The higth temperature and pressure condition in steel and iron material liquid forging decides that the friction between the core and the work piece is much bigger than the common non-ferrous metal liquid forging. The smallest strength of pulling out of the core should not to be considered as the sliding friction

but needs to rest on the rough peak adherence mechanism to do the static friction according to the high temperature to calculate. By now the smallest strength of pulling out of the core materially is the dot of adherence of rough peak or the biggest static friction of the cut of the rough peak. Though theoretical analysis

some quota relations such as the strenght of the pulling out of the core and the stripping temperature

the work piece and the yield strength and the critical shear stress of the core materials

as well as surface roughness and the rough peak quantity are obtained and according to the above the formula of strength of pulling out of the core is given out.