Research On Microstructures And Properties Of High Speed Steel Of Composite Roll

With the appearance of advanced roll mills and effective rolling techniques, the rolling industry has made a rapid development. At the same time, the manufacturing of roll is accelerated to stride into the new technique field. How to elevate the rolling life and quality to suit the mills is a new question for discussion for the persons who develop rolls. During the recent years the breakthrough of manufacturing the new type of composite roll has been made. It is possible for the composite roll of high Cr cast iron widely used at present to be replaced by the composite roll of high speed steel (HSS) in the ten years. The composite roll of high speed steel has the synthetical life of over ten times more than traditional high Cr cast iron roll. Now the research and production of the high speed steel for roll are very drastic.In this thesis, the microstructure, heat treatment and performance of high speed steel Fe-2%C-4%V-4%Mo-5%Cr-X%W (wt) are studied systematically by using metallograph, method of lattice, XRD, SEM, EDS and hardness. The high speed steel of composite roll is characteristic of high carbon and high alloy elements. So it is easy to form a lot of netlike carbides. These carbides which are very hard and fragile and are very difficult to be eliminated by heat treatment can depress the properties of high speed steel. It is pivotal to improve the formation and distribution of carbides in the solidification microstructure of composite rolls of high speed steel. On the base of the research of composition, the influence of modification with RE, Ti, Mg, RE-Ti-Mg (wt, %) on high speed steel of composite roll is investigated emphatically.The results show that the solidification microstructure of high speed steelFe-2%C-4%V-4%Mo-5%Cr- X%W (wt) contains of martensite, retained austenite structure, and MC, M6C, M2C carbides, and MC/ M2C complex carbides by XRD, SEM and metallographs. Most of the MC carbides locate in the cell and other carbides distribute along the boundary of the cell. By SEM and EDS, the solidification microstructure of Fe-2%C-4%V-4%Mo-5%Cr-2%W contains granular MC carbides which mainly include V (31.70%, atomic percent), featherlike or rodlike M2C carbides which mainly include Mo (6.47%) and W(1.57%), fishbone M6C carbides which mainly include W (12.07%) and Mo (5.07%), and complex carbides MC/ M2C which mainly include Mo (6.17%) and W(1.46%).In the Fe-2%C-4%V-4%Mo-5%Cr-X%W alloy system, the resistance to oxidation is poor when the content of W is 0.6% and the alloy which contains 2% W can have good resistance to oxidation and limit the formation of netlike carbides. Although the resistance to oxidation increases with the increase of tungsten, the netlike carbides distributed along the crystal boundary including M2C, M6C and complex carbides manifold by means of the analysis of optical microstructure, XRD and reckoning the volume fraction of carbides by lattice. When the content of W is 4% or 6%, M(,C carbides increase distinctly despite of the major carbides of M2C and MC/M2C distributed along the crystal boundary in the solidification microstructure. The element of niobium can accelerate the formation of MC carbides and form the composite carbides of V and Nb. When the high speed steel of Fe-2%C-5%V-4%Mo-5%Cr-2%W is added a little of niobium (1.5%,wt), MC carbides in the crystal cells increases.For the Fe-2%C-4%V-4%Mo-5%Cr-X%W alloy system, the peak value of quenching appears in 950-1100. The peak value increases with the increment of W. When the content of W is 2%, the temperature of temper hardening is 550. For the Fe-2%C- 4%V-4%Mo-5%Cr-2%W alloy, the appropriate quenching temperature can be controlled in 1000 1050 and the time of preserving heat is 12min. The temperingtemperature is adaptive at about 550 and the time of preserving heat is 25min. In order to improve the formation and distribution of carbides and the resistance to oxidation of high speed steel during the procession of heat treatment, the content of W and Mo should be limited and the content of V and Nb sh...