Surface inspection method, surface inspection device, manufacturing system, method of identifying defect formed area, and manufacturing method of steel pipe

1. A surface inspection method for inspecting a steel pipe emitting light as a result of hot rolling, the method comprising:

imaging a self-luminous image of a surface of the steel pipe in longitudinal direction and circumferential direction after the hot rolling, luminance of the self-luminous image depending on a surface temperature of the steel pipe;

correcting the luminance of the self-luminous image by making variation in the luminance more uniform in a circumferential direction of the self-luminous image; and

detecting a surface defect based on the self-luminous image whose luminance has been corrected.

2. The method according to claim 1 , wherein the self-luminous image is captured in a position behind a reducer.

3. The method according to claim 2 , wherein a diameter reduction rate applied by the reducer for the steel pipe is equal to or greater than 110%.

4. The method according to claim 1 , wherein the correcting step makes more uniform luminance variation in the circumferential direction of the self-luminous image by using a luminance distribution in the circumferential direction where luminance in a longitudinal direction of the steel pipe on the self-luminous image is averaged.

5. The method according to claim 1 , wherein the correcting step makes more uniform a luminance variation in the circumferential direction of the self-luminous image by using a difference in luminance between a plurality of self-luminous images captured with a position in a longitudinal direction of the steel pipe changed.

6. The method according to claim 1 , wherein the imaging step captures the self-luminous image by using a bandwidth of equal to or longer than 700 nm wavelength of near infrared light and equal to or shorter than 20 μm wavelength of infrared light.

7. A method of identifying a defect formed area on a steel pipe, the method comprising:

inspecting the steel pipe and detecting a surface defect on the steel pipe with the method according to claim 1 ;

determining that the surface defect is caused in a steel pipe manufacturing process when regularity is seen in a position where the surface defect has been detected; and

determining that the surface defect is caused at a steel making stage when irregularity is seen in the position where the surface defect has been detected.

8. A method of manufacturing a steel pipe, comprising:

changing a manufacturing condition in a steel pipe manufacturing process and/or an operation condition at a steel making stage to prevent generation of a further surface defect based on a cause of the surface defect identified with the method of identifying a defect formed area according to claim 7 .

9. A surface inspection device for inspecting a steel pipe emitting light as a result of heating, the device comprising:

an imaging device configured to capture a self-luminous image of a surface of the steel pipe, luminance of the self-luminous image depending on a surface temperature of the steel pipe;

a correcting device configured to correct the luminance of the self-luminous image by making variation in the luminance more uniform in a circumferential direction of the self-luminous image; and

a detecting device configured to detect a surface defect based on the self-luminous image whose luminance has been corrected.

10. A steel pipe manufacturing system comprising the device according to claim 9 .