I found that most of the specs I examined for HSLA steel include a specification for minimum bend radius, in terms of thickness.
Most mild steels, minimum bend is 1x thickness. HSLA's (high-strength low alloy) typically are 1.5x thickness. There are some exceptions, depending on your application. Thin gage work tends to go to near-zero creases.
Also, there is some dependence on processes. If you are bending with a prog die vs. brake press, you have the option of coining bends and getting tighter and more accurate bends.
K-factors are sometimes published in steel specs, but most die-builders I've known have their own secret recipies based on alloys, press force, bend radii, etc. Good guesstimate values typically around 0.35 for mild steel and 0.45 for harder grades.