Most of the SSTs require around 1850-1900 fps at impact to expand. Keep in mind that at that low of a velocity, expansion will be minimal unless bone is impacted.
An exception to the rule here is the 6.5mm/129 grain SST which opens very quickly and will still expand somewhat down to around 1650 fps. Hogs are a target of opportunity where I live, and for that reason, I use the standard Hornady 129 grain SP as the SST is too fragile, even at the modest speeds of my 6.5 Bullberry Improved rifle (a shade over 2600 fps from a 26" barrel).
But like Dave D noted, on deer, which typically don't overly tax the construction and integrity of a bullet, those same 129 grain SSTs are extremely lethal.