As an aluminum producer, I did try to organize an aluminum cartel. Utter failure. If the prices are too high, someone else will come in and buy it cheap and sell it cheaper then your cartel price. If you keep prices too high, people will say "Let's not focus on air forces, its too expensive", and you'll have to decommision some aluminum factories as demand withers(but prices might remain ok).
One way to implement this despite the limitations might be to allow players to join a cartel, and the cartel can set price restrictions that you must follow in the cartel. Joining or leaving a cartel cancels all world market orders for you. Cartels should be an entirely in-game entity, I don't want to have to join a cartel forum in addition to my alliance forum, so we need a basic "vote on prices" mechanism and "elect officers" thing.
The cartel would have no normal way to enforce its prices on non-members save warfare, a sphere ordinarily reserved for alliances on mass scale. Maybe give the cartel an embargo feature, where any nations selling outside its parameters can be embargoed, and all cartel members are forbidden from trading with those nations?