JP2016206008A - Billet surface defect inspection method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a billet surface defect inspection method that enables surface defect of a billet to be inspected with relatively high accuracy.SOLUTION: The billet surface defect inspection method of the present invention comprises a step of removing scale of a billet surface layer and a step of inspecting the surface defects of the billets after the scale removal step by magnetic particle testing or magnetic leakage flux testing. In the billet surface defect inspection method, the scale removal step is characterized by having a step of removing a scale layer of the billet surface by shot blast, and a step of removing scale remaining in a flaw of an exposed billet surface after the scale layer removal step. The residual scale removal step may be performed by wet blast for spraying liquid containing abrasive particles. The residual scale removal step may be performed by brushing. The residual scale removal step may be performed by spraying of high pressure water.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a billet surface defect inspection method.

  If a surface defect exists in a steel piece before being rolled into a wire, a steel bar or the like, harmful flaws (scratches) are generated in the product after rolling. Therefore, the presence or absence of surface defects in the steel slab before rolling is inspected by using, for example, a magnetic particle flaw detector, a leakage magnetic flux flaw detector, etc. After that, it has been subjected to a rolling process. However, in actual manufacturing sites, the accuracy of detecting flaws in steel slab surface defect inspection is insufficient, so that steel slabs having surface defects can be overlooked, reducing the quality and production efficiency of rolled products. It is a problem.

  As a method to improve the detection accuracy of the steel slab surface defect inspection by magnetic particle flaw detection, after removing the scale by performing shot blasting that collides the projection material with the surface, the steel slab is corrected and then the same as the steel slab surface again. It has been proposed to perform shot blasting that causes a projectile material to collide, and then to detect the surface of a steel slab (see, for example, JP-A-62-277551). However, even with this method, it cannot be said that the detection accuracy of the surface defect is sufficient, and the oversight of the surface defect may still occur.

JP-A-62-277551

  In view of the above problems, an object of the present invention is to provide a steel slab surface defect inspection method capable of detecting a surface defect of a steel slab with relatively high accuracy.

  The present inventors analyzed in detail about the steel piece which could not detect a surface defect by the surface defect inspection method by the conventional magnetic particle flaw detection. As a result, it was confirmed that the surface defects that could not be detected by the magnetic particle flaw detection were those in which a scale was formed as shown in the cross-sectional photograph of FIG. That is, it is considered that the scale existing inside the scratch suppresses the generation of the magnetic flux leakage when the steel piece is magnetized and inhibits the discovery of the scratch.

  As a result of intensive studies based on this new knowledge, the present inventors have completed the following present invention.

  That is, the invention made to solve the above-mentioned problems includes a step of removing the scale of the steel slab surface layer, and a step of inspecting the surface defect of the steel slab after the scale removing step by magnetic particle inspection or leakage magnetic flux inspection. A billet surface defect inspection method, wherein the scale removal step includes a step of removing a scale layer on the surface of the billet by shot blasting, and a scale remaining in a flaw on the surface of the billet exposed after the scale layer removal step. It is a steel slab surface defect inspection method characterized by having a process to remove.

  The billet surface defect inspection method includes a step of removing a scale layer on the billet surface by shot blasting, and a step of removing a scale remaining in a flaw on the billet surface exposed after the scale layer removing step. By providing the removal step, it is possible to suppress a decrease in surface defect detection accuracy due to the residual scale in the scratch disturbing the magnetic flux. For this reason, the said steel piece surface defect inspection method can detect the surface defect of a steel piece with comparatively high precision in an inspection process.

  The residual scale removing step may be performed by wet blasting with a liquid containing abrasive grains. In this way, by performing the residual scale removing step by wet blasting that sprays a liquid containing abrasive grains, the abrasive grains can collide with the residual scale inside the scratches by water pressure, so that the scratches are not enlarged. Residual scale inside the wound can be efficiently removed. For this reason, the influence of a residual scale can be excluded effectively and the surface defect of a steel piece can be detected with higher accuracy in the inspection process.

