Semi-solid forming of the high melting point alloys such as steel is a promising near-net shape forming process for decreasing manufacturing costs and increasing the quality of the final products. This paper presents the microstructure characteristics of SKH51 (high speed tool steel) during heating and holding in the mushy zone between $1233^{circ}C$ and $1453^{circ}C$, which has been measured by differential scanning calorimetry (DSC). The results of heating/holding experiments showed that the grain size and the liquid fraction increased gradually with temperature up to $1350^{circ}C$. The drastic grain growth occurred at heating above $1380^{circ}C$. The strain-induced melt-activated (SIMA) process has been applied to obtain globular grains in the billet materials. Working by mechanical upsetting and successive heating of SKH51 into the temperatures in the mushy zone resulted in globular grains due to recrystallization and partial melting.