| High speed steel is a kind of tool steel with high hardness,high wear resistance and good red hardness,which is mainly used to manufacture complex thin edges and various metal cutting tools.High speed steel contains high W,Mo,Cr,V and other alloy elements,and the total amount of alloy elements is up to 15%～25%.A large number of ledeburite eutectic carbides are precipitated in the solidification process of high-speed steel,which is easy to cause billet cracking and material toughness damage or fracture failure in the bearing process of high-speed steel.Therefore,controlling the amount and distribution of ledeburite carbide in the microstructure is always a key problem in the preparation of high speed steel.Although the ledeburite carbide in high speed steel can be broken and distributed uniformly in the conventional forging and heat treatment process,it also has the characteristics of high cost and long manufacturing cycle.It has been proved that the semi-solid casting technology has an important influence on the precipitation and distribution of the harmful liquid-phase in the high alloy steel.Therefore,in this paper,through the semi-solid deformation technology,the influence of different conditions on the carbide in the structure of the high-speed steel during solidification has been studied systematically.In this paper,the influence of different cooling rate on the solidification structure of high speed steel is studied firstly,then the influence of different liquid fraction deformation on the solidification structure of M2 high speed steel is studied by Semi-Solid Deformation Technology.Finally,the influence of Semi-Solid Deformation on the high temperature decomposition of carbide in high speed steel is studied by heat treatment of Semi-Solid Deformation samples.The main results are as follows:(1)With the increase of cooling rate,the as cast structure is refined,the dendrite arm spacing is reduced,and the size of carbide is refined.With the increase of cooling rate,the lamellar eutectic carbides in as cast structure bend and turn into rod and honeycomb carbides.The increase of cooling rate changes the composition of carbide in the structure.With the increase of cooling rate,the content of alloying elements in eutectic carbides decreased,the content of alloying elements in lamellar carbides was higher than that in rod carbides,the content of main forming elements W and Mo in M2C carbides decreased with the increase of cooling rate,and the content of alloying elements in matrix increased with the increase of cooling rate.(2)Compared with the as-cast structure,the semi-solid deformation can effectively cut off the carbide network in the structure,avoid the accumulation of carbides and reduce the area percentage of carbides in the solidified structure.The large deformation of the solid-liquid two-phase region at the end of solidification makes the solidified dendrite of M2 high speed steel fully broken and refined.The residual liquid phase between the broken dendrites is uniformly mixed due to convection,which makes the alloy elements in the residual melt between the broken grains of M2 high speed steel deviate from the eutectic reaction conditions,which is conducive to the re melting of elements into the austenite grains,thus reducing the solidification structure Precipitation of eutectic carbide.Compared with as cast structure,the deformation of solid-liquid two-phase region at 1300℃ and strain rate of 0.1/s has the most obvious effect on reducing the precipitation of eutectic carbide,and the area percentage of eutectic carbide is reduced from 5.6%to 1.9%in as-cast structure.(3)As the network structure of primary carbide is broken by the semi-solid deformed sample,the amount of primary carbide in the structure is reduced,the carbide in the deformed sample is decomposed more completely by heat treatment,the content of carbide alloy element in the as cast structure is lower than that in the semi-solid deformed sample,and the content of austenite alloy element in the deformed sample is high,which promotes the The precipitation of secondary carbide in the structure after heat treatment.In the sample deformed at 1260℃ at the end of solidification,some of the precipitated carbides are deformed and compressed,and the carbides are refined by the first deformation.In the process of heat treatment,the primary carbides are refined by the second transformation.The combination of the deformation refined carbides and the transformation refined carbides is beneficial to the high temperature decomposition of the primary carbides,which can effectively reduce the single carbide in the structure Size of carbide. |
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