| H13hot work die steel is one of the most wide used and most representative hot working die steel. There are certain gaps in domestic die steel compared with international advanced levels, which causes more than forty percent of upscale die steel have to be imported from abroad. To develop the high-quality of H13hot work die steel, the micro-alloying element Mg was used in H13die steel in this work and the effects of different Mg contents on the inclusions, carbides and properties of H13die steelwere systematically studied.In this work, the change of magnesium content in H13die steel and the volatilizing dynamics of magnesiumduring the electroslag remelting processingwere studied. The results show that, the content of magnesium is decreased by oxidization when transferring into the slag. During the electroslag remelting, the content of magnesium is decreased by53%at the formation stage of molten drop of the electrode. According tokinetics analysis, the loss limitative step of magnesium is the transferring of [Mg2+], from iron-slag boundary to the slag. The mass transfer coefficient of [Mg2+], namely DMg2+/σMg2+was calculated to be4.27x10’4mm/s.Considering the yield of magnesium,the optimal slag for electroslag slag of magnesium-containing H13steel is composed of50%CaF2-20%Al2O3-20%CaO-10%MgO.With the study of the inclusion and the carbide during the manufacturing process, the oxygen content can be decreased to the lower level by magnesium-aluminum deoxidant. Because of the large amount of the inclusions, namely MgO·Al2O3and MgS which are in the size of2μm, and the composite inclusion which contains magnesium, the inclusion size in the H13die steel by magnesium processing can reach the size of5μm. The magnesium-containingESR ingotmainly contains silicon-based composite inclusions, and a small amount of Al2O3-SiO2-CaO composite inclusions, and the amount of large size inclusion is smaller in the magnesium contained electroslag than in the magnesium uncontained electroslag. The carbides in the H13die steel by magnesium processing also get the smaller size. Magnesium is more likely to segregate under the annealing and tempering temperature, and generate the result, i.e. the decreasing of network chain developing of carbide. With the segregation of magnesium, the developing of the carbide is hindered, and forms into spheroidized carbide. The mismatch of MgAl2O4,γ-Fe, M(CN) and M6(CN) are4.0%,3.0%and2.8%separately. y-Fe, M(CN) and M6(CN) can nuclear more easily on MgAl2O4compared with Al2O3.The study of phase change and the performance impact of magnesium in the H13die steel shows that, in the CCT curve of the magnesium-containing H13die steel, perlitic transformation area moves left, bainite transformation area moves right. The magnesium-containing H13die steel has a good heat endurance. Withincreasing magnesiumcontent in steel, the amount of granulous pearlite in the electroslag remelting and annealing steel reaches higher level, and it leads to the fact that the strength and the hardness of the magnesium contained steel are lower than those of the magnesium-freesteel. The fracture mechanism of the magnesium uncontained steel is cleavage fracture With increasing amount of magnesium, the quasi-cleavage fracture changes into gliding fracture. Magnesium can increase the stability of the tempered martensite, i.e. the intensity and the hardness. After magnesium treatment, the inclusion in the fracture is consisted of MgOAl2O3and MgO-Al2O3-SiO2-CaO with small size. Carbide in the fracture changes into the shape of almost or approximately sphere, and it increases the tenacity of the steel. The friction mechanism of the magnesium uncontained steel is adhesive wear, and it changes into the oxidative wear when the content of magnesium increases. With increasing content of magnesium, the friction coefficient decreases, while the hardness increases. It can be concluded that the wear rate is inversely proportional to the content of magnesium. |
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