Any of you knowledgable lot have an idea what grade of bolt I should use in the hot side of a K03 turbo please?

i had to cut the heads off as they where in too tight, they are an m6 bolt of about 20mm ish.

I’ve got new cores coming for rebuild so I’d like to get a set of bolts ordered so I can get them finished, I’ve tried google but I’m not getting anything, they obviously get very hot so I want the right bolt as it’s engine out if they fail.