 22 Vicinity of turning section

Claims (6)

A method for removing oxide scale formed on a surface of a stainless steel material after hot rolling or cold rolling by irradiating the surface with a pulsed laser beam, comprising the steps of:
an irradiation step of irradiating the stainless steel material with the pulsed laser light while scanning it back and forth in an X direction and moving the stainless steel material in a Y direction perpendicular to the X direction;
and a pickling treatment step of performing pickling treatment after the irradiation step,
The pulse width of the pulsed laser light is 10 to 1000 ns,
a fluence of the pulsed laser light in a return portion of the reciprocating scan and in the vicinity thereof is made smaller than a fluence of the pulsed laser light in a main scanning portion of the reciprocating scan;
The irradiation step includes a first irradiation step of irradiating a portion A of the surface of the stainless steel material with the pulsed laser light, and a second irradiation step of irradiating a portion B of the surface of the stainless steel material with the pulsed laser light after the first irradiation step,
an overlap of an irradiation area of the pulsed laser light between the part A and the part B in the X direction is 1 to 50% of a scanning width in the X direction of each of the part A and the part B. The method for removing oxide scale according to claim 1, wherein the folded portion and its vicinity are an area of 1% or less of the scanning width in the X direction. The method for removing oxide scale according to claim 1 or 2, wherein the pulsed laser light in the main scanning section satisfies the following formula (1) when the fluence is F [J/cm ² ] and the pulse width is τ [ns].
1×(τ/100) ^1/2 ≦F≦15×(τ/100) ^1/2 ...(1) The method for removing oxide scale according to any one of claims 1 to 3, wherein the fluence of the pulsed laser light in the turning portion and its vicinity is 20 to 90% of the fluence of the pulsed laser light in the main scanning portion. The method for removing oxide scale according to any one of claims 1 to 4, wherein the stainless steel material is a stainless steel plate. A method for producing a stainless steel material, comprising the method for removing oxide scale according to any one of claims 1 to 5 .