The goal of this work is to reveal the dynamic mechanical properties and constitutive relationship of a novel low alloy ultra-high strength DT506 steel under high strain rates. The quasi-static state and dynamic compression behavior of the material at the strain rates of 10−3–103 s−1 were examined using an MTS landmark electro-hydraulic servo universal tester and a Split-Hopkinson bar (SHPB). The results show that DT506 steel is a strain-rate-sensitive material that shows an increase in strength with increasing strain rate. Based on the quasi-static and dynamic compression data, the parameters in the Cowper-Symonds (C-S) and the Johnson-Cook (J-C) models are determined. Since the previously developed models are cannot accurately predict the effect of the strain rate. A new theoretical model is obtained through the optimization of the parameters in the standard J-C model. The optimized model greatly improves the prediction accuracy of the true stress-strain dynamic behavior of materials. This new model is helpful for the verification of the underlying mechanism of the dynamic behavior of the material. Test results may provide basic data for future research on dynamic mechanical properties and constitutive relationship of metal alloys.
Owing to the continuous increasing of steel strength, mechanical properties including toughness and fatigue performance are becoming increasingly sensitive to inclusions in ultra-high strength steel. Rare-earth treatment is considered as an effective method to reduce the harmful effects of inclusions, but is rarely applied in secondary-hardening steel. In the present study, different amounts of cerium were added in a secondary-hardening steel to investigate the modification effect of Ce on non-metallic inclusions in steel. The characteristics of inclusions were observed experimentally using SEM-EDS and the modification mechanism was analyzed based on thermodynamic calculations. The results indicated that the main inclusions in Ce-free steel are Mg-Al-O + MgS. Thermodynamic calculation indicated that MgAl2O4 is firstly formed in liquid steel and then successively transformed into MgO and MgS during cooling process. When the Ce content is 0.0030%, the typical inclusions in steel were individual Ce2O2S and MgO + Ce2O2S complex inclusions. When the Ce content was increased to 0.0071%, the typical inclusions in steel were individual Ce2O2S- and Mg-containing inclusions. Ce treatment modifies the angular magnesium aluminum spinel inclusions into spherical and ellipsoidal Ce-containing inclusions, thus reducing the harmful effect of inclusion on steel properties.
Hot deformation behavior of as-cast 300M steel was investigated in the temperature range of 850-1200°C and strain rate range of 0.01-10 s-1 using Gleeble-3800 thermo-mechanical simulator. Based on the true stress-strain curves corrected for friction, flow stress behavior and deformation mechanism were analyzed, and the constitutive model of as-cast 300M steel was established based on the Arrhenius model and Zener-Hollomon parameter (Z). The microstructure after deformation was observed by Olympus GX51 microscope. The experimental results show that the flow stress of as-cast 300M steel decrease with the increase of deformation temperature and the decline of strain rate. The dynamic recrystallization is more likely to occur at higher temperatures and lower strain rates. By regression analysis, the hot deformation activation energy (Q) of the as-cast 300M steel was calculated to be 360.332 kJ/mol. Microstructure evolution is greatly affected by deformation temperature and strain rate. The dynamic recrystallized grain size increases with the enhancement of deformation temperature and the decrease of strain rate.
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