I would recommend talking to a tool shop who specializes in extrusion dies. My references say to use AISI H21, H23,or H26. All these grades are tungsten tool steels. Backers behind the die are made of H12 or H11. You may also need to make a bolster.
Mobster, point of clarification, do you mean to say as in backward IMPACT extrusion? If so, you may want to look at the range of carbide products rather than tool steels.
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
Dies for cold forming (also called impact extrusion, cold forging, or cold extrusion) are always reinforced, meaning that there is an inner piece (usually called the die insert or die nib) that is reinforced by at least one stress ring (also called shrink rings). Some times the outer ring is called the die case. The inserts are interference fit into the rings/case.
Typical tool materials for die inserts are D2, M2, and M4. They are hardened and tempered to hardness of 58-64 HRC. Cemented carbides are also used, as are a range of new tool steels specifically developed for this process that have improved fracture toughness (resistance to chipping) and fatigue strength. Stress rings are made from hot work tool steels like H11 or H13 heat treated to 45-50 HRC. Punches usually start with M2 or M4 at 62-66 HRC, although cemented carbides are also commonly used.
