Is it considered a divorce if the husband says to his wife "You are divorced" – while not intending divorce – during a quarrel to prevent her hysteria, knowing that she threatened to expose him?

The divorce has occurred, and your unintended desire for it not to happen, despite your intentional utterance of the divorce pronouncement, holds no weight. However, you may take back your wife without a new contract or witnesses, and without needing her consent or approval, provided it is before the expiration of her waiting period (iddah), unless this was the third divorce. One should avoid uttering divorce pronouncements. If you wish to threaten her, you can do so through insinuation or words from which divorce is understood but not intended. It is better to abandon all of that and instead resort to what Allah Almighty has legislated for addressing disobedience (nushooz).