Experimental Research And Numerical Simulation On Dynamic Properties Of Ultra-high Strength Low Alloy Steels

Ultra-high strength low alloy steels have been widely used in bearing strong dynamic loads of structures, such as girder and landing gear, for the high strength, excellent toughnessd and high hot formability. Studies of mechanical response of ultra-high strength low alloy steels under dynamic loading have guiding significance to material application.Firstly, the microstructure and common mechanical properties of 300M and 300M-Nb steel under different quenching temperature was studied and the effect of Nb on microstructure and properties of 300M steel was analysed. The results show that microstructures were refined, the yield strength and tensile strength and hardness were improved in the quenching temperature of 870-1020℃, and plasticity and impact toughness were enhanced slightly by the addition of Nb to 300M steel when the quenching temperature overtops 900℃.Research on dynamic mechanical properties of 300M steel and 300M-Nb steel was carried in this paper. The Dynamic compression experiments of all testing steels under different quenching temperature at the same impact loading and under different strain rate at the same quenching temperature were studied by SHPB. The research suggests that 300M steel had higher resistance to deformation in the low temperature quenching (<930℃) compared with that in the high temperature quenching (≥ 930 ℃) (the rheological stress is more higher and relative compressibilities is more lower), and 300M-Nb steel had more excellent resistance to deformation in the whole quenching temperature range. In the strain rate range of 1000-4000s-1, all of the dynamic yield strength of the testing steels were increased with the increasing of strain rate at the same quenching temperature, moreover, the strain rate sensitivity of 300M steel was higher than 300M-Nb steel.Then, constitutive equation of the testing steels under quenching for 1h at 870℃ oil cooling(OQ) and tempering for 2h at 260℃ air cooling(AC) were fitted basing on static and dynamic experiment results and combining with Johnson-Cook model: 300M steel:σ=(1610+3372.8εp0.152)(1+0.0251nε*) 300M-Nb steel:σ= (1672+3006εp0.121)(1+0.018lnε*)The results uniformity of theory and test illustrate the J-C equation can describe the constitutive relation of material better.Finally, based on the experimental results, under the condition of considering the strain-rate effect, the SHPB dynamic compression test were simulated through the finite element method. And the finite element analysis results were in accordance with the test results.