One man's junk..

[Frank S]

MeJason T is correct the plate does have the appearance of having been used as a bolster plate. As for paying too much well .33 cents a pound in today's market is not out of the range for used steels or cut off drops I visited a metal place a while back and thye had gone to .60 for drops and $1.00 for any cut shapes they thought folks might want.
Trike road poet is on the right track for annealing however this process is not quite as simple as just heating a hunk of metal to 1650°f and packing it in sand. while the method does work it sometimes is not optimal for thicker or larger pieces of steel.
First off the metal needs to be heated evenly and slowly all over then held to a specific temp for a period of time. The length of time depends on shape , size and type of steel. A good starting point when the grade is unknown is to bring the steel up 50 to 100°f per minute in an oven until the desired temp is reached then held there for at least half an hour per half inch thickness then turn off or remove the heat source form the oven and leave it in the oven until the temperature is below 450°f remove and allow to air cool. this will fully and evenly anneal most medium carbon steels
However most of us do not own or have access to an oven for this process the next best would be a charcoal grill build a large fire in the grill as if you were going to grill a 72oz Amarillo TX sized steak. let the bed of coals get fully red put the steel directly on the coals and cover it with more burning coals close the lid making sure to have really good air circulation to keep the coals burning and forget about it until tomorrow.
The last resort method would be to use a #10 or larger rose bud torch act. or propane actually propane is better for this because it is cheaper make the flame as large as is sustainable with a minimal oxygen leaving a long neutral flame blue feather not orange place the metal on another thick steel surface or support it on small diameter pins about 4 inches above a steel surface or fire bricks so you can apply heat all the way around if placed directly on the surface be aware that this surface is also going to be heated very hot if this is a work table the table may become warped even if as thick as 1/2" Wave the torch all over the part and slowly bring it up to 1650° or just becoming almost orange all over keep it at that heat for at least half an hour after the color has stabilized all over. then place a 4 sided structure around it and fill with heated sand or lime, forget about it until tomorrow. Your steel will be as workable as can be.
There are some short cuts to either of these processes when the grade of steel is known.
OR if you want to leave it as is just surface grind it to smooth flat and square on all sides and have a nice block of steel for what ever you need to use a square block of steel for.