Andrzejsaw
It's best if you do publish free chapters when applying for contract because the editors want to check your work and if there's nothing, they'll reject your application.
Also you can only start locking chapters after 30k words, well that's recommended, but you can lock the second chapter after getting the contract (only the first chapter is restricted to having to be free). Of course if you do the latter, you won't get a lot of readers - it's why most lock between chapter 20-40.