A heat number is an identification coupon number that is stamped on a material plate after it is removed from the ladle and rolled at a steel mill.
Industry quality standards require materials to be tested at the manufacturer and the results of these tests be submitted through a report, also called a mill sheet, mill certificate or mill test certificate (MTC). The only way to trace a steel plate back to its mill sheet is the heat number. A heat number is similar to a lot number, which is used to identify production runs of any other product for quality control purposes.
Numerical significance
Usually, but not universally, the numbers indicate: the first digit corresponds to the furnace number the second digit indicates the year in which the material was melted the last three (and sometimes four) indicate the melt number.