Dred-Scott did not have anything to do with expanding slavery in to northern states.
It certainly did!
Certainly in the minds of Northerners it did, and it's legal language was absolutely clear: Africans were not human beings and therefore could never be citizens.
So slaves would remain slaves regardless of where they were taken -- to other slave states, or to non-slave states -- they were still slaves regardless.
In practical effect, that meant Northern states' acts outlawing slavery were themselves unconstitutional, according to the Dred-Scott Supreme Court.
According to Lincoln, Dred-Scott required only one more such decision confirming it to make slavery again lawful in every state, just as it had been back on July 4, 1776.