  The residual scale removing step may be performed by brushing. Thus, by performing the residual scale removal step by brushing, the hair of the brush enters the scratch and scrapes the residual scale inside the scratch, so that the residual scale is efficiently removed without enlarging the scratch. be able to. For this reason, the influence of a residual scale can be excluded effectively and the surface defect of a steel piece can be detected with higher accuracy in the inspection process.

  The residual scale removing step may be performed by jetting high pressure water. In this way, by performing the residual scale removal step by jetting high-pressure water, the residual scale in the scratch can be scraped out by the high-pressure water, and the residual scale can be efficiently removed without enlarging the scratch. For this reason, the influence of a residual scale can be excluded effectively and the surface defect of a steel piece can be detected with higher accuracy in the inspection process.

  The steel slab surface defect inspection method of the present invention can detect a surface defect of a steel slab with relatively high accuracy.

It is a microscope picture which shows an example of the cross section of the surface defect of a steel piece. It is a flowchart which shows the procedure of the steel piece surface defect inspection method which concerns on one Embodiment of this invention. It is a microscope picture which shows the cross section of the artificial defect of the steel piece used for the steel piece surface defect inspection experiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

[Slab surface inspection method]
In FIG. 2, the procedure of the steel piece surface defect inspection method which concerns on one Embodiment of this invention is shown.

  The steel slab surface defect inspection method is roughly divided into a step of removing the scale of the steel slab surface layer (step S1: scale removal step), and a step of inspecting the surface defect of the steel slab after the scale removal step by magnetic particle inspection. (Step S2: inspection process) and two processes.

[Scale removal process]
The scale removal process of step S1 remains in the process of removing the scale layer on the surface of the steel slab by shot blasting (step S11: scale layer removal process) and the scratch on the surface of the steel slab exposed after the scale layer removal process. And a step of removing the scale (step S12: residual scale removing step).

<Scale layer removal process>
In the scale layer removing step of step S11, the scale layer on the steel slab surface is scraped off by shot blasting in which the particulate projection material is projected and collides with the steel slab surface. Thereby, the flaw which exists in the steel piece surface is exposed.

  Examples of the method for projecting the projection material in shot blasting include a mechanical method in which the projection material is projected by the rotational force of an impeller, and an air method in which the projection material is projected by a jet of compressed air.

The lower limit of the average hardness of the projection material used for shot blasting is preferably 200 kgf / mm ^{2 and} more preferably 300 kgf / mm ² in terms of Vickers hardness (Hv). In contrast, the upper limit of the average hardness of the projection material used for shot blasting, preferably ^{800kgf / mm 2, 600kgf / mm} 2 is more preferable. If the average hardness of the projection material used for shot blasting is less than the lower limit, the removal efficiency of the scale layer may be lowered. On the contrary, when the average hardness of the projection material used for shot blasting exceeds the above upper limit, the steel piece under the scale layer may be damaged. The “Vickers hardness” is a value measured according to JIS-Z2244 (2009).

  Examples of such a projection material include sand, iron, and steel.

  Further, the shape of the projection material is not particularly limited, and may be an angular grit shape, a shot shape without corners, or the like, but a shot shape that hardly damages a steel piece under the scale layer is preferable.

  As a minimum of an average diameter of a projection material, 0.8 mm is preferred and 1 mm is more preferred. On the other hand, the upper limit of the average diameter of the projection material is preferably 3 mm, and more preferably 2 mm. When the average diameter of the projection material is less than the above lower limit, the impact energy of the projection material is reduced, which may reduce the removal efficiency of the scale layer. On the contrary, when the average diameter of the projection material exceeds the upper limit, there is a possibility that the removal of the scale layer may be uneven or the steel piece under the scale layer may be damaged. The “average diameter” means a value that is 50% of the mass integrated value obtained from the particle size distribution measured according to JIS-Z0311 (2004).

