The microstructures of B bearing cast steel (B steel) containing 0?8-1?2 wt-%B, 0?8-1?2 wt-%Cr, 1?0-1?5 wt-%Mn, 0?6-1?0 wt-%Si and 0?10-0?25 wt-%C have been characterised by means of optical microscopy, scanning electron microscopy, electron probe microanalyser and X-ray diffraction. The solidification structure of B steel consists of pearlite, ferrite, martensite and boride (Fe 2 B), while the hardness is 1430-1480 HV. Borides distribute along the grain boundary in the form of eutectic. Fine lath martensite and eutectic Fe 2 B can be obtained by water quenching at 1223-1273 K. The hardness and impact toughness of the B steel exceed 55 HRC and 150 kJ m 22 respectively. The abrasion resistance determined using a pin abrasion tester is obviously higher than that of the martensitic cast steel and nears to the high chromium white cast iron.

The effect of quenching temperature and cooling rate on the structures of cast Fe-B-C alloy containing 1.0wt.%B and 0.2wt.%C was researched. The results showed that, under the same quenching temperature, the microstructures of the metallic matrix are transformed from the mixture of the pearlite, ferrite and martensite to the martensite along with the increase of quenching cooling rate. Under the water cooling condition, excessively low or excessively high quenching temperature did not favor to obtain the single martensite matrix. In the Fe-B-C alloy, the stability of Fe 2 B was good. It still had not been dissolved while heating up to 1050 o C. The dissolution of boride appeared heating at the high temperature. The higher the quenching temperature was, the more the boride dissolved obviously. Along with the dissolution of boride, the boride morphology changed from network to broken-network and isolated shape. When the heating temperature was 1050 o C, the boride transformed the isolated shape completely. Quenching at 950$1000 o C, cast Fe-B-C alloy transformed into the compound structures of fine lath martensite and boride in the water cooling condition.

The effects of austenitising temperature and temper treatment on the microstructure, hardness and abrasive wear resistance of a high carbon high vanadium high speed steel roll of composition 2?2-2?4C, 5?5-6?5V, 4?5-5?5Cr, 14?0-16?0(Wz2Mo), 1?0-1?4Ni and 1?2-1?5Nb (wt-%) were investigated by light optical microscopy, scanning electron microscopy, backscattered electron image, X-ray diffraction and hardness and wear testing. It was found that the hardness increases as the austenitising temperature increases to a maximum of 1050uC, and then begins to decrease because of the increase in retained austenite. Moreover, retained austenite begins to transform into martensite with increasing tempering temperature. The retained austenite in the quenched structure leads to the temper hardening of the rolls, which promotes improved abrasive wear resistance. The roll life of high carbon high vanadium high speed steel compound rolls austenitised at 1050-1080uC and tempered at 540-560uC increased by 400% (single groove trials) compared to high nickel-chromium white cast iron rolls when the rolls were used in a K1 mill housing of a bar mill.

High-boron high-speed steel (HSS) is a cheap roll material. In the paper, the authors research the effect of heat treatment on the microstructure and properties of high-boron high-speed steel HSS roll containing 0.54% C, 1.96% B, 3.82% W, 7.06% Mo, 5.23% Cr and 2.62% Al by means of the optical microscopy (OM), the scanning electron microscopy (SEM), X-ray diffraction (XRD) and hardness test. The results showed that as-cast structure of boron-bearing high-speed steel HSS consisted of martensite, pearlite, M 2 (B, C), M 3 (B, C) and M 23 (B, C) 6 type borocarbides. After quenching, the matrix transformed into the lath martensite, and M 3 (B, C) dissolved into the matrix. When quenching temperature is lower than 1050 8C, the hardness is increased with the increase of quenching temperature under oil cooling, while quenching temperature excels 1100 8C, the hardness will decrease with the increase of quenching temperature. Under the condition of salt bath and air cooling, the effect of quenching temperature on the hardness is similar to the above law, but the quenching temperature obtaining the highest hardness is higher than that of oil cooling. The highest hardness is obtained while tempering at 525 8C. The hardness of high-boron high-speed steel HSS roll is 66.5 HRC, and its impact toughness excels 13.1 J/ cm 2 . Using in pre-finishing stands of high-speed hot wire-rod rolling mill, the wear rate of highboron HSS rolls is 0.26 mm/one thousand tons steel. However the manufacturing cost of highboron HSS rolls is obviously lower than that of powder metallurgy hard alloy rolls, it is only 28% of that of powder metallurgy (PM) hard alloy rolls.Keywords: High speed steel roll / boron alloying / heat treatment / rolling steel / wear resistance /

The effect of quenching temperature from 900 to 1050°C on the microstructures, mechanical properties and abrasion resistance of modified high born cast steel containing 0·3 wt-%C and 3·0 wt-%B was studied by optical microscopy, scanning electron microscopy, X-ray diffraction analysis, tensile tester, impact tester, hardness tester and abrasion tester. Quenching at 900°C resulted in structures containing a small amount of pearlite. The existence of pearlite led to poor hardness and wear resistance of modified high born cast steel. Quenching at temperatures between 900 and 950°C resulted in the decrease in pearlite and the increase in hardness and abrasion resistance in comparison with 900°C quenching. The metallic matrix all transformed into the martensite quenching at 1000°C; the modified high born cast steel had high hardness, tensile strength, impact toughness and excellent abrasion resistance. The hardness, tensile strength and impact toughness of modified high born cast steel had no obvious change quenching over 1000°C. The increase in quenching temperature also led to the transformation of boride from continuous shape to isolated shape and promoted the coarsening of boride.