Influence Of Magnesium On The Microstructure And Performance Of M7 High Speed Steel Under Pressurization

M7 high-speed tool steel is a kind of tool steel commonly used in Mo high-speed steel.Appropriately increasing the content of carbon and vanadium on the basis of M1 steel not only retains the high toughness characteristics of Tungsten high speed steel,but also improves its cutting performance.It becomes a general-purpose steel grade with good red hardness.toughness,and wear resistance.And it is widely used in cutting field and has an irreplaceable position.However,there are many problems in the solidification structure of the M7 high speed steel prepared by traditional smelting method,e.g.coarse cast dendrites,eutedtic ledeburite and carbides,etc.which seriously affects the processing performance of M7 and restricts its application and popularization finally.For example,compared with the typical of the M2 highspeed steel,it is poor and needs to improvement that the impact performance of the M7 high speed steel.As we know,Magnesium is a kind of alkaline earth metal element with very active chemical properties.Research indicates that Mg treatment technology can purify liquid steel.refine dendritic structure,improving carbide distribution,microalloying and so on.The microalloying element Mg was used in M7 high-speed steel in this work and the effect of different Mg contents on the carbides and properties of M7 high-speed steel were systematically-studied.Thermodynamic calculation results show that the presence of trace magnesium is sufficient to complete the transformation of oxide and sulfide inclusions.With the increase of Mg content in steel,the conversion behavior of oxides:Al2O3?MgO·Al2O3?MgO;the conversion behavior of sulfide:MnS?MgS.At the same time,the calculation results of FactSage software show that the more MgO inclusions were formed in steel,and the lower dissolved oxygen content in steel.Four steel was smelted in this work.In order to improve the microstructure and properties of M7 high speed steel,magnesium modification was carried out under the conditions of pressure during the experiment.In this paper,under the condition of pressurization,magnesium molybdenum alloy with a lower magnesium content was used as a carrier for adding magnesium,so as to weaken the intensity of the transient reaction of Mg addition,and then to increase the yield of Mg.The results show that the magnesium treatment has strong deoxidation ability.By By magnesium modification on the M7 high-speed steel,the total oxygen mass fraction was only 0.0002%.In addition,magnesium has certain desulfurization and denitrification effect in molten steel.With the increase of Mg content in steel,the inclusions to be more fine and diffuse in steel.The proportion of inclusions less than 2?m in steel samples increases from 76.34%to 81.23%,more than 5 ?m in steel samples decrease from 6.87%to 1.46%and the number of inclusions per unit area increases gradually.This can prove that the addition of Mg can promote to generate small size inclusions.Mg treatment also has a significant effect on the as cast microstructure and carbide structure of M7 steel.'In details,with the conten of Mg increases,the paimary and secondary dendrite arm spacing decresses,and the average thickness of eutectic ledeburite decreases accordingly.The results show that by magnesium modification,carbides distributed uniformly and semi mesh carbides replacing reticular carbides in M7 high speed steel increase the continuity of the matrix.After annealing,the distribution of carbides is improved and large carbides are refined and spheroidized,and the proportion of small carbides is increased.However,the abnormal growth of carbides maybe caused when the magnesium content in steel is further increased.The results of EBSD analysis show that the peak value of Mg in carbides is much higher than that in matrix,which indicates that Mg tends to segregate and enrich in carbides.In addition,magnesium treatment had slight effect on the type of carbides in annealing steel,and a partial effect for the content of various carbides.Magnesium treatment has the effect on the properties of M7 steel.The experimental results show that the magnesium modification can obviously improve the plasticity and the toughness of the steel.Furher more,in terms of properties,the impact energy of steel increased gently and then decreased with the increase of magnesium mass fraction.When the Mg content of 0.0160%,the elongation after fracture is 24.18,the shrinkage after fracture is 10.0%,the impact energy is 75.0 J.It is the best of all the four steel.