"We are the shields of the Imperium, and we shall defend it promptly and until death, lest I and we all have failed. Is it is written, let it be so."
―Dantio Selenius, High Shield of the Steel Guardians.
The Steel Guardians are one of many Chapters of Space Marines and were raised in a time near Millennium Thirty-Nine in the Galactic Fringe near the Ghoul Stars but their distance from civilized space hasn't stopped them from aiding in many engagements around the Imperium beyond. They are however, heavily sedentary and keep to their own realms very often, only sending the occasional excursion force of marines to aid in various engagements(Though they've committed 5 of their 7 Battalions to the defense against the 13th Black Crusade in recent years) around their home segmentum.
They are a chapter of high numerical value and as such have been looked upon with scrutiny but have credited the thin spreading of Astartes to such rampant activity of enemy forces as a rebuttal to the Inquisitors and Imperial Officials who have openly questioned them on this. Due to their heavy numerical strength they've reorganized their chapter structure into a stronger more cohesive grouping of units containing 200 Marines known as Battalions.