Title Raspodjela naprezanja u zoni deformacije niobijem mikrolegiranog čelika Title (english) The stress distribution in the deformation zone of niobium microalloyed steel Author Ivan Jandrlić Mentor Stoja Rešković (mentor)Committee member Ladislav Lazić (predsjednik povjerenstva)Committee member Željko Alar (član povjerenstva)Committee member Toma Udiljak (član povjerenstva)Granter University of Zagreb Defense date and country 2015-11-17, Croatia Scientific / art field, TECHNICAL SCIENCES Universal decimal classificationUDC ) 669 - Metallurgy Abstract U ovom doktorskom radu provedena su usporedna istraživanja na čeliku mikrolegiranom niobijem koji ima sitne precipitate niobija, čeliku istog baznog kemijskog sastava bez dodatka niobija i niobijem mikrolegiranom čeliku s krupnim precipitatima niobija. Uz statički vlačni pokus istovremeno su primijenjene metode termografije i digitalne korelacije slike, koje omogućuju konstantno mjerenje promjena temperature i deformacije.
Primjenom programskih paketa IRBIS3 Professional i MatchID 2D omogućena je kvalitativna i kvantitativna analiza rezultata mjerenja promjena temperature i pomaka tijekom deformacije. Provedena istraživanja su pokazala da se čelik mikrolegiran niobijem značajno različito ponaša pri deformaciji u odnosu na čelik istog baznog sastava bez niobija. Utjecaj sitnih niobijevih precipitata naročito je izražen na početku plastičnog toka materijala. Nakon granice proporcionalnosti, niobijem mikrolegirani čelik ima reološki kompleksno ponašanje. Istraživanja su pokazala da je to ponašanje povezano s pojavom Lüdersovih linija. Lüdersova linija se formira na način da plastična deformacija započinje na jednom kraju uzorka, pri čemu se ostvari deformacija od 0,004 mm/mm. Nakon toga, Lüdersova linija propagira pod kutom od 45. Utvrđeno je da na fronti Lüdersove linije dolazi do porasta temperature od 4,5 do 6C, što je povezano s porastom naprezanja. Iza fronte linije ostvaruje se Lüdersova deformacija od 0,05 mm/mm. Nakon toga, u formiranoj zoni deformacije nastavlja se daljnja plastična deformacija, a naprezanja se koncentriraju u sredini zone deformacije, kao kod niskougljičnog čelika.
Predložen je matematički model koji povezuje naprezanja i deformaciju s izmjerenim promjenama temperatura, s kojim je moguće iz izmjerenih vrijednosti deformacije i temperature odrediti iznose naprezanja u promatranim točkama. Usporedbom rezultata istraživanja i rezultata dobivenih razvijenim modelom, pokazalo se kako se vrijednosti podudaraju sve do brzine 30 mm/min, gdje su zabilježena značajna odstupanja.
Abstract (english) In this doctoral thesis a comparative research were conducted on niobium microalloyed steel which has fine niobium precipitates, steel with same base chemical composition but without the addition of niobium and on niobium microalloyed steel with large precipitates of niobium.
With the tensile testing, at the same time the methods of thermography and digital image correlation were applied, which enables the continuous measurement of temperature changes and strain.
By using software packages IRBIS 3 Professional and MatchID 2D is enabled a quantitative and qualitative analysis of measured changes in temperature and strain. Research results have shown that microalloyed niobium steel has a significantly different behavior upon deformation in relation to the steel of the same base composition without niobium. Effect of the fine precipitates of niobium is especially pronounced at the beginning of plastic flow of material. After yielding the niobium microalloyed steel has rheological complex behavior. Research has shown that this behavior is associated with the occurrence of Lüders bands. Lüders band is formed in such way that plastic deformation starts at the one end of sample, in doing so the strain of 0,004 mm/mm is carried out. After that Lüders band propagates at the angle of 45°. It was found that on the Lüders band front there is increase in temperature of about 4.5 to 6 °C, which is associated with increases in stress. Behind the Lüders front the strain of 0.05 mm/mm is achieved, Lüders deformation. After that, in the formed deformation zone, further plastic deformation continues and stresses are concentrated in the centre of the deformation zone, as in the low-carbon steel.
Proposed mathematical model links measured stress and deformation with the measured temperature changes, with which is possible from the measured values of deformation and temperature changes determine the amount of stress in the observed points of deformation zone. By comparing the results of the research and the results obtained by the developed model, it was shown that the values match up to the speed of 30 mm / min, where is shown a significant discrepancies.
Keywords
mikrolegirani čelik
zona deformacije
raspodjela naprezanja
statičko vlačno ispitivanje
termografija
vizioplastičnost
digitalna korelacija slike
Keywords (english)
microalloyed steel
deformation zone
stress distribution
static tensile testing
thermography
visioplasticity
digital image correlation
Language croatian URN:NBN urn:nbn:hr:115:612314 Study programme Title: Postgraduate university doctoral study in the field of Metallurgy Type of resource Text Extent 160 str. ; 30 cm File origin Born digital Access conditions Closed access Terms of use Created on 2017-05-18 08:39:59
