Ludo:
What was the original use of this material, was it a spring etc? If you know that, you can consult junkyard steel lists and or Machinery's Handbook and get some idea of the type of alloy. You can heat a piece of it to non magnetic and then quench in oil. After it is quenched try cutting the quenched area with a file. If the file skates, it's high carbon, if the file will cut it, reheat it to non magnetic and quench in water try the file again. If a file will cut it at this point, it is mild steel. You can also heat to non magnetic quench in oil and then try to bend the quenched area, if it bends, mild steel, if it breaks, high carbon steel. You can also spark test it and there are those who swear by spark testing, but I find it to be not much use. As for what if any alloy mateial is in the steel, that's anybodies guess unless you can determine the original use of the steel.
Before half the blacksmith world goes of on me with a rant about the advantages and the preciseness of spark testing, save your effort, I don't care.