Optimizacija parametrov poboljšanja debelih plošč iz jekla S890QL : magistrsko delo

Bernik, Matic (author), Nagode, Aleš (mentor), Skumavc, Andrej (comentor)

Glavni namen magistrskega dela je bila karakterizacija in optimizacija procesa poboljšanja dveh debelinskih razredov jekla S890QL. V ta namen smo izbrali tri slabe iz dveh debelinskih razredov, ki smo jih zvaljali v plošče treh različnih debelin. Vročemu valjanju je sledilo kaljenje na industrijskem agregatu, temu pa odrez vzorcev za laboratorijsko popuščanje. Karakterizacija mikrostrukture je bila izvedena v valjanem, kaljenem in popuščenem stanju. Med mikrostrukturami različnih debelinskih razredov ni bilo pomembnih razlik. V valjanem stanju je bila mikrostruktura bainitno-martenzitna, v kaljenem smo po celotnem volumnu dobili martezit, v popuščenem stanju pa smo imeli popuščeni martenzit. Mehanske lastnosti so bile ne glede na stanje najboljše na najtanjši plošči, kar je posledica najvišje hitrosti ohlajanja. Preizkus po metodi Jominy je pokazal, da ima preiskovano jeklo zelo dobro prekaljivost, saj globine, kjer je v mikrostrukturi le še 50 % martenzita, nismo dosegli. Z uporabo dilatometra je bil izrisan CCT diagram. Določili smo premenske temperature preiskovanega jekla, feritno-perlitno premeno, začetek in konec bainitne premene ter začetek in konec martenzitne premene. Na diagramu v magistrskem delu sta prikazani še premenski temperaturi Ac1 in Ac3. Z mikrostrukturno analizo smo določili tudi kritično ohlajevalno hitrost, ki znaša 5 °C/s. Upoštevajoč rezultate preiskav mehanskih lastnosti laboratorijsko popuščenih vzorcev smo izrisali popuščne diagrame za posamezne plošče ter glede nanje določili optimalne temperature popuščanja na industrijskem nivoju. Za ploščo, debelo 25 mm, je ta temperatura znašala 575 °C, za srednje legirano ploščo debeline 50 mm je znašala 555 °C in za tisto debeline 60 mm 595 °C. Laboratorijsko določene temperature popuščanja smo preverili v industrijskem okolju in vse tri so se izkazale za ustrezne, saj so mehanske lastnosti vseh testnih plošč ustrezale predpisanim zahtevam.

• Language: Slovenian
• Keywords: Jeklo S890QL, HSLA, popuščanje, mikrostrukturna analiza, mehanske lastnosti
• 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: M. Bernik
• Year: 2024
• Number of pages: XVII, 64 f.
• PID: 20.500.12556/RUL-162569
• UDC: 669
• COBISS.SI-ID: 212615427
• Publication date in RUL: 25.09.2024

Secondary language

• Language: English
• Title: Optimisation of quenching and tempering parameters of S890QL steel plates : master's thesis

Abstract

The main objective of this Master’s Thesis was to characterize and optimize the quenching and tempering process for two thickness grades of S890QL steel. For this purpose, three slabs from two thickness grades were selected, which were rolled into three different plate thicknesses and hot rolled, which was followed by quenching on an industrial unit and then by cutting samples for laboratory tempering. A microstructure characterization was performed in the rolled, quenched, and tempered states. No significant differences between the microstructures of the different thickness grades were found. In the rolled state, the microstructure was bainitic-martensitic, in the quenched state, the entire volume consisted of martensite, and in the tempered state, tempered martensite was present. The mechanical properties were found to be the best when the plate was thinnest regardless of the state it was in, which was due to the fastest cooling rate. The Jominy test showed that the examined steel had excellent hardenability as the depth where only 50% martensite is present in the microstructure was not reached. A CCT diagram was plotted using a dilatometer. The phase transformation temperatures of the investigated steel, the ferrite-pearlite transformation, the start and endpoint of the bainite transformation, and the start and endpoint of the martensite transformation were determined. The diagram also shows the Ac1 and Ac3 transformation temperatures. Through microstructural analysis, the critical cooling rate was also determined, which is 5 °C/s. From the results of the mechanical properties of the laboratory-tempered samples, tempering diagrams were drawn for individual plates, and optimal tempering temperatures at the industrial level were determined from them. For the 25 mm thick plate, this temperature is 575 °C, for the medium alloyed 50 mm plate, it is 555 °C, and for the 60 mm plate, it is 595 °C. The tempering temperatures determined in the laboratory were checked against an industrial setting, and all three proved to be appropriate given that the mechanical properties of all test plates met the prescribed requirements.

• Keywords: S890QL steel, HSLA, tempering, microstructural analysis, mechanical properties