On the Origin and Distributions of the Inclusions in Production-scale ESR and PESR Remelted Ingots and Materials from Different Ingot Sizes and Solidification Structures
Time: Fri 2021-05-28 14.00
Subject area: Materials Science and Engineering
Doctoral student: Ewa Eva Sjöqvist Persson , Materialvetenskap, Uddeholms AB email@example.com
Opponent: Professor Torbjörn Carlberg, Mittuniversitetet, Sundsvall
Supervisor: Professor Pär Jönsson, Materialvetenskap
The study was carried out with the aim to evaluate the origin, morphology, and distribution of the non-metallic inclusions (NMI) in electro-slag remelted (ESR) steels and in electro-slag remelted steels using a pressured controlled inert atmosphere (PESR). In addition to the NMI studies, the solidification structure in different ingot sizes were studied in order to define the influence of the solidification on the NMI characteristics. The steel grade chosen for the studies was a common martensitic stainless steel. The focus is on the origin and the distribution of oxide inclusions with the assumption that sulfides and nitrides are secondary inclusions in remelted material.
In order to get a good statistical basis, a large number of SEM samples from different axial positions were taken from both an electrode and several ESR and PESR remelted ingots as well as processed (rolling/forging) materials. The inclusions were investigated by using both two-dimensional (2-D) and three-dimensional (3-D) methods. Especially for steels with a higher cleanliness, as for example remelted steels, a large analyzed area is important in order to get a true picture of the inclusion morphology. As an attempt to localize the origin of the inclusions, a pilot trial using a La2O3 as a tracer in the ESR process slag was performed. To study the influence of the solidification structure on the inclusions, horizontal slice/slices were cut from different positions from the electrode as well as from ESR and PESR remelted ingots of different sizes. Beside inclusions and chemical composition determinations across the diameter of the slices, also the second dendrite arm spacing (SDAS) and the angles of the dendrites towards the axial plane were measured.
The result gave rise to a new classification of the inclusions present in ESR or PESR remelted steels, i) Primary Inclusions. They survive from the electrode because they were trapped inside a steel drop or a fallen steel fragment, without having contact with the ESR/PESR process slag. The size depends on the size of the inclusions in the electrode and the size of the steel droplets. ii) Semi-Secondary Inclusions, primary Al-Mg oxides covered by process slag. Normal size class is ≈ < 30 µm. iii) Secondary Inclusions, precipitated during solidification of the liquid steel as a result of the reactions between alloying elements and the dissolved oxygen. Normal size class is < 10 µm.
The structure study showed that the transition from a columnar-dendritic to an equiaxial structure (CET) in the center of the ingot have a strong effect on the number and size of the inclusions. As long as the center of the ingot solidifies in a columnar-dendritic manner, the increase of the inclusion number and size is almost linear with an increasing ingot size. However, after the CET transition in the center, the inclusion number and sizes are much larger. For this steel grade, the transition from a columnar-dendritic to an equiaxial is between the 800 mm in diameter (PESR-800) ingot and the 1050 mm in diameter (PESR-1050) ingot. The primary arms growth rate needed for the CET transition is less than 4 x 10-7m/s. In order to undertake the transition, the temperature gradient must be less than approximately 103 °C/m.
On the whole, the results illustrated that the overall cleanliness of the electrode (as well as the composition of the inclusions in the electrode) has an extremely large influence on the cleanliness in ESR and PESR remelted steels. The majority of the failure critical inclusions originates direct or indirect from the inclusions in the electrode. Moreover, the solidification structure (ingot size) also has a direct bearing on the inclusion sizes and contents present in ESR and PESR ingots.