Manga where the main character kills a demon lord but his brother takes credit and casts him out of the country

This sounds like Yuusha no Gawari ni Maou Toubatsu Shitara Tegara wo Yokodoroi Saremashita by Sekijou Hijiri and Kimizuka Shou, which I believe translates to "False Hero Has No Place in this World" in English.

Description from online sources:

As a member of the hero's party, Arga hid his identity and was on a journey to subjugate the Demon King with the hero. At last the party chased the demon king, and Arga brought him down. However, the hero insists that he was the one who killed the Demon King, and then charges Arga with an innocent sin and banishes him from the Imperial Palace. The warrior who devoted his life to the hero as a part of his corp would walk a new life in a strange country town.

Arga constantly wears a suit of plate armor in the manga, as I recall.