These are blanks by definition (first reference). Also, for a stamping press, either pre-dimensioned blanks or coils of steel sheeting would be used (the latter in the case of progressive dies). Since a lift-truck operator brings the blanks, they would be in a stack, presumably for a feeder mechanism on the machine or production line. Steel sheets are generally raw materials delivered by a steel vendor, in standard sizes. Sheets are then cut into blanks suitable for automated production machinery.

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Note added at 3 hrs (2015-04-15 13:59:59 GMT)
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If you Google "flans d'acier" images, you will see blanks of varioius thicknesses. For a stamping press, they would be relatively thin -- as opposed to gear blanks which would be as thick as the gear itself and would be fed into a machining centre. The term "blanks" implies that some operation is to be performed. Blanks can be any shape, pre-cut to minimise waste or anticipate some later operation on the same piece. Steel sheets may be correct in some cases, but it commonly implies that they are rectangular.

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Note added at 3 hrs (2015-04-15 14:09:10 GMT)
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Sheet steel would be more commonly referred to as "tôle d'acier".

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Note added at 3 hrs (2015-04-15 14:11:23 GMT)
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Dictionnaire Malgorn (Gouadec, 7th ed) defines "flan" uniquely as "blank".