In this paper

the auther extends the theory of equilibrium carbon. According to this theory

eleven kinds of new alloy steels are designed for improving Cr

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MoV steel by adjusting car-bon and alloy element content in Cr

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MoV steel and adding approp-eriate rare-earth element.By the synthetic study of smelting

heat treatment and mechanical inspection

the optimum content of alloy element is found out. The experimental results show that

the new tool steels in acord with equilibrium carbon theory have fine eutectic carbides. Compared with Cr

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MoV steel

the new tool steels have no obvious variation of the hardness

but the toughness and ware resistant are improved remarkably. By adding approperate rare-earth element to the tool steels

good metallurgical quality can be obtained and mechanical property can be improved.