Researches On Application Of Oxides To High Performance Steels

Adding strong carbide-forming elements such as Mo,V,and Ti into steels can effectively suppress growth of austenite grains and remain fine grains,but production cost of the steels goes up.Oxides such as Al₂O₃and Ti O₂possess high thermal stability,and they can also play a role in refining grains of the steels under a condition of homogeneous distribution of the fine oxide particles.In this paper,oxide-added"master alloy"were initially prepared.Using it as raw mateials,oxide-added 45 carbon structural steel and high carbon&low alloy steel were prepared.Influence of the oxide particles on the microstructure,ambient-temperature mechanical properties and high-temperature hot deformation properties of the materials were studied.Our results proved that:（1）Adopting the oxide-containing master alloy as the raw material results in that the added oxide particles break into nano-sized particles（20-50 nm）during smelting of the steel,and their absorptivity is about 33.5%.These nano-oxide particles as heterogeneous nuclei can effectively refine the microstructure of the steel,and change grain size grade of the 45steel from 7 to 10.Its hardening radius is larger than 10.5 mm,which is about 4 times that of conventional 45 steel.After quenching,average size of the martensitic packets is about 8.7μm,and that of the martensite blocks was 0.71μm.They are about 30%and 31%,respectively,lower than those of the conventional 45 steel.After quenching and tempering at high temperature,the oxide-added 45 steel presents the yield strength at least 53%,the tensile strength at least 20%,and the impact energy at least 56%higher than the conventional 45steel,under similar hardness and plasticity.Among them,the oxide added 45 steel quenched at 850°C and then tempered at 620°C presents yield strength of 773 MPa,tensile strength of830 MPa and hardness of 20HRC,together with impact energy of 140 J,elongation of 19.6%and percentage reduction of area of 65%.The yield strength are at least 53%higher,the tensile strength at least 20%higher and the impact energy at least 97%higher than those of the conventional one.（2）The addition of oxides in high-carbon&low-alloy steels is favorable to refine their pearlite structure.Moreover,using aluminum as its alloying element is more conducive to suppress formation of the network cementite,and it is easy to obtain fine spherical carbides during spheroidization.UHCS-1.8Cr steel obtains fine carbide and ferrite microstructures after quenching at 850°C and tempering at 680°C for 2h.The steel with fine spheroidized carbides shows superplasticity（m>0.3）only when deformation at 700°C and at a strain rate of 5×10^-4 s^-1,and its an elongation is about 351.9%.On the other hand,UHCS-1.8Cr-1.5Al steel can obtain uniformly distributed carbide particles on the ferrite matrix after quenching at 850°C and then tempering at 620°C and 660°C.Among them,the sample tempered at620°C is composed of the carbide particle size of only 0.25μm,and it exhibits high elongation of 604.4%when deforming at 750°C and a strain rate of 5×10^-4 s^-1.This value is about 20%higher than that of the sample tempered at 660°C.This sample presents the activation energy of deformation of 207.9 k J·mol^-1 in the range of 725°C to 775°C.Its m values are larger than 0.3,and the sample deformed at 750°C shows the maximum m of 0.46.Those results indicate that the sample has good necking-resistant ability when deformation at this temperature.In conclusion,addition of the oxide nanoparticles into steels by using the master alloy is an effective way to improve the room-temperature mechanical properties and high-temperature deformation properties of materials.