Method for measuring flatness of sheet, device for measuring flatness of sheet, and production method for steel sheet

A device for measuring flatness of a plate material, comprising:

• a projection unit situated between adjacent rolling stands, the projection unit projecting a light and dark pattern composed of light portions and dark portions onto a surface of the plate material that is traveling between the rolling stands;
• an image capture unit situated between the rolling stands, the image capture unit acquiring a pattern image by capturing an image of the light and dark pattern;
• and
• an image analysis device that measures the flatness of the plate material by analyzing the acquired pattern image,
• wherein if
• L0 represents a distance in a horizontal direction between an intersection of respective optical axes of the projection unit and the image capture unit and, of the adjacent rolling stands, the rolling stand that is disposed at the opposite side of the intersection with respect to a looper that controls tension of the plate material between the rolling stands,
• α represents an angle formed between a vertical direction and a straight line formed by projecting the optical axis of the image capture unit onto a plane that contains a direction parallel to a rolling direction and the vertical direction, a value of α being positive if the optical axis of the image capture unit is inclined from the vertical direction toward the looper side,
• β represents an angle formed between the vertical direction and a straight line formed by projecting the optical axis of the projection unit onto the plane, a value of β being positive if the optical axis of the projection unit is inclined from the vertical direction toward the rolling stand side,
• hc represents a distance in the vertical direction between a pass line, which is a straight line passing through crest points of respective lower side work rolls of pairs of upper and lower work rolls that are respectively disposed at the adjacent rolling stands, and an image principal point of a lens of the image capture unit, and
• hp represents a distance in the vertical direction between the pass line and an image principal point of a lens of the projection unit,
• then L0, α, β, hc and hp satisfy the following mathematical expression (1)
• 0.75
• 
• L
• 0
• ≤
• tan
• 
• 
• α
• +
• tan
• 
• 
• β
• 1
• /
• h
• C
• -
• 1
• /
• h
• P
• ≤
• 1.5
• 
• L
• 0
• .
• (
• 1
• )