For spring steels heat treated between 48 to 51 HRC, you might want to consider carbide drill bits. They are readily available, but you need to be careful because they are brittle.
A coated carbide bit will work best- PVD TiN is common (gold color), but others will work as well. Use lots of lubricant during drilling if you intend to make more than 1 hole in 1 part.
Many years ago I worked as an X-RAY repairman. There were large (man killer) flat wound springs which supported the X-RAY tube. Once in a while the end of the spring would break in which case we had to drill new holes for attachment. My mentor showed me how to drill into this spring steel. Use water instead of oil. We did this with regular drill bits. It worked every time.
It's been 20+ years. I forgot to mention that you also had to use a variable speed drill set at a low speed. You also must lay the spring steel on another piece of regular steel to give to give the bit a hard surface to drill into. You must drill both through the spring steel and into the regular steel.
I worked in a spring shop for several years. If you only have a few holes to drill, high speed steel drill bit will work. Carbide is better and use carbide for sure if you have more than a few holes. If this is a production item then punch as was suggested which is common practice which we did almost daily with 1075.