The lower limit of the shot-blasting of the projection density (mass of the shot material is projected per unit area of the billet), preferably from 20 kg / ^{m 2,} more preferably 30kg / ^{m 2.} On the other hand, the upper limit of the projection density of shot blasting, preferably ^{120kg / m 2, 100kg / m} 2 is more preferable. If the shot blast projection density is less than the lower limit, the removal of the scale layer may be insufficient. Conversely, when the shot blast projection density exceeds the above upper limit, the steel piece under the scale layer may be damaged.

  The lower limit of the average shot blasting time (time for projecting the projection material to the same position) is preferably 0.5 seconds, and more preferably 1 second. On the other hand, the upper limit of the average shot blasting time is preferably 10 seconds, and more preferably 3 seconds. When the average shot time of shot blasting is less than the above lower limit, there is a possibility that a sufficient projection amount cannot be secured, or the projected projection materials collide with each other, and there is a possibility that removal of the scale layer may be hindered. If the average projection time exceeds the upper limit, productivity may be hindered due to excessive projection.

  The lower limit of the average shot blasting speed is preferably 20 m / sec, and more preferably 50 m / sec. On the other hand, the upper limit of the average projection speed of shot blast is preferably 100 m / sec, and more preferably 80 m / sec. When the average shot blasting speed is less than the lower limit, the removal of the scale layer may be insufficient. On the other hand, when the average shot blasting speed exceeds the above upper limit, there is a risk of scratching the steel piece below the scale layer.

<Residual scale removal process>
In the residual scale removing step in step S12, the scale in the scratch is removed using a medium that can enter the scratch on the surface of the steel slab.

  Specific methods for removing the residual scale as described above include, for example, wet blasting, brushing, high-pressure water injection, pickling, shot blasting using a projection material having a smaller diameter than the scale layer removing step, and the like. Among these, the residual scale removal step is preferably performed by wet blasting, brushing, or high-pressure water jet.

(Wet blast)
The wet blasting is a method of scraping off the residual scale in the scratches on the surface of the steel slab with abrasive grains by spraying a liquid (slurry) containing fine abrasive grains on the surface of the steel slab.

  Examples of the material of abrasive grains used for wet blasting include steel, alumina, silicon carbide and the like.

  As a minimum of the average diameter of the abrasive grain used for wet blasting, 5 micrometers is preferred and 50 micrometers is more preferred. On the other hand, the upper limit of the average diameter of the abrasive grains used for wet blasting is preferably 200 μm, and more preferably 150 μm. When the average diameter of the abrasive grains used for wet blasting is less than the lower limit, the effect of scraping off the residual scale may be insufficient. On the contrary, when the average diameter of the abrasive grains used for wet blasting exceeds the above upper limit, the abrasive grains cannot enter the flaws, and the residual scale may not be sufficiently scraped off.

  The liquid used for wet blasting is not particularly limited, but water is generally used.

  As a minimum of the ratio of the abrasive grain in the slurry used for wet blasting, 5 mass% is preferred and 10 mass% is more preferred. On the other hand, the upper limit of the proportion of abrasive grains in the slurry used for wet blasting is preferably 40% by mass, and more preferably 35% by mass. When the ratio of the abrasive grains in the slurry used for wet blasting is less than the lower limit, the effect of scraping off the residual scale may be insufficient. Conversely, when the proportion of abrasive grains in the slurry used for wet blasting exceeds the above upper limit, it may not be easy to inject the slurry at a sufficient speed.

  The lower limit of the average blasting time for wet blasting (the time during which the slurry is projected at the same position) is preferably 15 seconds, and more preferably 30 seconds. On the other hand, the upper limit of the average projection time of wet blasting is preferably 180 seconds, and more preferably 120 seconds. If the average projection time of wet blasting is less than the lower limit, the residual scale may not be sufficiently removed. Conversely, when the average projection time of wet blasting exceeds the above upper limit, productivity may be hindered due to excessive projection of wet blasting.

