| Enhancing the strength and toughness of steels and developing new weathering steels for containers have become main ways to cut down the cost of transportation and expand the manufacturing share of containers. In this study, the effects of trace element Ce on the microstructures and mechanical properties of SPA-H steel were studied, aiming at improving the microstructures and increasing the impact performance. The basic experimental data was provided for developing the smelting process, contour machining and engineered application technique of low-carbon and high-strength containers. As is well-known, SPA-H weathering steel is one of the main materials for manufacturing the containers. The strength is highly required for this kind of steels, since they chronically work under complicated conditions. Adding rare earth elements into the steels is an effective way to improving the strength and toughness of the material. As an elementary study for enhancing the strength and toughness of the steels and broadening their range of application, the widely used SPA-H weathering steels were investigated in the study. Based on the previous metallurgical research, the following four tasks were completed:1. Estimate the influence of Ce content on the degree of super-cooling of molten steels on the basis of metallurgical thermodynamics and calculate the thermodynamics conditions of the produced Ce, O, and S inclusions.2. Prepare the0.08C-0.6Si-0.45Mn-0.3Cu-0.09P (mass%) molten steel under Ar atmosphere of800Pa in the vacuum induction furnace by melting3kg industrial pure iron and adjuvant materials. Add Ce elements into the molten steel and refine at1873K. Prepare four groups of specimens by no-oxidation casting and forging after refining.3. Study and characterize the effects of Ce element on the compositions, inclusions, microstructures and mechanical properties of the SPA-H container steels, employing the methods of component analysis, metallographic observation, scanning electron microscope observation, XRD analysis, tensile test and impact test.4. Analyze and explore the reactions between [Ce] and [O],[S], and the inclusions produced by the possible reactions, as well as the mechanisms. The results show that:(1) The contents of [O] and [S] in the molten steel tended to decrease with the increase of [Ce] content. When the Ce content was0.043%, the contents of [O] and [S] reduced to0.0009%and0.0030%, respectively. However, the content of [P] stabled at0.09%.(2) Without Ce addition, irregular elongated MnS inclusions and Al2O3-SiO2inclusions with a size of7μm were observed in the specimens. When the Ce content was in the range of0.008%-0.020%, the inclusions observed in the specimens were mainly ellipsoidal MgO-Al2O3-Ce2O2S(Ce2O3) inclusions, whose size of minor axis was about0.3μm, and the long axis was about1μm. When the content of Ce was0.043%, ellipsoidal FePCe-Ce2O3inclusions with a size of about2μm were observed.(3) The statistical results of the SEM images at the fractures of impact specimens indicate that when the Ce content was in the range of0.008～0.043%, the amount of inclusions with large sizes and the total amount of the inclusions decreased markedly with the increasing of Ce content. When the content of [Ce] was0.020%, the residual area of inclusions in the steel reduced by58.1%.(4) After Ce addition into the steels, the strength and toughness were improved evidently, the yield ratio decreased and the antiknock characteristics of the steels were enhanced. When the Ce content was0.020%, the steels’tensile strength, yield strength and extensibility at room temperature reached766.1MPa,661.1MPa and45.8%, respectively. These figures were increased by21.9%,32.2%, and96.6%, respectively, compared with those of specimens without Ce addition. In addition, the impact energy Ak reached281J, increasing by75.6%in comparison with that of Ce-free specimens. |
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