Cleanliness Assessment of Steel Bars Produced From a High Frequency Induction Furnace Frank Hayford Master’s Thesis Division of Applied Process Metallurgy Department Of Materials Science and Engineering Royal Institute of Technology, Stockholm August 2011 Uddeholms AB, Hagfors, Sweden Master’s Thesis in Applied Process Metallurgy Cleanliness Assessment of Steels Bars Produced From a High Frequency Induction Furnace Frank Hayford KTH, Royal Institute of Technology Stockholm, August 2011 Supervisors: Lars Ekman and Bert Nilsson Uddeholms AB, Hagfors, Sweden Diana Janis Division of Applied Process Metallurgy KTH, Royal Institute of Technology, Stockholm Examiner: Prof. Pär Jönsson Head, Department of Materials Science and Engineering KTH, Royal Institute of Technology, Stockholm i To God be all the glory ii Abstract To stay competitive in the steel industry, steelmakers are utilizing different production methods to reduce production cost without compromising on the quality of their products. In steelmaking, the production process plays a significant role on the steel cleanliness. Recent increasing demands on the cleanliness level therefore require optimization of production process to meet the requirement. Often, the types and distribution of non- metallic inclusions in steel determines the steel cleanliness. In order to optimize the production process, complete assessment of the non-metallic inclusions in the steel is necessary, leading to implementation of measures to control and/or remove non-metallic inclusions in the steel. The present study was performed to investigate the cleanliness level of steel bars produced from a high frequency induction furnace (HF) route at Uddeholms AB. Experimental studies were carried out and characteristics such as number, composition, size distribution and morphology of non-metallic inclusions were investigated. Total oxygen and total nitrogen content were also measured for indirect assessment. Further, the production operations at the HF were observed and evaluated to determine their influence on the inclusion characteristics. The characteristics obtained were compared with characteristics of inclusions in steel bars produced from an electric arc furnace production (EAF) route at Uddeholms AB and a competitor producer sample. The results showed that the level of cleanliness varies from different production routes and is hence dependent on the process at each production route. The number, maximum and mean size of inclusions were found to be higher in the HF route compared to the other routes. More so, there were differences in the types of oxide inclusions observed from each process route. However, sulphide inclusions exhibit similar characteristics from the different process routes. Further, the compositions of oxide inclusions observed from the HF route were found to be closely related to the steel chemistry. More importantly, the types of inclusions formed in the HF route were found to be sensitively affected by the extent of aluminium and calcium contents in the steel. Thus, the oxide inclusion types in the HF samples could be traced to the extent of different additions and operations such as deoxidation and calcium treatment that were carried out during the steelmaking process. Keywords: high frequency induction furnace, non-metallic inclusions, inclusions assessment, deoxidation, calcium treatment iii Acknowledgement First of all, I will like to express my sincere appreciation to Ulrika Svensson and the High Performance Steel division of Uddeholms AB for given me the opportunity to carry out this work with Uddeholms AB. My sincere gratitude also go to my examiner and the head of the Materials Science and Engineering department, KTH, Professor Pär Jönsson, for allowing me to coordinate this work with the Applied Process Metallurgy division of the department. Many thanks go to my supervisor Diana Janis of KTH for everything especially for your time, patience and fruitful discussions. I am indebted to Dr Andrey Karasev of KTH for his time and patience, and for everything he did for me. I am really grateful. To my coordinators Berth Nilsson and Lars Ekman of Uddeholms AB for the enormous support they showed me during my stay in Hagfors, I say a big thank you. Berth, I really appreciate all the times you were there for discussions about all and nothing. My sincere appreciation goes to Dr Mselly Nzotta of Uddeholms AB for his constructive criticism, endless support and helpful discussions. Dr Nzotta, you really helped me to understand metallurgy and I appreciate it a lot. Special thanks to Lennart Lindstöm of Uddeholms AB for the numerous discussions, decisive questions and support he gave me at the plant. I thank Christer Söderström also of Uddeholms AB for his patience, smiles and support with operating the scanning electron microscope. I am also grateful to all the workers of Uddeholms AB for their support and friendliness during my stay there. To all my friends, I say thank you for being there for me. You are all part of this success. Lastly, I will like to say a big thank you to my dear family: my parent, my sisters and my brothers for always being there for me. I appreciate all the love, prayers and the supports you gave me. To my dearest, I say thank you for your never-ending love, support and encouragement. Frank Hayford Stockholm, August 2011 iv TABLE OF CONTENT 1.0 INTRODUCTION .............................................................................................................................. 1 2.0 LITERATURE REVIEW ...................................................................................................................... 3 2.1 CLEAN STEEL ........................................................................................................................................ 3 2.2 PRODUCT REQUIREMENT ........................................................................................................................ 4 2.4 STEELMAKING OPERATIONS AT UDDEHOLMS .............................................................................................. 5 2.4.1 The EAF route ............................................................................................................................... 5 2.4.2 The HF route................................................................................................................................. 5 2.5 CALCIUM TREATMENT OF STEEL ................................................................................................................ 9 2.5.1 Sulphides modification by calcium addition ............................................................................... 10 2.5.2 Modification of oxides inclusions by calcium addition ............................................................... 11 2.5.3 Estimation of Calcium recovery in steel ..................................................................................... 12 2.5.4 Influence of Calcium treatment on steel properties ................................................................... 12 2.6 INCLUSIONS AND MACHINABILITY ............................................................................................................ 12 2.7 TYPES OF NON-METALLIC INCLUSIONS IN STEEL ......................................................................................... 13 2.7.1 CLASSIFICATION BASED ON SOURCE ......................................................................................................... 13 2.7.2 CLASSIFICATION BASED ON COMPOSITION ................................................................................................ 14 2.8 INCLUSION DETERMINATIONS IN STEEL ..................................................................................................... 14 2.9 INCLUSION RATINGS IN STEEL ................................................................................................................. 15 3.0 EXPERIMENTAL WORK ................................................................................................................. 17 3.1 SAMPLING ......................................................................................................................................... 17 3.1.1. Final product samples ............................................................................................................ 17 3.1.2 Lollipop samples .................................................................................................................... 18 3.2 SAMPLE PREPARATION ......................................................................................................................... 18 3.3 2D INVESTIGATIONS OF INCLUSIONS BY CROSS SECTION METHOD .................................................................. 18 3.4 3D INVESTIGATIONS OF INCLUSIONS BY EXTRACTION METHOD ...................................................................... 19 3.4.1 Procedure ................................................................................................................................... 19 3.4.2 SEM/EDS analysis on film filter .................................................................................................. 20 4.0 RESULTS ....................................................................................................................................... 21 4.1 TYPES AND COMPOSITIONS OF INCLUSIONS ............................................................................................... 21 4.2 MORPHOLOGY OF INCLUSIONS ..............................................................................................................