Abstract
Solidification structure of two kinds of B alloyed high speed steel for rolling mill rolls, which carbon contents are 0.50% and 1.00% respectively, and the changes in the microstructure and mechanical properties after quenching from 1050 °C and tempering at 200-600 °C respectively are studied. The results of metallographic, scanning electron microscopy analyses, X-ray diffraction analyses, hardness and impact toughness measurements of as-cast and heat-treated B alloyed high speed steel are discussed. As-cast matrix of B alloyed high speed steel consists of martensite and troostite. There are 19-26 vol.% M₂₃(B,C)₆, M₃(B_0.7C_0.3) and M₂(B,C) type carboborides in the matrix. Microhardness of M₂₃(B,C)₆, M₃(B_0.7C_0.3) and M₂(B,C) are 1940-2030Hv, 1380-1460Hv and 1460-1530 Hv respectively. Macrohardness of as-cast B alloyed high speed steel reaches 58-60 HRC. After quenching from 1050 °C, the eutectic M₂₃(B,C)₆ and M₃(B_0.7C_0.3) carboborides dissolve into the matrix, and many granular M₂₃(B,C)₆ precipitate from the matrix, and the whole matrix transforms into martensite. Microhardness of matrix and macrohardness of quenched B alloyed high speed steel have a slight increase comparing with as-cast sample. Hardness of the B alloyed high speed steel remains constant while tempering temperature is below 550 °C, and then, with increasing temperature, decreases considerably. Impact toughness of the B alloyed high speed steel increases slightly until tempering temperature reaches 525 °C, and then increases considerably. B alloyed high speed steel has higher hardness and will have excellent wear resistance after tempering under 550 °C that can be attributed to the effect of boron.

Key words
High speed steel for rolling mill roll, B alloyed steel, carboboride, quenching; tempering, hardness, impact toughness

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