Fundamental Research on Fluorine-Free Ladle Furnace Slag for Axle Steel of Electric Multiple Unit Vehicles

Fluorine-bearing refining slag (FBS) is used to produce axle steel for electric multiple unit vehicles. To avoid environmental pollution caused by fluorine, a fluorine-free ladle furnace slag (FFS) was designed based on an industrial FBS. The effects of main components on the physical and metallurgical properties of slag were investigated via theoretical analysis and laboratory tests. The composition range of components of the designed FFS are w(CaO) = 40–55 wt.%, w(SiO2) = 2–6 wt.%, w(Al2O3) = 30–40 wt.%, w(MgO) = 6–8 wt.%, and w(CaO)/w(Al2O3) = 1.25–1.50. Industrial-scale test results indicate that the FFS has similar deoxidation and desulfurization capabilities to industrial FBS.

Optimizing Slag Content to Control Ds-Type Inclusions in 10B21 Cold Heading Steel

Ds-type inclusions during production are an important factor affecting the performance and quality of manufactured 10B21 steel. To minimize Ds-type inclusions in steel and improve the production qualification rate of steelmaking plants, a refining slag system optimization scheme was proposed based on the analysis of current inclusion evolution during the steelmaking process, and industrial tests were conducted to verify improvements resulting from application of the proposed scheme. The results showed that the composition of Ds inclusions in 10B21 steel are mainly CaO–Al2O3–MgO–CaS–TiN, which exists in the form of calcium–magnesium aluminate coated with titanium nitride and calcium sulfide. The main reason for the formation of Ds inclusions is the poor fluidity of the refining slag and its low capability to absorb inclusions. The poor coverage of the refining slag on the molten steel during refining can easily cause secondary oxidation of the molten steel. Thus, the formation and growth of Ds-type inclusions are aggravated after the calcium feeding line and soft blowing operation. Here, we propose to minimize Ds inclusions using our optimized refined slag system. The mass percentage of the optimized slag system is CaO: 55–60%, Al2O3: 20–35%, SiO2: 3–7%, MgO: 4–8%, (MnO + FeO) < 1%, and the basicity is controlled within the range of 7–11. We observed that our optimized refining slag system has a significantly improved ability to remove inclusions, particularly Ds inclusions, which improves the qualification rate of 10B21 steel.

Influence of Hydrothermal Pretreatment Temperature on the Hydration Properties and Direct Carbonation Efficiency of Al-Rich Ladle Furnace Refining Slag

The influence of hydrothermal pretreatment temperature on the hydration products and carbonation efficiency of Al-rich LF slag was investigated. The results showed that the carbonation efficiency was strongly dependent on the morphology of hydration products and the hydration extent of the raw slag. Hydrothermal pretreatment at 20 °C or 80 °C favored the formation of flake-shaped products with a higher specific surface area and therefore resulted in a higher CO2 uptake of 20 °C and 80 °C-pretreated slags (13.66 wt% and 10.82 wt%, respectively). However, hydrothermal pretreatment at 40 °C, 60 °C or 100 °C led to the rhombohedral-shaped calcite layer surrounding the unreacted core of the raw slag and the formation of fewer flake-shaped products, resulting in a lower CO2 uptake of 40 °C, 60 °C and 100 °C-pretreated slags (9.21 wt%, 9.83 wt%, and 6.84 wt%, respectively).

Effect of Mn Content on the Reaction between Fe-xMn (x = 5, 10, 15, and 20 Mass Pct) Steel and CaO-SiO2-Al2O3-MgO Slag

Medium- and high-Mn steels have excellent properties but are very difficult to be commercially produced because of the high content of some alloy elements. To enhance the understanding of the reaction between medium/high-Mn steel and refining slag which is significantly different from the conventional steels, steel and slag composition and the inclusions were investigated by equilibrium reaction between Fe-xMn (x = 5, 10, 15, and 20 mass pct) and CaO-SiO2-Al2O3-MgO top slag at 1873 K in the laboratory. Furthermore, the effect of Mn content on inclusion transformation and steel cleanliness was also explored. After slag–steel reaction, both contents of MnO in slag and Si in steel increased. Most MnO inclusions in master steel transformed to MnO-SiO2 and MnO-Al2O3-MgO. With the increase in Mn content, the amount share of MnO type inclusions decreased and that of MnO-Al2O3-MgO type increased. In addition, both the number density of observed inclusions and the calculated oxygen content in inclusions increased. Thermodynamic analysis indicates that the composition change of steel and slag and the transformation of inclusions are mainly the consequence of the reaction between Mn in molten steel and SiO2 and MgO in top slag. The dissolved Mn in medium/high-Mn steel presents a strong reactivity.

Molecular Dynamics Study of Structural Properties of Refining Slag with Various CaO/Al2O3 Ratios

SiO2-Al2O3-CaO-MgO is the main type of refining slag in a ladle furnace. Here the effects of the CaO/Al2O3 mass ratio on the structural properties of the refining slag system are studied by molecular dynamics simulations. The pair distribution function, coordination number, micro-structure unit and diffusion capacity were analyzed. An increase in the CaO/Al2O3 ratio did not change the bond lengths of these units. However, an increase of the CaO/Al2O3 ratio caused more charge compensators to be introduced into the refining slag system, which led to conversion of free oxygen and non-bridging oxygen to tricluster oxygen and bridging oxygen in the system. Moreover, this augmented the content of relatively stable 4- and 5-coordinate Al. As the micro-structure of the system became more complex, the overall diffusion capacity of the refining slag became poorer. In the micro-structure, the diffusion capabilities of different atoms decreased in the following order: Mg > Ca > O > Al > Si.

The Influence of Different Natural Fibers on the Mechanical Properties of Geopolymer

Geopolymers can effectively deal with construction waste disposal problems, achieving resource recycling and higher strength than normal cement concrete. However, natural defects make geopolymer concrete difficult to apply in actual construction. This paper aims to make up for the natural defects of geopolymers while maintaining environmentally friendly materials. The low-calcium fly ash and blast furnace refining slag powder are used as raw materials, sodium silicate and sodium hydroxide are used as alkali activators, and chopped natural fibers are added as reinforcing materials to prepare natural fiber reinforced low-calcium fly ash and blast furnace refining slag base. Polymeric composites were used to investigate the effects of different natural fibers on the workability, compressive strength, tensile strength, water absorption, shrinkage and microstructure of geopolymer composites.