This is directed to: steel novices, steel challenged, and people who might otherwise cause harm to themselves, those around them, or pieces of steel.
So, will steel be affected by temperature? That depends. What Temperature? That depends.
Let’s define temperature as “Service Temperature”. That is, the temperature the steel will encounter where it is being used. It is worth mentioning that service temperature may be “Intermittent”, or “Constant”. If the steel is exposed to intermittent temperatures, it is not exposed long enough to thoroughly take upon itself the service temperature. (It just passes in and out of a furnace but not long enough to get as hot as the furnace.) If the steel is exposed to constant temperature, it takes-on the service temperature.
When the steel mill hardens steel to obtain specific properties, it involves heating the steel and cooling it to a very specific formula. If you are now going to expose it to temperatures that approach those used in the original recipe, you increase the chances of changing the original properties (hardness, brittleness, ductility, etc.). That is reason for caution if you are intending to do anything other than drop it or throw it.
The temperature to which the steel was originally heated were specific to the elements that were in the steel. The temperature it was cooled to, as well as the rate of cooling and even the time required to move the steel from one process to another affected the properties obtained.
Heat will affect steel based on the composition of that steel and relative to the past thermal processing that steel has undergone.
Give or take a country mile; steels will melt around 3000°F. Whereas aluminum will melt around 1200°F. Short of those temperatures, you should not have to worry about your steel leaking off the shelf. Steels will begin to soften, however, at a wide range of temperatures based on their chemical composition and the thermal processing that got them to the current hardness.
Temperatures need not be extremely high to begin to lower the properties of the steel. Some of the very hard wear plates found in industrial applications (near diamond hard) will begin to soften at 280° to 350°F. You can cook a pork butt at 280°F.
In very general terms, if you have a very hard piece of steel that will be exposed to elevated temperatures, there is a good chance it may soften. Conversely, if you have a soft steel and expose it to elevated temperatures, you may cause hardening.
In all cases, with known grades or unknown grades of steel; when heat is involved and the steel you are using may be hardened or may be hardenable, exercise caution. (safety glasses, hard hat, gloves, etc.)