The top cap appears to be plastic. The top cap's only function is to allow you to preload the bearings before tightening the stem. After that, you could remove it and it wouldn't make any difference. It is not a load bearing component hence why it's sometimes made of plastic. More expensive headsets use aluminum top caps which are slimmer and look nicer (IMO).
I'm almost certain that the crown race is steel for that headset.