  The lower limit of the average projection speed of wet blasting is preferably 50 m / sec, and more preferably 80 m / sec. On the other hand, the upper limit of the average projection speed of wet blasting is preferably 1200 m / sec, and more preferably 150 m / sec. When the average projection speed of wet blasting is less than the above lower limit, there is a risk that removal of residual scale may be insufficient. Conversely, when the average projection speed of wet blasting exceeds the above upper limit, the equipment cost may be unnecessarily increased.

(Brushing)
The brushing is a method of scraping off the residual scale in the scratch on the surface of the steel slab with a brush in which a small-diameter strand capable of entering into the scratch on the surface of the steel slab is implanted. This brushing can be performed using, for example, a rotating brush.

  The lower limit of the average diameter of the strands planted on the brush used for brushing for residual scale removal is preferably 1 μm and more preferably 5 μm. On the other hand, the upper limit of the average diameter of the strands implanted in the brush is preferably 200 μm, and more preferably 150 μm. If the average diameter of the strands planted on the brush is less than the lower limit, the strands may have insufficient rigidity, and the residual scale may not be sufficiently scraped off. On the other hand, when the average diameter of the strands implanted in the brush exceeds the above upper limit, the strands cannot enter the flaws on the surface of the steel piece, and there is a possibility that the removal of the residual scale may be insufficient.

  Examples of the material of the brush wire include a resin such as nylon and a resin mixed with abrasive grains. Moreover, as a rotation speed in the case of using a rotating brush, it can be set to 1000 rpm or more and 1500 rpm or less, for example. In addition, as the relative movement speed of the central axis of the rotating brush with respect to the steel piece (the moving speed of the steel piece when the position of the rotating brush is fixed, the moving speed of the rotating brush center axis when the steel piece is moved), For example, it can be 20 m / min or more and 80 m / min or less. Moreover, as a press-contact force per unit length with respect to the steel piece of a rotating brush, it can be 10 kN / m or more and 30 kN / m or less, for example.

(High pressure water injection)
The high-pressure water injection is a method of injecting high-pressure water onto the surface of a steel slab and causing the high-pressure water to enter a scratch to scrape off the residual scale. Such high-pressure water injection can be performed using, for example, a flat pattern spray nozzle that is commercially available for scale removal.

  As a collision pressure with respect to the steel piece surface of high pressure water, it shall be 10 MPa or more and 50 MPa or less, for example. For this reason, as an average pressure (supply pressure to a nozzle) before injection of high-pressure water, it can be 10 MPa or more and 50 MPa or less, for example. The average flow rate per spray nozzle depends on the specifications of the spray nozzle. For example, when the spray angle is 32 degrees, the distance from the steel slab surface of the spray nozzle to 100 mm or more and 300 mm or less is 60 L / min. It can be set to 200 L / min or less.

(Pickling)
The pickling is a method in which a steel piece is immersed in an acid solution to dissolve and remove residual scale in the scratches.

  Examples of the acidic liquid used for pickling include hydrochloric acid and sulfuric acid.

  The concentration of the acidic solution may be 10% by mass to 35% by mass with hydrochloric acid and 20% by mass to 60% by mass with sulfuric acid. Moreover, as temperature of an acidic liquid, it can be set as 15 to 85 degreeC, for example. Further, the immersion time of the steel slab in the acidic solution may be, for example, 20 seconds or more and 3 minutes or less, depending on the concentration of the acidic solution.

(Shot blast)
The conditions for the shot blasting performed as the residual scale removing step may be the same as those for the shot blasting in the scale layer removing step, except that the average diameter of the projection material is smaller than the shot blasting in the scale layer removing step. it can.

  The lower limit of the average diameter of the projection material in shot blasting performed as the residual scale removing step is preferably 0.1 mm, and more preferably 0.3 mm. On the other hand, the upper limit of the average diameter of the projection material is preferably 0.7 mm, and more preferably 0.5 mm. When the average diameter of the projection material is less than the lower limit, there is a possibility that the residual scale cannot be sufficiently removed due to the collision energy of the projection material becoming small, or the projection material may not be stably projected. On the other hand, when the average diameter of the projection material exceeds the above upper limit, it is not possible to enter the inside of the scratch and the residual scale may not be sufficiently removed.

