Effects Of Rare Earth Y On Microstructures And Properties Of H13 Steel

The application of rare earth(RE)mainly focuses on the modification of non-metallic inclusions in steel and the purification of molten steel.With the development of smelting technology,the micro-alloying mechanism of RE in steel has become an urgent problem to be explored in the regulation of mechanical properties.This article by adding rare earth yttrium(Y)of H13 steel was taken as the research object.The effects of different Y contents on the microstructures and mechanical properties of H13 steel were systematically studied by using scanning electron microscopy and electron back scattering diffraction,carbon-replica extraction,transmission electron microscopy,three-dimensional atomic probe,thermal expansion instrument and multifunctional internal friction.And the key factors of Y micro-alloying on its property control were revealed.The main results are as follows:(1)The addition of appropriate amount of Y is beneficial to the fracture of strip Cr₂₃C₆carbides along grain boundaries in H13 steel,and makes it chain distribution.On the one hand,RE inclusions become the core of heterogeneous nucleation of carbides and the breaking point of carbide fragmentation during forging in H13 steel.In addition,submicron spherical RE inclusions also induce the precipitation of carbides rich in Y.On the other hand,the solid solution of Y atoms leads to lattice distortion,increases the nucleation point of VC particles,and reduces the lattice mismatch between VC particles and the matrix.When the addition amount of Y was 130 ppm,the number of VC particles increased by nearly 70%,and the precipitation strengthening effect was enhanced.Therefore,the work-hardening index of 0.013Y-H13 steel was the largest,and the work hardening ability was the best.(2)With the increasing Y content(0?440 ppm),the toughness and plasticity of H13steel are firstly enhanced and then slightly weakened,and the optimal addition amount of Y element is 130ppm.When the addition of RE is 130 ppm,the impact energy,tensile strength,and elongation are increased by 35.7%,7%(50.2 MPa),and 5.6%respectively.The micro-fracture morphology analysis shows that RE inclusions cause the fracture mode change from brittle to ductile.(3)The addition of the appropriate amount of Y refined the primary austenite grain and martensite microstructure in H13 steel,enhanced Snoek-K(?)ster-Kêpeak,and the interaction between an interstitial carbon atom and dislocation.With the increasing Y content,the Ms temperature of H13 steel decreases first and then increases slightly.After solid solution,RE Y atoms provide more nucleation points and extra transformation power for martensitic transformation,leading to a relatively high transformation rate of RE-modified H13 steel before 70%martensitic transformation.The transformation rate of RE-modified H13 steel is relatively low with the reduction of phase transformation area.Besides,the formation of Nano-twin martensite is induced by the preferential selection of the 0.013Y-H13 steel variant.The tensile strength of the tempered H13 steel decreases with the increasing Y content.At this time,the second phase particles have become the key factor to control the tempering mechanical properties of the RE-modified H13 steel.