A lot of factors play into it in my experience. Target hardness, thickness, design, distance, etc.
Most mild or standard construction steel is 130-180 Brinell, and in 3/8" thick generally won't hold up long at all to anything above a .22 LR. At 1/2" thick, it will be marked by non magnum rounds, depending upon caliber, velocity, bullet type, distance, etc, but will last a while. I have some stationary targets that have taken a LOT of rounds, mainly 9mm, .38 Super, .40, and .45. They even went through a Magnolia full auto shoot. They are beat up but still going with just a bunch of small "pock" marks. In general, the hot Supers left more damage than the .40's and .45's.
T-1 or A514 has been a very popular handgun target steel choice for years. It has a Brinell hardness of about 230-300. This is what most of the 3/8" thick steel targets used in the USPSA matches in Jackson are made out of. It will bow over time but most non-magnum rounds don't do much to it.
AR400 is an "abrasion resistant" steel that is heated and oil-quenched. Hardness is roughly 360-450 Brinell. It is great for handguns and I imagine 1/2" thick AR400 would be a decent choice with some lighter magnums, i.e. no .454 Casull or .500 S&W, with a good target design, but I don't have any experience to say for sure.
AR500 or T520 is the same as AR400 but with a hardness of roughly 460-540 Brinell. This is roughly 52 on the Rockwell scale, is in the level of armor plate hardness, holds up great to handgun use, and is suitable for rifle targets. A high powered rifle will put a visible mark on it at 100 yards, and of course caliber, speed, and power are again factors. From 200 yards on, this stuff should take .308 ball or .223 for thousands of rounds.
All steel will bow some with use. The softer mild steel will cave in, while the harder steels will become convex because the surface is actually stretching. Any design that allows reversal of the target is therefore preferred so it can be kept relatively flat.
Large magnum handguns are going to be a challenge for just about any steel target at close range. Besides hardness, other major factors include the design and weight. If the target is completely stationary or designed not to move, the target has to absorb all of the bullets' energy and is going to be damaged more than one that moves or falls down upon impact.
My brother sold steel targets for a while some years ago, but steel and shipping prices went up and business went down, so he sold off his programmable cutter and is no longer doing it. My $.02; hope it helps some.
