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It is known that steels of the RtM5 type containing over 0.1% 13 become brittle due to the formation of a coarse eutectic along grain boundaries. In previous works we have shown that methods of hardening from the liquid state can be used to increase the boron content in this steel to 0.5 -1% without increasing the brittleness. A special heat treatment involving controlled transformation from an amorphous-crystalline state to a microcrystalline one made is possible to increase the ductility and curing properties of the steel. However, the mechanism of this transformation was not considered earlier. The present article is devoted to this problem.It has been shown in [1 -3] that high-speed steels of the R6M5 type that contain over 1% boron can be fabricated in an amorphous state by hardening from the liquid slate. The same steels with a lower boron concentration have an amorphous-crystalline structure after hardening. The alloys are ductile in the amorphous initial state and are embrittled after high-temperature annealing. On the other hand, the alloys with an amorphous--crystalline initial state are brittle and become ductile after heat treatment by a special regime. Here ductility is understood as the capacity of a rapidly quenched ribbon to withstand bending to 180 ° without failure.In the previous work [3] we established a relation between the carbon and boron content in steel and the temperature of its transformation to a ductile state. It has been shown that the higher the concentration of these elements in the steel the higher the temperature of the transformation. However, the mechanism of the corresponding structural change in the metal has not been considered.In the present work we investigated structural transformations occurring upon heating in the tested steel (1.13% C, 5.8% W, 4.9% Me, 4.0% Cr, 1.7% V, 0,5% B, the remainder Fe) hardened from the liquid state. The metal was melted in a vacuum induction furnace in an alundum crucible in an argon atmosphere. The ingots were cut into 30-g pieces, placed in quartz ampoules, and melted by a high-frequency inductor. After this the melt was squeezed out of the ampoule under an excess argon pressure through an opening 0.5 mm in diameter, and the resulting stream was cooled on the external sur-

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