Industrial experiment and thermodynamic calculation have confirmed that calcium treatment can modify alumina inclusions into 12CaO*7Al2O3 and CaS. SEM mapping also shows that a special form of the CaS inclusion is wrapped by calcium aluminates.
The "liquid zone" range for inclusion modification by calcium is influenced by oxygen and sulfur content in the molten steel. Low sulfur level is necessary for better shape control of sulphides.
Thermodynamic calculation is an important tool in materials science, engineering, and industrial process optimization. It is the scientific study of the transfer of energy from one system to another, with emphasis on the concept that energy can neither be created nor destroyed but can only be transferred between systems with different energy levels.
A thermodynamically derived M S formula is required to account for the effect of alloying elements on the martensitic transformation behavior of HSS. The current study evaluates and adapts such a formula by comparing calculated M S with dilatometrically measured values for HSS with varying alloy composition.
For this purpose, two different Thermo-Calc databases, namely an older TCFE2 and the recently developed TCFE7 Steels/Fe-alloys database were used. Calculated M S was compared with data obtained from dilatometer measurements for the investigated alloys at two austenitizing temperatures. The calculated M S showed good agreement with the measured ones. This is mainly due to the improved impact of the strong carbide-forming elements V and W in the used equations.
Industrial experiments are powerful tools for tackling manufacturing and quality problems. They are based on experimental designs that explore the response surface of a process and help engineers determine which factors have the greatest influence on the measured responses. However, not many engineers are familiar with these approaches.
This study examines formability of St14 low carbon steel sheet by systematic uniaxial tensile tests. Microstructures of the as received and heat treated samples were examined by scanning electron microscope (SEM) before and after forming and by X-ray diffraction. The results show that the as received samples have a spheroidized structure, while those after forming have more lamellar and granular ferrite with a smaller amount of transformed martensite. Both of these microstructures can be used to generate formability curves. However, no model considering all geometrical, loading and microstructural parameters can predict acceptable forming limits in real cases. Thus, it is necessary to carry out a series of experiment to evaluate the reliability of any model.
While thermodynamic calculation and industrial experiment support the conclusion that calcium content is important, a further factor influencing steel formability is the sulfur content of molten steel. The sulfur content influences the size and morphology of MnS inclusions. The higher the sulfur content, the more elongated the MnS inclusions become, which reduces toughness by decohesion of the inclusions and reducing inter-inclusion contacts.
The elongation of MnS also affects crack propagation in fatigue testing. The long axis of MnS inclusions in resulfurized steel is parallel to the specimen length, and when this axis is perpendicular to the load direction, fatigue cracks cut across the flattened inclusions, which reduces the endurance limit.
This phenomenon can lead to unexpected failures of components, if designers do not systematically consider loading conditions and loading directions when specifying resulfurized steels. For example, longitudinal tensile properties are commonly specified, but transverse fracture toughness is not taken into consideration. Using the correct property values for the transverse direction would avoid these problems.
The high strength-formability trade-off has been a long-standing issue in the metals industry. It is essential to develop a low-cost Mg alloy with good formability. However, the formation of strong textures with a preferential alignment of basal planes in sheet plane limits the strain accommodation and leads to limited RT formability.
SPD is a process that can modify the microstructure of magnesium alloys. It can refine the grain, partially desulfurize and change the composition, morphology, and quantity of inclusions. This can improve the corrosion resistance, wear resistance, high-temperature performance and impact toughness of steel.
The results of thermodynamic calculation and industrial experiment indicate that the calcium content in molten steel determines the extent of inclusion modification. With the increase of sulfur content, the critical calcium contents corresponding to the "liquid zone" range of inclusion transformation decreases and the difference between upper and lower calcium contents widens. The calcium content in molten steel should be strictly controlled, so that the liquid phase ratio of modified inclusions can reach more than 50%.
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