Title Pojava Lüdersovih linija kod niobijem mikrolegiranoga čelika Title (english) Appearance of Lüders bands in niobium microalloyed steel Author Tin Brlić Mentor Stoja Rešković (mentor)Committee member Ladislav Lazić (predsjednik povjerenstva)Committee member Mladen Franz (član povjerenstva)Committee member Tomaž Rodič (član povjerenstva)305524575 Granter University of Zagreb Granter University of Zagreb Defense date and country 2020-09-30, Croatia Scientific / art field, TECHNICAL SCIENCES Universal decimal classificationUDC ) 669 - Metallurgy Abstract Posljednjih godina provode se intenzivna istraživanja nehomogenih deformacija na početku plastičnog toka materijala tijekom hladne deformacije. Prethodna istraživanja pokazala su da postoji razlika u ponašanju mikrolegiranih čelika na početku plastičnog toka s obzirom da kod čelika s dodatkom mikrolegirajućega elementa niobija dolazi do pojave Lüdersovih linija, dok čelici istog osnovnog kemijskog sastava bez dodatka mikrolegirajućega elementa niobija ne pokazuju pojavu nehomogenih deformacija, odnosno pojavu Lüdersovih linija.
U ovoj doktorskoj disertaciji provedena su istraživanja na niobijem mikrolegiranom čeliku s različitim dodacima mikrolegirajućega elementa niobija dobivenima u industrijskim i poluindustrijskim uvjetima te niskougljičnom čeliku bez dodatka mikrolegirajućega elementa niobija. Primjenom suvremenih metoda istraživanja termografije i digitalne korelacije slike, zapaženo je da su nehomogene deformacije povezane s pojavom Lüdersovih linija. Statičkim vlačnim ispitivanjem uz suvremene metode, termografiju i digitalnu korelaciju slike te metodama za mikrostrukturna ispitivanja, istražen je utjecaj mikrolegirajućega elementa niobija na pojavu i propagaciju Lüdersovih linija. U svrhu mikrostrukturnih ispitivanja prije i nakon određenog stupnja deformacije korištene su metalografija, pretražna elektronska mikroskopija uz EBSD analizu, transmisijska elektronska mikroskopija te analiza rendgenskom difrakcijom.
Posebna pozornost u istraživanju posvećena je formiranju i propagaciji Lüdersovih linija na početku plastičnog toka niobijem mikrolegiranoga čelika tijekom hladne deformacije. U cilju utvrđivanja utjecaja mikrolegirajućega elementa niobija, početne mikrostrukture i brzine deformacije na pojavu i propagaciju Lüdersovih linija te lokalne iznose deformacije provedena su ispitivanja na čelicima različitog kemijskog sastava, početne mikrostrukture i brzine deformacije. S obzirom da je kod niobijem mikrolegiranih čelika prethodnim istraživanjima uočena prisutnost sitnih, deformacijom induciranih, niobijevih precipitata ovim istraživanjem istražen je njihov utjecaj zajedno s dislokacijama na formiranje i propagaciju Lüdersove linije u različitim pozicijama zone deformacije kako bi se utvrdio njihov mehanizam nastajanja.
Istraživanja su pokazala da utjecajni faktori različitog sadržaja mikrolegirajućega elementa niobija, početna mikrostruktura i brzina deformacije imaju utjecaj na propagaciju Lüdersove linije. Međutim, brzina deformacije nema utjecaj na pojavu Lüdersove linije dok dodatak mikrolegirajućega elementa niobija, početna mikrostruktura te veličina i sastav niobijevih precipitata imaju utjecaj na pojavu Lüdersove linije. Sitni, deformacijom inducirani, niobijevi precipitati i gustoća dislokacija pokazali su se kao parametri koji imaju značajan utjecaj na nastajanje i propagaciju Lüdersove linije. Detaljno je, po prvi puta, postavljen mehanizam formiranja i propagacije Lüdersove linije kod niobijem mikrolegiranoga čelika.
Na kraju su postavljeni jednostavni matematički modeli za opisivanje utjecajnih parametara na formiranje i propagaciju Lüdersove linije. Postavljenim matematičkim modelima moguće je predvidjeti ponašanje niobijem mikrolegiranoga čelika s obzirom na pojavu Lüdersove linije na početku plastičnog toka tijekom hladne deformacije.
Abstract (english) Intensive research into inhomogeneous deformations at the start of the plastic flow in cold deformation of different metal materials has been carried out during the last few years. Previous research has clearly shown that there are differences in deformation behaviour of microalloyed steels at the start of the plastic flow. Lüders bands appear only in low carbon steels with addition of microalloying element niobium while low carbon steels with the same basic chemical composition without niobium do not show inhomogeneous deformations, i.e. Lüders bands.
This research was carried out on low carbon steels without niobium and niobium microalloyed steels with various additions of the microalloying element niobium obtained in industrial and semi-industrial conditions. Connection of Lüders bands with inhomogeneous deformations was observed using modern methods of thermography and digital image correlation. Influence of microalloying element niobium on Lüders bands formation and propagation was researched by static tensile tests with thermography and digital image correlation, and modern methods of microstructural testing. Metallography, scanning electron microscopy with EBSD analysis, transmission electron microscopy and X-ray diffraction analysis were used for the purpose of microstructural testing before and after a certain deformation degree.
In this research particular attention was given to the formation and propagation of Lüders bands at the start of the plastic flow of niobium microalloyed steels during cold deformation. Tests were conducted on steels with various chemical composition, initial microstructure and strain rate in order to determine the influence of the microalloying element niobium, the initial microstructure and strain rate on the appearance and propagation of Lüders bands as well as local strain amounts. Previous studies have shown a presence of fine deformation induced niobium precipitates in niobium microalloyed steels. This doctoral thesis presents a research of their influence along with dislocations on the formation and propagation of Lüders bands in various positions of the deformation zone to determine the mechanism of Lüders bands formation.
The research showed an influence of various contents of the microalloying element niobium, initial microstructure and strain rate on the propagation of the Lüders bands. However, the strain rate do not have influence on the appearance of Lüders bands while the addition of the microalloying element niobium, initial microstructure, size and chemical composition of niobium precipitates have influence on the appearance of Lüders bands. Fine deformation induced niobium precipitates and dislocation density showed a significant impact on the formation and propagation of Lüders bands. The mechanism of formation and propagation of Lüders bands in niobium microalloyed steel is set out in detail for the first time.
Simple mathematical models for the parameters influencing the formation and propagation of Lüders bands were developed at the end of the doctoral thesis. Behaviour of niobium microalloyed steels with Lüders bands can be predicted by the defined mathematical models at the start of the plastic flow during the cold deformation.
Keywords
Lüdersove linije
niobijem mikrolegirani čelik
termografija
digitalna korelacija slike
mikrostruktura
Keywords (english)
Lüders bands
niobium microalloyed steel
thermography
digital image correlation
microstructure
Language croatian URN:NBN urn:nbn:hr:235:149550 Study programme Title: Doctoral study mechanical engineering, naval architecture, aeronautical engineering, metallurgical engineering Type of resource Text File origin Born digital Access conditions Open access Terms of use Created on 2020-12-14 10:57:27
