• Nicoli, Cécile

Abstract

The objective of this work is to improve the mechanical properties of a low-carbon steel (0.2%) without the use of expensive secondary metallurgy processes. For this, a method of inoculation, already used during the development of the cast iron, is transposed to our steel grade. It consists in adding very small quantities in liquid steel in order to modify the solidification process, thus the microstructure and consequently the properties of the steel, especially resilience. The addition of elements in very small quantities does not modify the grade of steel. They must be added at the end of melting of the metal in the form of powder in the casting stream. The effects of these elements will be seen on the microstructure, particularly in terms of inclusions and grain size. The "good candidate" is an element that will lead to a homogeneous distribution of inclusions of small size and spherical shape. It must also make it possible to reduce the size of the grains. These structural modifications are supposed to improve the mechanical properties of the steel and in particular the impact resistance. A reference piece is made to study the microstructure. The retained shape is a parallelepiped ingot sized using a casting simulation software to prevent major foundry defects. The size is adapted to the laboratory scale (furnace capacity 120 kg). The load corresponding to the grade studied are provided by the SAFE Metal, the partner company. The first step is to obtain a suitably deoxidized bath; that means adding aluminum in order to trap the dissolved oxygen and to evacuate it. To demonstrate any significant effects of the various inoculants tested, it is necessary to start from a reference sample containing a relatively high number of inclusions. This is achieved by adding sulfur to the liquid bath. This element acts directly on the number of inclusions present in the steel by forming sulphides. For the inoculation stage, it was necessary to create specific tools that could be adapted to the laboratory scale. Trials are then carried out with different products at varying concentrations. The samples obtained are analyzed by various techniques: chemical analysis of steel (spark spectrometry and ICP), microstructure and grain size analysis (optical micrograph) and inclusion counting. In order to detect inclusions, count and classify them by their nature, shape and size, two possibilities exist. Either using an optical microscope but the risks of error are significant and the process is long and tedious, either using a specific software (AZtec) coupled to a scanning electron microscope (SEM). It is this choice that was made, because besides the fact that it allows a considerable saving of time thanks to an automation of the process, it is possible to know via an EDS probe, the chemical composition of each particle. For all the products tested, it was shown that inoculation had little effect on the size of the inclusions and that they became more complex by containing several chemical elements. For some products, clusters of inclusions appear. These clusters are likely to promote the fragility of steel by forming cracking primers. For others, the inclusions decrease sharply, have a globular shape and the grain size is refined. These effects tend to improve the mechanical properties of these steels. The introductory content of the inoculant is also determined for maximum effectiveness. The most efficient inoculant can be used for a possible industrialization.

Topics

• impedance spectroscopy
• cluster
• Carbon
• grain
• inclusion
• grain size
• scanning electron microscopy
• simulation
• Oxygen
• aluminium
• steel
• chemical composition
• casting
• forming
• iron
• Energy-dispersive X-ray spectroscopy
• cast iron
• spectrometry
• solidification