  The lower limit of the ratio of the average diameter of the projection material in shot blasting in the residual scale removal step to the average diameter of the projection material in shot blasting in the scale layer removal step is preferably 0.05, and more preferably 0.1. . On the other hand, the upper limit of the ratio of the average diameter of the projection material is preferably 0.5, and more preferably 0.3. When the ratio of the average diameter of the projection material is less than the lower limit, the projection material in the residual scale removal process becomes excessively small, and the residual scale removal process may take time. On the contrary, when the ratio of the average diameter of the projection material exceeds the upper limit, the projection material in the residual scale removing process becomes excessively large, and the residual scale may not be sufficiently removed.

[Inspection process]
In the inspection process in step S2, the surface defect (scratch) of the steel piece from which the scale layer and the residual scale have been removed in step S1 is detected by magnetic particle inspection or leakage magnetic flux inspection.

(Magnetic particle inspection)
Magnetic particle flaw detection is an inspection method that makes it easy to visually recognize a surface defect by magnetizing a steel piece and attaching magnetic powder to a portion where the surface defect exists by utilizing a leakage magnetic flux generated by the surface defect. In this magnetic particle flaw detection, the adhesion of magnetic particles due to surface defects may be determined visually, or the adhesion of magnetic particles may be determined using an image processing technique. The magnetic particle flaw detection is preferably a fluorescent magnetic particle flaw detection that confirms the adhesion of the magnetic powder by irradiating with ultraviolet rays using the tendency magnetic powder. This magnetic particle flaw detection can be performed in accordance with JIS-Z2320-1 (2007).

(Leakage magnetic flux inspection)
Leakage magnetic flux inspection is an inspection method in which a steel piece is magnetized and leakage magnetic flux caused by surface defects is detected by a sensor, and can be performed using a commercially available leakage magnetic flux inspection device.

[Process after inspection of surface defects on steel slab]
In addition, the steel slab in which the surface defect was found by the method for inspecting a surface defect of the steel slab is to scrape the entire surface by chipping etc. Can be used.

〔advantage〕
The billet surface defect inspection method includes a step of removing a scale layer on the billet surface by shot blasting, and a step of removing a scale remaining in a flaw on the billet surface exposed after the scale layer removing step. By providing the removal step, it is possible to suppress a decrease in surface defect detection accuracy due to the residual scale in the scratch disturbing the magnetic flux. For this reason, the said steel piece surface defect inspection method can detect the surface defect of a steel piece with comparatively high precision in an inspection process.

[Other Embodiments]
The said embodiment does not limit the structure of this invention. Therefore, in the above-described embodiment, the components of each part of the above-described embodiment can be omitted, replaced, or added based on the description and common general knowledge of the present specification, and they are all interpreted as belonging to the scope of the present invention. Should.

  Hereinafter, although this invention is explained in full detail based on an experiment example, this invention is not interpreted limitedly based on description of this experiment example.

<Slab>
As a steel slab to be a specimen in steel slab surface defect inspection experiments 1 to 7 to be described later, it is a hot rolled steel slab corresponding to SWRCH20A defined in JIS-G3507-2 (2005), having an average width of 90 mm and an average What was formed in the rectangular parallelepiped shape of length 300mm and average thickness 70mm was used.

  Ten artificial flaws as shown in FIG. 3 were formed on the steel piece. This artificial flaw is formed by imitating a surface defect that could not be detected by magnetic particle flaw detection after shot blasting once on the surface of a steel piece, and reproduced a surface defect that bites a scale into the flaw. It is.

<Slab surface defect inspection experiment 1>
As a steel piece surface defect inspection experiment 1, a single shot blast (scale layer removal process) was performed on the steel piece on which the artificial flaw was formed, and the magnetic flaw detection (inspection process) was performed as it was to detect the artificial flaw.

