How Flat is FLAT ?

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HOW FLAT is FLAT?

Most asked question in the steel sheets market is this – How flat is the material? But very often the person asking the question has no idea how much flat they want their work to be! Or indeed what the word FLAT means to them. Shiny (specular) means flat to them and quite often that is all they really need.




But is there a measure for flatness? Can we really “measure” a FLAT sheet? Well here’s the answer – we cannot measure flatness. As flatness can only be compared. Flatness is a ratio of the “length of wave” to the “height of wave”. This ratio is termed as the “I” Value. The following is a table showing the various “I Values”.

 

 Steel has the tendency to cling to its original shape and molecular composition. Why? Because it was given a unique configuration when it was rolled, cooled, and coiled at the steel mill. In its natural state, steel is not at all flat.


In fact, a hot-rolled sheet coil generally reveals some or all symptoms of poor flatness. The illustrations show some of the states of defects in coils present from steel mills.

 

The ASTM has internationally set forth a standard for flatness as “variation from FLAT”. It compares the flatness of sheet to a “table of flatness”, this table specifies the range between which a sheet can be termed as FLAT.

The “I-unit” however is a more precise measure of flatness than the “variation from flat” standards set forth by the ASTM. It takes both amplitude and frequency of shape deviations into account. The “I unit” is not a table comparison, but it can mathematically calculated based on the height of wave and the length of the wave.

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