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Evaluations of Non-metallic Inclusionsin Ca-treated Steels and Their Effecton the Machinability

Time: Thu 2021-04-08 10.00

Location:, Stockholm (English)

Subject area: Materials Science and Engineering

Doctoral student: Hongying Du , Processer

Opponent: Prof.Em. Lauri Holappa, Aalto University, Finland.

Supervisor: Docent Andrey Karasev, Materialvetenskap; Professor Pär Jönsson, Processer

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In recent decades, a considerable development of steel with respect to the performance of steel has taken place, which also has resulted in large challenges to process these steel grades. Therefore, it is essential to make suitable modifications of non-metallic inclusions (NMI) in the steelmaking process and to have a good control of its characteristics to meet the target mechanical properties and to obtain a good machinability. 

Based on a case of 316L stainless steel trials with a calcium modification to improve the machinability of steel, the influence and contribution of different NMIs on the machinability were discussed. First, based on the Thermo-Calc calculation results with respect to the appropriate range of Ca additions, steel is produced by an additional Ca treatment at the end of the ladle treatment. In order to evaluate non-metallic inclusions and their influence on machinability tests, steel samples were collected from rolled bars produced by the conventional production route (316R) and an experimental trial with Ca treatment (316Ca). 

The metal chips generated during the machining test were also collected for the evaluation of chip breakability and NMIs characteristics after machining. In addition, the Electrolytic extraction (EE) technology is used to extract NMIs from steel and chip samples. Then, a three-dimensional (3-D) study is performed on the inclusions collected on a membrane filter using a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). The morphology, size, number, frequency, and composition of non-metallic inclusions are studied. Four main types of inclusions were found in the 316Ca steel: Type I (elongated MnS), Type II (oxy-sulfides with hard oxide cores), Type III (soft elongated oxides), and Type IV (hard undeformed oxides).

The results show that the morphologies of NMIs in stainless steel chips were significantly changed after cutting. Overall, three different main shapes of NMIs were found: i) Group I having similar shapes, ii) Group II stretched inclusions having very thin film-like (Group II-a) and fractured stretched morphologies (Group II-b), and iii) Group III brittlely fractured inclusions. The total areas of MnS and SO inclusions in the secondary deformation zone of the chips were significantly increased (by up to 2-3 times) compared to that of the reference steel sample before the cutting test. It was found that the morphologies of NMIs during machining depend on the location in chips, the workpiece material, as well as the applied cutting speed. This results in different temperatures and metal matrix deformation degrees during machining. 

In addition, the chip breakability and chip tool contact length of the reference steel and the experimental steel were evaluated and compared with the characteristics of NMIs in the two steels. A new weight-measurement-based method was developed. The results show that the 316Ca steel generally has a better machinability compared to the reference 316R steel. However, the chip-tool contact length results show that the modification of NMIs for machinability improvements is only beneficial in some machining processes. The 316R steel was preferred at low cutting speeds, whereas the 316Ca steel was preferred at high cutting speeds. The different characteristics of NMI in the various cutting conditions and materials lead to different behaviors and functions of NMI during processing.  

Finally, the possible application of PDA/OES in the steelmaking process was also evaluated. This online survey method developed in the industry during recent years provides a high possibility for implementing a rapid screening of the NMI content and shows the potential of establishing an online control of NMI during the processing of steel.