I had planished it at 4mm of thickness, so that I could grind it without too much worrying. The issue is that I tried to forge the bevel (don't know if that's the proper word) instead of fully grinding the edge, and I shouldn't have done this, since the edge have pits aswell. My other problem is that I use a very old and worn-out belt grinder, and the belt is discontinued on the market. It makes the grinding process very long since the belt is exhausted, combined with a lack of power.
I do not know what kind of steel it is. I know how dumb it sounds, but I started by gathering remains from the house and from friends. I've been throwing away a lot of steel as I discovered how bad some of it was (some cracked in many thin layers as I heated and worked it ; other simply never hardened after quenching, ...). I am a bit worried actually, because I'm running out of good steel and I simply cannot find a way to purchase some. I visited and contacted many companies dealing in steel around where I lived, whether it was second-hand material or industrial distributor, and no one sells steel capable of sustaining thermal treatment ... Some professionals even have never heard of the type of steel I've been looking for (french / european standard : XC75. And the industry apparently only deals with S235).
And to finish answering you, Frazer, I'm not sure what you imply by "plan for hardening". I use vegetable oil (sunflower) for the quenching, and temper it once in the oven. Since I lack a lot of knowledge in metallurgy (in fact I am lacking in many other aspects), I adopt an empirical approach : 220°C for 2 hours in the oven, and observe the results. If I feel it didn't work, I change the parameters for the same steel on future projects.