On Dec 18, 10:37 pm, wrote:
\
I´ve the following situation: 4 mm mild steel plate in which I made 6
mm holes with thread.
{and want to close the holes with steel plugs}

Three options:

Use rivets (i.e. put the overlength bit of threaded rod into place
then hammer it, pneumatic riveter recommended, against a
steel backing block). Sealant goo optional.

Use glue (there are silicone adhesives that wouldn't have a problem
with grinder cutoff temperatures).

Use a shear plate. Basically, a hard steel 'nut' that snugs up
on the allthread rod after you've plugged the hole, that takes a
sidewise hammer blow and shears off the not-in-the-hole
length of rod from the plug.