Can somebody recommend a steels wich can attain surface hardness of 700Hv and core hardness of 35Hrc after heat treatment. the application is "tapping screws"
You have several different material options for your application. You can go with Cr-Ni-Mo low alloy steels (AISI type 4340 or AISI Type 8620) or martensitic stainless steel (AISI Type 410) to achieve core hardenability, and use a surface treatment method like nitriding, carbonitriding or chemical vapor deposition to achieve your surface hardness requirement.
With all of the self tapping screw material available, why do you need such a high surface hardness?