Research On Microstructure And Subsequent Treatment Of High Speed Steel Cast-strip Produced By Twin-Roll Strip Casting

As one of the most advanced technologies in the fields of ferrous metallurgy and material processing in the 21st century, twin-roll strip casting technology could cast liquid metal into thin strip. Compared with the conventional technique, the twin-roll strip casting technology has obvious advantages in processes and microstructure. The twin-roll strip casting technology is the optimizational development trend of manufacturing technology of flat iron & steel and one of the main technologies for shortening process in iron & steel enterprises.In this study, the M2 high speed steel cast-strips were produced by industrial-scale twin-roll strip caster, which achieved the leap of producing high speed steel cast-strips by twin-roll technogoly from laboratory studies to trial production. The microsturcture characteristics of M2 cast-strip were studied with such experimental methods as metallographic analysis, SEM, XRD and TEM[0]. The effect of sub-rapid solidification on the size, type and distribution of eutectic carbides in M2 cast-strip was analyzed. The effect of heat treatment temperature and its holding time on the eutectic carbides in M2 cast-strip was explored. The eveolution of metastable phase of M2C eutectic carbide during the heat treatment period and the decomposition dynamics concerned were researched. The M2 cast-strips were rolled into thin strips and the effect of deformation ratio on carbides in M2 cast-strip was examined. Then these thin strips were comparied with those that were produced by conventional technology on carbide, hardness, red-hardness and bend strength. At last the rolled M2 cast-strips were made into saw blades and their cutting performance was experimented on in tools factories and then compared with that of similar products produced by conventional technology.The results show that the solidification rate of twin-roll strip casting technology is between 102K/s and 104 K/s, much faster than that of conventional technology, that is, 10-1K/s to 10K/s. Therefore, it belonges to sub-rapid solidification, which improves the solid solution coefficient of C and alloy agents in basal bodies of M2 cast-strip, thus inhibits the segregation of C and alloy agents in M2 cast-strip. The eutectic carbides are refined and the spacing of eutectic carbides lamina is shortened by twin-roll strip casting technology. The eutectic carbides in the edge of M2 cast-strip are composed of fine and grainy granular carbides and strip carbides. The eutectic carbides in the center of M2 cast-strip are distributed around the austenite with a net-like or half net-like structure. They are only 1/10 to 1/4 the size of electric ingot. The eutectic carbides are full of W, Mo and Fe, but lack of V. The eutectic carbides in M2 cast-strip are dominated by M2C and M6C carbides, but the MC carbides within only make up a small percentage.The microstructure of M2 cast-strip could be improved by heat treatment because the M2C carbide decomposes at a high temperature. The effect of heat treatment's temperature and holding time on the eutectic carbides in M2 cast-strip is interactive. With the increase of heat treatment's temperature and holding time, the net of eutectic carbides gradually gets disconnected and then disappears. Many secondary carbides dissolve out from M2 cast-strip after the heat treatment and the forms of the primary carbides are converted from a lamina or an abnormal lamina into a linear chain and every unit in the chain is equiaxed, linked to weakly and even isolated from each other. The main components of carbides in M2 cast-strip change from W and Mo into W and Fe after the heat treatment, which confirms that Fe has had chemical reaction with M2C eutectic carbides, with the resultants of M6C and MC carbides. The carbides in heat treated M2 cast-strip are mainly M6C carbides and MC carbides, but no M2C carbides are found.The eutectic carbide net is somewhat distorted and broken under machining stress, and the carbides are elongated in the rolling direction with obvious directivity. With the increase of deformation ratio, the microstructure of M2 cast-strip is being improved dramatically. But the limited deformation ratio and weak deforming force are not able to make break the eutectic carbide net completely, thus it is necessary to process M2 cast-strip with the heat treatment before they are hot-rolled. After the M2 cast-strip are processed at the temperature of 1150℃and with the holding time of 40 minutes, and are hot-rolled, the carbides in the resultant cast-strip are generally distributed homogeneously as fine particles in the basal bodies concerned. The hardness, red-hardness and bend strength of rolled M2 cast-strip are all higher than that those of conventional M2 thin strip. The cutting speed and service life of the saw blades produced by the twin-roll strip casting technology are better than those of conventional saw blades; besides, its cutting speed recuction is less than that of the conventional ones.