Microstructure And Mechanical Properties Of Oxide Dispersion Strengthened Low Carbon Steels

In this study,the Q345 and Q235 low carbon steels were used as raw materials.The ZrO₂ and TiO₂ oxide particles were added into molten steels using cored-wire injection and prefabricated block punch technologies,respective,to prepare ZrO₂ dispersion strengthened Q345 steel and TiO₂ dispersion strengthened Q235 steel.The microstructural evolution and its effects on mechanical properties of those two dispersion strengthened steels during casting,normalizing,quenching and thermomechanical treatments were investigated.The optical microscope was used to investigate the microstructural evolution after various treatments.Instron testing machine was used to test mechanical properties of dispersion strengthened steels.Fracture feature was observed using scanning electron microscopy to analyze the fractural mechanism.The bonding interface between dispersion particles and substrate and microstructure were characterized using transmission electron microscopy to analyze the strengthening mechanism.The main conclusions are as followed:The strength of Q345 increases with the addition content of ZrO₂ particles with size of submicron.The ZrO₂ dispersion strengthened Q345 steel with 1.2wt%ZrO₂ has the maximum strength of 845MPa after normalization and 1398MPa after quenching treatment,respectively.The improvement in strength comparing to the Q345 without reinforcement steel,which strength is 686MPa after normalization and 1086MPa after quenching treatment,is approximately 23.2%and 28.7%,respectively.The mechanical properties of ZrO₂ dispersion strengthened Q345 steel after thermo-mechanical treatment with 30%deformation at temperature of 850??900?and 950?,respectively,were tested.Results showed that the strength and toughness were improved significantly after thermo-mechanical treatment at temperature of 900?.The yield strength,tensile strength and elongation of the ZrO₂ dispersion strengthened Q345 stee with 1.2 wt.%ZrO₂ after thermo-mechanical treatment at temperature of900?followed by 550?tempering is 932 MPa,977MPa and 15.4%,respectively.The microstructural evolution of TiO₂ dispersion strengthened Q235 steel during casting,normalizing,quenching and thermomechanical treatments were investigated.Results indicate that the grain size of TiO₂ dispersion strengthened Q235 steel is obviously fined comparing to the Q235 without reinforcement steel after both normalization at910?for 15 min and quenching treatment at 925?for 15 min.The quenched TiO₂dispersion strengthened Q235 steel has finer and more uniform grains.The mechanical properties are also improved and enhanced.The TiO₂ dispersion strengthened Q235 steel were underwent the thermomechanical treatment with 20%,25%and 30%deformation at 850?,900?and 950?,respectively.After thermomechanical treatment,the specimens were tempered at 570?,600?and 630?,respectively.Tensile test on tempered TiO₂ dispersion strengthened Q235 steel with and without thermomechanical treatment showed that the thermomechanical treatment can obviously improve and enhance the strength and toughness of the TiO₂ dispersion strengthened Q235 steel.After thermomechanical treatment with 30%deformation at 900?followed by tempering at various temperatures,the TiO₂ dispersion strengthened Q235 steel has yield strength of 346 MPa for 570?tempering,328 MPa for 600?and 323 MPa for 630?;fracture strength of 547 MPa for 570?tempering,535 MPa for 600?and 521 MPa for 630?;and elongation of 15.5%for 570?tempering,16.2%for 600?and 20.8%for 630?,respectively.