Admittedly steam engines could be used for battering rams. But once you've got them, you'll have an easier time manufacturing what you could use instead.
The only 'real battering rams' I could think of being used after the invention of steam engines were boarding / storming vehicles / ships, which not only broke through the wall or gate, but also delivered troops to hold the breach until reinforcements could arrive. In that way, they weren't 'pure battering rams', but hey, who gives a damn?