First, did you encrypt the entire hard drive
/dev/sdf or a partition
cryptsetup luksOpen /dev/sdf test from a root terminal to see if maybe you encrypted the entire volume and not the first partition in it. Which is OK, you'll just have to remember that each time you mount it and ignore the partition warnings. (You probably can't mount it through the Disk Utility GUI - you'll need to use the command line each time.)
If you really overwrote the partition table, you may need to use a tool to recreate. It shouldn't be too hard if you only had 1 partition in it, and a utility such as
cfdisk may be able to help. Don't format the partition afterward, obviously.
Something that may be safer than putting a new partition table on it, if you indeed overwrote the partition table, is to get
cryptsetup to look at not sector 0 of that volume but the sector that your encrypted volume begins at. I'm sure that it's possible somehow but can't think of a tool that does that (you can do that with "loopback" files easily but I'm not sure if
losetup works with a raw device
You can probably
dd if=/dev/sdf | hexdump -C | less and scroll through until you find the text
LUKS - then divide that by 512 to get the beginning block of your LUKS partition.