(Shot blast)
For shot blasting, a steel shot (iron ball) having a Vickers hardness of 390 kgf / mm ² or more and 510 kgf / mm ² or less was used as a projection material. Shot blasting was performed by adjusting the projection capacity to 720 kg / min and the projection density to 50 kg / m ² .

(Magnetic particle inspection)
Magnetic particle inspection was performed according to JIS-Z2320-1. The intensity of the magnetic field for magnetizing the steel slab was set to a magnetic field intensity of 25 Oe or more recommended by JIS. Moreover, as the magnetic powder adhered to the leakage magnetic flux due to surface defects, a powder dispersed in water so as to have a concentration of 0.2 g / L or more and 2.0 g / L or less was used. Confirmation of the adhesion of magnetic powder, that is, detection of surface defects was performed by visual observation with irradiation with ultraviolet light having an illuminance of 1000 μW / cm ² or more.

  In this steel piece surface defect inspection experiment 1, it was possible to detect 6 artificial flaws out of 10 artificial flaws.

<Slab surface defect inspection experiment 2>
After performing the same shot blasting (scale layer removing step) as the steel slab surface defect inspection experiment 1 as the steel slab surface defect inspection experiment 2, the above steel slab is subjected to the same shot blasting again. Magnetic particle flaw detection (inspection process) similar to the surface defect inspection experiment 1 was performed to detect the artificial flaw. Also, in the inspection process, the amount of magnetic particles adhering to the entire steel piece, that is, magnetic particles that adhere to parts other than the artificial flaw and become noise that makes it difficult to detect the flaw is confirmed, and a steel piece surface defect inspection experiment 3 described later. It was set as a reference for evaluating the amount of noise in the inspection process at ˜7.

  In this steel piece surface defect inspection experiment 2, eight artificial flaws out of ten artificial flaws could be detected.

<Slab surface defect inspection experiment 3>
As the steel slab surface defect inspection experiment 3, after performing the same shot blasting (scale layer removal process) as the above steel slab surface defect inspection experiment 1, except that the average diameter of the projection material was changed to 0.3 mm. After performing shot blasting (residual scale removing step) under the same conditions as in the scale layer removing step, magnetic particle flaw detection (inspecting step) similar to that in the steel piece surface defect inspection experiment 1 was performed to detect the artificial flaw.

  In this steel piece surface defect inspection experiment 3, nine artificial flaws out of ten artificial flaws could be detected. Moreover, the noise in the inspection process in the steel slab surface defect inspection experiment 3 was less than the noise in the steel slab surface defect inspection experiment 2.

<Slab surface defect inspection experiment 4>
After performing the shot blasting (scale layer removing step) similar to the above-described steel piece surface defect inspection experiment 1 as the steel piece surface defect inspection experiment 4, and further performing pickling (residual scale removing step), A magnetic particle flaw detection (inspection process) similar to that in the steel piece surface defect inspection experiment 1 was performed to detect the artificial flaw.

(Pickling)
In pickling, the steel piece was immersed for 120 seconds in hydrochloric acid having a concentration of 35% by mass at a temperature of 25 ° C.

  In this steel piece surface defect inspection experiment 4, all ten artificial flaws could be detected. Moreover, the noise in the inspection process in the steel slab surface defect inspection experiment 4 was larger than the noise in the steel slab surface defect inspection experiment 2 (same as the steel slab surface defect inspection experiment 3).

<Slab surface defect inspection experiment 5>
As the steel slab surface defect inspection experiment 5, after performing one shot blasting (scale layer removal process) similar to the above-described steel slab surface defect inspection experiment 1, further brushing (residual scale removal process) is performed, and then Magnetic particle flaw detection (inspection process) similar to the steel piece surface defect inspection experiment 1 was performed to detect the artificial flaw.

