The morphology, quantity and position of inclusions can significantly influence the properties of AHSS grades. This is because they can create privileged locations for damage.
The calcium content in the molten steel influences its reaction with oxygen, sulfur and arsenic. When the critical calcium content is reached, the reaction with these elements is in equilibrium.
The Ca content has a significant effect on the formation of inclusions in liquid steel. Inclusions in molten steel are mainly composed of oxygen, sulfur and arsenic. Oxygen is the first to react with calcium, and then sulfur, and lastly arsenic. The atomic ratio of calcium and oxygen is therefore critical in controlling the concentration of these elements.
In the case of AZX611 alloy, the addition of 1% Ca significantly improved the impact strength and near-threshold fatigue crack growth rate in the as-rolled and quenched and tempered condition. This was attributed to the control of the size and shape of sulphide inclusions.
However, it should be noted that excessive reoxidation during the Ca treatment process favours the formation of high melting point calcium aluminates (CaO) and SiO2 which are just as harmful to continuous casting operations in terms of nozzle blockage as the alumina and silicate inclusions that the Ca was supposed to remove or modify.
While the effect of sulphur on steel properties has been studied extensively, less attention has been given to the effect of calcium alloys. It has been seen, for example, that the behavior of sulphide inclusions in low-sulfur steel is highly dependent on the calcium content of the alloy that forms the steel matrix. This is because calcium can affect the size, morphology, and distribution of these inclusions, as well as their ability to form a second phase.
It is therefore important to have a control of the sulphur content of the calcium-based additive in order to optimize the steel properties. This is because the level of sulphur in the raw material used for forming cold rolled products can influence the impact resistance of the finished product, even if the Charpy test results obtained are within the specification limits. In the case of high sulphur levels, the steel can be prone to transverse stress cracking and poor weld penetration.
In addition to the formation of nonmetallic inclusions and microporosity, oxygen also influences impact toughness. In a previous study, it was found that a substantial decrease in impact toughness occurred when the oxygen content in the weld metal increased from low to high concentrations. This was attributed to the presence of fine inclusions, which decreased the size of intragranular acicular ferritic grains, and as a result, reduced impact toughness [3].
Oxygen content can be directly related to the microstructure of an alloy, as it causes elements with lower surface energies and negative heats of solution to segregate to alloy surfaces. This is particularly evident during forming operations such as forging and hot isostatic pressing, which introduce a large amount of oxygen into the bulk of the metal.
In oxygen cutting (also known as oxy-flame cutting), a preheat flame is directed to the area of the steel that needs to be cut and then the steel is blasted with a stream of pure oxygen. This rapidly oxidizes the iron in the steel, creating a stream of molten metal and sweeping away the oxide product.
Arsenic is an impurity that is difficult to remove during steelmaking. When its content reaches a certain value, it will segregate to phase and grain boundaries and significantly reduce the strength, plasticity, and weldability of steel products. It also contributes to hot brittle cracks in the tempering process, which is the primary cause of defective products.
Arsenic binds weakly with oxygen and sulfur in molten steel, making it more difficult to oxidize and remove than Fe. Therefore, it is important to maintain the oxygen and sulfur concentrations in molten steel within a narrow range for arsenic removal.
The research results showed that the calcium alloy with a low Ca content (0.01%) could effectively perform the dearsenication reaction in molten steel, resulting in improved yield strength and elongation to fracture than those of the 2% Ca-containing sample. This improvement may be attributed to the formation of small and well-dispersed Ca2Mg6Zn3 inclusions. The morphology of these inclusions may be beneficial for forming composite inclusions, which are more resistant to fracture than pure crystals.
English
Write a Message