Calcium treatment can modify the morphology of B-type inclusions. It can transform oxide inclusions into spherical semi-liquid or liquid calcium aluminates and sulfide inclusions into dispersed inclusions with fine morphology.
This is beneficial to lowering the threshold stress and improving fatigue resistance. It also reduces the damage nucleation sites in the ductile zone.
Inclusions form during the steel making process and can cause clogging of the submerged entry nozzle (SEN). The presence of these inclusions affects the quality of the cast products. In particular, the brittle inclusions degrade toughness and ductility. The angular shape of the included phases increases the strain amplification at the interface with the matrix.
Oxides inclusions are classified into two main groups based on their mineralogical composition: free oxides (FeO, MnO, Cr2O3, SiO2, Al2O3 and TiO2) and spinels which consist of multivalent elements. The latter group includes ferrites, chromites and aluminates.
Using SEM-EDS and thermodynamic calculations, the optimum window of calcium addition during the ladle furnace was determined. This window depends on the sulphur and total oxygen content of the liquid steel bath. The predicted equilibrium inclusion compositions were compared with the measured compositions to confirm that reaction with the steel occurred. The average composition and size of the inclusions were also monitored. The evolution of these parameters was analyzed during the heat treatment.
Steel inclusions are characterized by their size, composition, shape and morphology. Some of them are detrimental to the quality of steels due to the kinetic and thermodynamic phenomena they cause at different stages of the steelmaking process. They can be classified in two main groups, endogenous and exogenous.
The formation of the second group of inclusions is a consequence of trapping non-metallic materials from the slag and ladle refractory during the steelmaking process. Exogenous inclusions are usually characterized by their large size, which makes it difficult to recognize their origin.
In order to reduce the harmful effects of these inclusions, they must be refined and changed into more suitable forms. One way to do this is by calcium treatment. When done properly, the alumina and silica inclusions are converted into liquid calcium aluminate and calcium silicate. These inclusions have a lower melting point and can be plastically deformed. They can also be transformed into oxide-sulphide duplex inclusion with a shell of CaS, which can prevent the formation of damage at the steel surface during hot deformation.
It is well known that the behaviour of hard inclusions (inclusions with low bonding strength to the steel matrix) during hot working and forming is not easy to predict. They tend to break and redistribute under these conditions, causing nozzle clogging and reducing the plastic deformation of the steel.
They can be divided in two categories based on their origin; endogenous inclusions like alumina and magnesium-spinel, which form from the precipitation within liquid steel and exogenous inclusions, entrapments of materials from refractory interfaces or slag. The latter are difficult to eliminate but can be minimized.
The calcium treatment significantly reduces the size of these brittle inclusions by a process of reaction with sulphur. This results in a sulphide crown precipitated on an oxide core, which prevents their plastic deformation. This system configuration mutually compensates the expansion coefficient of the non-metallic inclusion with that of the steel metal matrix. This allows to avoid nozzle clogging and improve the plastic deformation of the steel part.
The aim of calcium treatment is to transform brittle inclusions into glassy and ceramic ones, in order to prevent the problems of nozzle clogging and of damage initiation during hot forming. The control of inclusion morphology is particularly important. It is necessary to avoid the precipitation of sulphide phases at the interface between the steel matrix and the inclusion.
In the case of high-carbon hard wire steel, calcium treatment was able to change the composition and morphology of the inclusions. Sample 1 (without calcium treatment) contained alumina inclusions with irregular morphology, while sample 2 was characterized by alumina inclusions transformed to a calcium aluminate and by a more regular morphology.
The inclusions in group 3 were also changed. The core of the oxide-sulfide duplex inclusion was converted to a CaS, while the shell remained CaO, and its morphology was modified from sharp and angular to spherical. The melting point of CaS is lower than that of the aluminates, and it has a corresponding relationship with the plastic yielding ability.
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