I want to mill steel with this machine.

That's funny. 4 times don't do it and than the fifth saying no problem, this machine can do it easily.

I would ofcourse use helix to go down. Not plunge it in the metal.

It's something to think about. The mills I have are HRC60.

i think the first thing you need to figure out is how slow the spindle can run since you want to be working with steel. smaller end mils can run faster but you want to have an end mill with enough rigidity to deal with the hard material your trying to cut. 1/8 inch is about as small as i would even consider and probably won't hold up well since it's on the small side. another thing is the length of the tool. the longer the tool the more trouble you are going to have. so how thick of material do you really need? on small end mills the length can mean quite a bit as far as how well it does with the cuts.

if you run these cutters to fast they will burn up without a doubt. we are not talking about doing one hole here you will have quite a few so tool life is going to be a factor. the only chance of getting a cutter in the right speed range will be with carbide. HHS or Cobalt tooling is not even on the table because the speeds needs are way to low for your spindle to get into most likely. mist coolant would probably be a big help if you can do it.

i agree with a ramping tool path to control the load on the tool. but there are other things you have to consider like i said before. the machine is big and heavy but the spindle is what i would focus on with it's speed range to see how much trouble you could get into.

A HRC of 60 is HSS a 1/8 HSS end mill needs to run around 2800 rpm a 1/4 HSS end mill is running around 1400 rpm in steel the bigger you go in diameter the slower they keep getting. the machine is not the factor the spindle range of speeds are very much so a factor when working with steel. this is why i said you need carbide at the very least it might get you into a range of speed your spindle can do? if i knew what you were working with better i could probably help better with speeds and feeds.

I did say it is possible, and the rest seem to forget you are talking about routing on already plasma cut parts, so HSS is out of the window.
You should find an end mill with inserts, they can handle high speeds and high cutting forces and might be able to remove the hardened part.

Only possible way I see is to use a high speed cutting strategy. Some CAMs have that option bullt in:

Looking at your z axis set up and how far out your spindle is mounted I doubt if your machine is really rigid enough to handle the forces. And then there is the issue with the hardened edges from the plasma cutter. I know people with real machines like the one in the above video who will not mill into a plasma cut edge. So If you really want to mill steel on your router you might be better off milling the holes all the way.

even with that video if you pay attention at one point the tip of the cutter is red hot and tossing sparks. the video is not straight through so in those breaks i know the tool is being swapped out for a new one before the finish pass is done and possibly during roughing as well.

when you get into high speed machining your tools and holders have to be balanced to run at those speeds. they do often run with just air in these applications because coolant would trow the tool into thermal shock. in the video with a VMC those DOC. are really light and they don't look like more than .005 to .010 deep (imperial)

i still think spindle speed or lack of lower ranges of speed is the thing that will hinder cutting steel the most. feed mills work well with harder material with the right inserts but i doubt spending $200 on a holder and another $100 on a box of inserts is that attractive to most. the down side to those kinds of cutters are the are not flat bottom cutters so they don't get all the way through the part unless you have the clearance to do so in the set up.

because the edges are plasma cut most likely that will need slower speeds as well just because of that part of the plan.for the whole plan to be cost effective the tooling needs to last somewhat. with carbide if the rigidity is not there in the machine you will find out fast enough. carbide is great but it loves rigidity. so if the rigidity is not there tooling will chip or break pretty easily.

i have worked with all types of burn outs over the years they are tough no matter what tooling or machine you use. so being able to adjust things to get the job done is important. when you loose the ability to slow things down speed wise the task is even harder yet.

this is all just my 2 cents since this is what i deal with in my trade.