Looking at the CASTI Black Book, 4th ed.:
1018 & 1020 are the same chemically except, carbon. 1018 has a range from 0.15 - 0.20; while 1020 has a range from 0.18 - 0.23.
1018 has a UNS of G10180, 1020 has G10200.
G10180 is included in ASTM A29, A510, A512, A513, A519, A576, A611, A635, A659, A794 & A830
G10200 is included in ASTM A29, A510, A512, A513, A519, A544, A575, A576, A635, A659, A787, A794, A827 & A830.
Charles
Another difference between wealthy, rich first world countries and us, poor, third world ones !!!
Here in Brazil, all you can get is AISI 1010, 1020, 1030 and 1040. If you're lucky enough and find some supplier who's got a surplus from a previous sale, you'll be able to get also 1050 and, who knows, miracles sometimes happen, also 1060.
For us it's unthinkable to specify AISI 1018, we're not accostumed to such luxuries.
So. Mr. Gary, don't worry, there's no reason for not making the change.
Cordially
Giovanni S. Crisi
Sao Paulo - Brazil
P.S. Apart from AISI materials, here in Brazil there's a small variety of ASTM materials as well.