(Brushing)
For brushing, a brush roll “C-32” (wire diameter 20 μm) manufactured by Hotani Co., Ltd. was used at a rotational speed of 1200 rpm and a vertical pressing force of 1700 N against the steel slab, and the steel slab conveying speed was 40 m / min. As a result, a reaction force of 875 N was applied to the brush roll in the horizontal direction.

  In this steel slab surface defect inspection experiment 5, all ten artificial flaws could be detected. Moreover, the noise in the inspection process in the steel slab surface defect inspection experiment 5 was less than the noise in the steel slab surface defect inspection experiment 2.

<Slab surface defect inspection experiment 6>
As the steel slab surface defect inspection experiment 6, after performing one shot blasting (scale layer removing process) similar to the above steel slab surface defect inspection experiment 1, further spraying high-pressure water (residual scale removing process) From the above, a magnetic particle flaw detection (inspection process) similar to that in the steel piece surface defect inspection experiment 1 was performed to detect the artificial flaw.

(High pressure water injection)
The high-pressure water injection was performed using a descaling nozzle “Everloy DNH1132” manufactured by Kyoritsu Alloy Manufacturing Co., Ltd. as the spray nozzle, and the supply pressure to the high-pressure water spray nozzle was 15 MPa (injection flow rate 82.9 L / min).

  In this steel piece surface defect inspection experiment 6, all ten artificial flaws could be detected. Moreover, the noise in the inspection process in the steel slab surface defect inspection experiment 6 was less than the noise in the steel slab surface defect inspection experiment 2 (similar to the steel slab surface defect inspection experiments 3 and 5).

<Slab surface defect inspection experiment 7>
After performing one shot blasting (scale layer removal process) similar to the above-described steel slab surface defect inspection experiment 1 as a steel slab surface defect inspection experiment 7, further performing wet blasting (residual scale removal process), A magnetic particle flaw detection (inspection process) similar to that in the steel piece surface defect inspection experiment 1 was performed to detect the artificial flaw.

(Wet blast)
Wet blasting uses a general-purpose wet blasting machine manufactured by Macau, using alumina having an average diameter of 0.2 mm as abrasive grains, and dispersing the abrasive grains in water to produce a slurry having an abrasive grain content of 15% by mass. It adjusted and injected on the steel piece surface by injecting from a nozzle. The distance from the steel slab surface of the nozzle was 100 mm, and the slurry projection speed was 100 m / sec. The projection time of wet blasting was 60 seconds.

  In this steel slab surface defect inspection experiment 7, all ten artificial flaws could be detected. Moreover, the noise in the inspection process in the steel slab surface defect inspection experiment 7 was significantly less than the noise in the steel slab surface defect inspection experiment 2.

  As described above, in the steel slab surface defect inspection experiments 3 to 7 in which the residual scale removal process was performed, the surface defect detection rate was improved as compared with the steel slab surface defect inspection experiment 1 in which the residual scale removal process was not performed. . In particular, in the steel piece surface defect inspection experiments 5 to 7 in which the brush removal, high-pressure water injection or wet blasting was performed as the residual scale removal process, all surface defects can be detected in the inspection process, and the noise is low. Judgment was easy.

  The billet surface defect inspection method according to the present invention is suitably used for the inspection of a rolled billet.

S1 Scale removal process S2 Inspection process S11 Scale layer removal process S12 Residual scale removal process

Claims (4)

A billet surface defect inspection method comprising a step of removing a scale of a billet surface layer and a step of inspecting a surface defect of the billet after the scale removal step by magnetic particle flaw detection or leakage magnetic flux flaw detection,
The scale removal step is
Removing the scale layer on the surface of the billet by shot blasting;
And a step of removing scale remaining in the scratches on the surface of the steel slab exposed after the scale layer removing step.   The steel slab surface defect inspection method according to claim 1, wherein the residual scale removing step is performed by wet blasting with a liquid containing abrasive grains.   The steel slab surface defect inspection method according to claim 1, wherein the residual scale removing step is performed by brushing. The slab surface defect inspection method according to claim 1, wherein the residual scale removing step is performed by jetting high-pressure water.