If you look at B31.1, Table A-1, you will see that only seamless A53-B pipe is allowed.
Using section 104.1.2 and solving for minimum wall thickness the following are so:
S = 15,000 (up to 650F, from Table A-1)
E = 1
y = 0.4
P = 350
D = I did it for 2", 6" and 10" pipe
A = corrosion allowance. We normally see 1/16" (0.062")
t for 2" = 0.089"
t for 6" = 0.139"
t for 10" = 0.187"
It looks like sch 40 is OK; I've just never seen anything below sch 80 used in steam lines.
Please check my math before relying on the numbers posted.
Charles