Good question, not getting consent and doing it against their wills can cause significant harm to their mental well being, in Islam you must treat slaves in good way not like that.
You Can't even Slap slave, Let Alone Raping Them
You are not allowed to inflict physical harm on slave, let alone raping them
I came to Ibn 'Umar as he had granted freedom to a stave. He (the narrator further) said: He took hold of a wood or something like it from the earth and said:(freedom of a slave) has not the reward evert equal to it, but the fact that I heard Allah's Messenger (way peace be upon him) say: He who slaps his slave or beats him, the expiation for it is that he should set him free.
Source : https://sunnah.com/muslim:1657a
That being said, you can verbally order them around but if they resist it you can't use physical aggression to force them into it and later Rape them, it's similar to the hadith about angels cursing a wide who reject sex but the husband can't force himself on her if she refuses, the punishment is between her and God not her husband physically forcing her, same thing for here, raping them is contradicting several ahadiths and verses severely and goes against how it's commanded of Muslims to treat them well, he needs to take set of order and cannot immediately force himself upon her.
Major Shafi'i scholar al-Haleemi (d. 403) said when commenting on Q. 4:36 which speaks of kindness to slaves:
"If she disliked being touched or intercourse, then he shouldn't touch her or have intercourse with her without her permission."
al-Minhaj fi Shu'ab al-Iman 3/267