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The best way to do this is to use a professionally developed reference, such as Stahlschlüssel, ASM's Worldwide Guide to Equivalent Irons and Steels, or ASTM's Handbook of Comparative World Steel Standards: Second Edition.  Some suppliers of steel products have partial information on their websites, but they are not necessarily up-to-date or broad in their coverage.  Here are some links that you can use to obtain these references:

http://www.stahlschluessel.de/en/home__lang.html

http://www.astm.org/BOOKSTORE/PUBS/1377.htm

http://asmcommunity.asminternational.org/portal/site/www/menuitem.2b9d1953d012ee1480a3c01026e110a0?vgnextoid=3bb55860ae0f8110VgnVCM100000701e010aRCRD&itemId=

Grade S550MC (1.0986) according to DIN EN 10149-2 is equivalent to ASTM A 656 Grade 80.  This standard covers all flat products, not just plate, so there are some thickness restrictions on the high strength grades in the plate range (16 mm max).  DIN EN 10025-4 also covers structural plates in HSLA grades, but the highest strength available is S460ML (1.8838) which is roughly equivalent to ASTM Grade 70.  Producers such as voestalpine Grobblech produce higher strength plate using their tradename alform® (alform 550 M would be Grade 80). Grade S355GP according to DIN EN 10248-1 is equivalent to ASTM A 572 Grade 50 for structural shapes, piling, etc.  Use the following links for more information:

https://www.dillinger.de/imperia/md/content/dillinger/publikationen/baumaschinen/kundeninfo/en_10025_en.pdf

http://www.vitkovicesteel.com/wysiwyg/file/katalogy_plechu/catalogue_of_plates_new.pdf

http://www.voestalpine.com/etc/picprod/heavy_plates/structural_steel/grobblech/en.PICPARSYSContentBeschreibung.7346.File.tmp/ConstructionMobile_E_15052008.pdf

http://www.arcelormittal.com/sheetpiling/page/index/name/steel-grades-for-hot-rolled-sections

http://iron.casti.ca/titanium/store/books/view_book/1
http://iron.casti.ca/titanium/store/books/view_book/3
http://iron.casti.ca/titanium/store/books/view_book/5
http://iron.casti.ca/titanium/store/books/view_book/8

http://www.klsteel.com/sdp/229183/4/pd-1029274/4664518-516042.html

A 656-80 look for the 1.0986,
A 572-50 look for the 1.0045

• http://home.arcor.de/volkhard.kuhn/astm.html

Wow! Thank you all for the assistance. These are all good references to keep.