What type of steel is usually used for .22lr bolts?

btt

any steel, really. You could probably make a bolt for a 22lr out of aluminum.

actually i think the bolts are made of the same steel as the rest  [or at least the majority of the rest ] of the gun, which would be 4140 [or 4130 or 4150] which is a chromium-molydenum type of carbon steel. the chromium lends a little corrosion resistance but can made an alloy a little brittle so the molybdenum makes it a little more workable and the carbon gives it it's hardness. i believe the weight of the bolt does play a part , seeing as it flys backwards against the spring pressure behind it when it's fired. heavier bolt equals weaker springs , whereas lighter bolt equals stronger springs. there's probably a scientific and/or mathamatical equasion behind the weight ratio and pressure of the round being fired. i know the barrels are hardened to an extent, somewhere's around the range of 28-36 rockwell "C" scale and i assume the bolt would be to, quite likely around the same range. with the steel being 4140 [ or thereabouts] it would be an oil hardened steel. hope this helps, it's been some time since my tool + die working days and heat treating days.