Razvoj nekovinskih vključkov od taline do valjanca za jeklo 20MnV6 EXEM : magistrsko delo

Fir, Brina (author), Knap, Matjaž (mentor), Drofelnik, Nejc (co-mentor)

EXEM jekla so zaščitena blagovna znamka podjetja Štore Steel. Glavni razlog za razvoj teh jekel je bila potreba po boljši obdelovalnosti, saj ta podaljšuje življenjsko dobo rezilnega orodja. Za obdelovalnost je pomembna optimalna vsebnost s kalcijem modificiranih aluminatnih vključkov in kalcijevih aluminatov, obdanih s sulfidnim ovojem. Cilj magistrskega dela je bil ovrednotiti možnost napovedovanja vrste in vsebnosti nekovinskih vključkov po valjanju na podlagi rezultatov vzorcev, odvzetih med litjem, ter uvedba metode za podajanje sestave vključkov v jeklu 20MnV6 EXEM. Analizirali smo vzorce šestih šarž, odvzetih v različnih korakih obdelave, in sicer pred in po dodatku modifikatorja CaSi v ponovčno peč, 30 minut po začetku litja in v končnem valjancu. Na vseh vzorcih smo z vrstičnim elektronskim mikroskopom izvedli ploskovno EDS analizo za karakterizacijo posameznih vključkov in avtomatsko analizo. Ta nam je na podlagi računsko določene sestave vključkov in izrisanih sestav vključkov v ternarne fazne diagrame sistemov MnS-Al2O3-CaO, MgO-CaO-MnS ter CaO-CaS-Al2O3 omogočila spremljanje tipa in sestave vključkov skozi različne stopnje v procesu izdelave jekla 20MnV6 EXEM. Izkazalo se je, da je na podlagi vzorca, vzetega 30 minut po začetku litja, mogoče napovedati tudi sestavo vključkov v valjancu. Zanimal nas je tudi najprimernejši čas odvzema vzorca od začetka litja, da bo prikaz sestave vključkov v valjancu najbolj verodostojen. Ugotovili smo, da so odstopanja v sestavi in tipu vključkov pri različnih časih odvzema od začetka litja zelo majhna. Zato velja, da rezultat avtomatske analize vzorca, vzetega kadarkoli po začetku litja, kaže dobro ujemanje z rezultati avtomatske analize v valjancu. Pojasnili smo tudi povezavo med vsebnostjo nekovinskih vključkov kalcijevega sulfida (CaS) in obdelovalnostjo preiskovanega jekla.

• Language: Slovenian
• Keywords: jeklo 20MnV6 EXEM, nekovinski vključki, avtomatska analiza vključkov, modifikacija, obdelovalnost
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Place of publishing: Ljubljana
• Publisher: [B. Fir]
• Year: 2021
• Number of pages: XVIII, 65 f.
• PID: 20.500.12556/RUL-131696
• UDC: 669
• COBISS.SI-ID: 85817347
• Publication date in RUL: 01.10.2021

Secondary language

• Language: English
• Title: Development of non-metallic inclusions from melt to rolled piece for 20MnV6 EXEM steel : master's thesis

Abstract

EXEM steels are a registered trademark of the company Štore Steel. The main reason for the development of these steels was the need for better machinability, as this prolongs the lifetime of the cutting tool. The optimum content of calcium-modified aluminate inclusions and calcium aluminates surrounded by a sulphide coating is important for machinability. The objective of the master's thesis was to evaluate the possibility of predicting the type and content of non-metallic inclusions after rolled piece on the results of samples taken during casting, and the introduction of a method for specifying the composition of inclusions in 20MnV6 EXEM steel. We analysed the samples of six batches taken at different treatment steps, namely before and after the addition of the CaSi modifier to the ladle furnace, 30 minutes after the start of casting, and in the final rolled piece. Surface EDS analysis for characterisation of individual inclusions and automated analysis were performed on all samples with a scanning electron microscope. Based on the calculated composition of inclusions and plotted compositions of inclusions in ternary phase diagrams of MnS-Al2O3-CaO, MgO-CaO-MnS and CaO-CaS-Al2O3 systems, the latter enabled monitoring of the type and composition of inclusions through various stages in the manufacturing process of 20MnV6 EXEM steel. It turned out that based on a sample taken 30 minutes after the start of casting, it is also possible to predict the composition of inclusions in the rolled piece. We were also interested in the most appropriate sampling time from the start of casting, so that the display of inclusions' composition in the rolled piece would be the most credible. We found that the deviations in the composition and type of inclusions at different sampling times from the start of casting are very small. Therefore, the result of the automated analysis of the sample, taken at any time after the start of casting, is considered to show a good agreement with the results of the automated analysis of the rolled piece. We also explained the relationship between the content of non-metallic calcium sulphide (CaS) inclusions and the machinability of the investigated steel.

• Keywords: 20MnV6 EXEM steel, non-metallic inclusions, automated analysis of inclusions, modification, machinability