The agreement increases wages and maintains seniority, benefit and pension provisions.
The union represents some 250 workers at the company who have been working without a contract since last July.
It says the new deal includes a lump sum payment of $750, improvements in vacations and an increase in life insurance for retirees.
The new agreement, which is retroactive to last year, runs until July 31, 2017.
AltaSteel, which is owned by Australia-based mining and steel company Arrium, is a scrap-based mill operation with both melting and casting facilities.