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Stress and strain distribution at the onset of cleavage fracture during fourpoint bending testing at liquid nitrogen temperature of two commercial mediumcarbon V and TiV microalloyed forging steels, with predominantly acicular ferrite structure, was examined using finite element analysis. The finite element models were based on notched four-point bending Griffiths-Owens’s type specimens, while the material mechanical properties data input was based on stress-strain curves obtained by tensile testing at liquid nitrogen temperature. Results of the modeling showed that there were no distinct differences in strain distribution along the distance from the notch tip between the two steel samples, aside from strain magnitude which stems from the differences in stress-strain curves. Based on the recorded load at fracture, the corresponding displacement calculated by finite element analysis was somewhat larger for the V steel. While the TiV steel breaks at crosshead displacement between 0.3 and 0.5 mm, the V steel breaks between 0.6 and 0.9 mm. Plastic strain at the cleavage initiation site for the TiV steel ranges from 0.0595 to 0.1612, while for the V steel these values range from 0.3694 to 0.6338. Observed differences in plastic deformations near the notch root, where cleavage initiation sites were detected, seem to reflect differences in deformation behavior at liquid nitrogen temperature. Moreover, such difference could be ascribed to the observed differences in structure, primarily in the volume fraction of acicular ferrite. It was concluded that deformations are more uniformly distributed when acicular ferrite is predominant in the structure of the steel. It could be ascribed to the effect of “gradual yielding” related to the high dislocation density in acicular ferrite, somewhat lower yield stress and higher ductility of the TiV steel with predominantly acicular ferrite structure.

Cleavage fracture of commercial medium-carbon V-microalloyed forging steel with predominantly acicular ferrite structure was studied by means of fourpoint bending testing at liquid nitrogen temperature in conjunction with finite element analysis. Both two-dimensional and three-dimensional finite element models were used for the purpose of comparison. Cleavage fracture was initiated by high plastic strains in the vicinity of notch tip. Strains calculated by threedimensional modeling were higher up to 80% than for two-dimensional model. Calculated local cleavage fracture stresses rendered effective surface energy values of 49J/m2 and 60J/m2, for two-dimensional and three-dimensional numerical model, respectively. Both results are in good agreement with previously published data for medium carbon microalloyed steels.

Two medium carbon vanadium microalloyed foring steels with different content of carbon and titanium were investigated. Broad goals were to examine transformational behavior of the steels on continuous cooling at still air directly from austenitisation temperatures, to determine optimal regime to produce predominantly acicular ferrite structure and to examine it both by means of mechanical behavior and fracture properties at liquid nitrogen temperature. Main goal was to establish micromehanism of cleavage fracture of the steels with predominantly acicular ferrite structure, and to measure critical parameters of cleavage fracture – critical fracture stress and effective fracture surface energy. The influence of vanadium and nitrogen on microstructure and mechanical properties of medium-carbon steels has been studied by means of metallography and mechanical testing. Vanadium addition to the low nitrogen steel suppresses the formation of ferrite-pearlite following the low reheating temperatures and microstructure consists of bainitic sheaves. For high reheating temperatures, dominantly acicular ferrite structure in both the low nitrogen and the high nitrogen vanadium steels is obtained. The results suggest that vanadium in solid solution promotes the formation of bainite, whereas the effect of nitrogen is related to the precipitation of VN particles in austenite with high potency for intragranular nucleation of acicular ferrite and to the precipitation of V(C,N) particles in ferrite with high potency for precipitation strengthening. Addition of both vanadium and nitrogen considerably increases the strength level. The aim of this work was to establish deformation behaviour of two vanadium microalloyed medium carbon steels with different contents of carbon and titanium by tensile testing at 77 K. Samples were reheated at 1250°C/30 min and continuously cooled at still air. Beside acicular ferrite as dominant morphology in both microstructures, the steel with lower content, U radu su ispitivanana dva srednjeugljenična čelika za kovanje, mikrolegirana vanadijumom, koji se međusobno razlikuju po sadržaju titana i ugljenika. V– mikrolegirani čelik sadrži 0,26%C, označen je kao „V19“, i Ti–V– mikrolegirani čelik mikrolegiran je titanom i sadrži 0,31%C, označen kao „TiV22“. Cilj rada bio je da se ispitaju transformacione karakteristike srednjeugljeničnih mikrolegiranih čelika pri kontinuiranom hlađenju na vazduhu, direktno sa temperature austenitizacije, da se odrede uslovi postizanja strukture acikularnog ferita i da se odrede kritični parametri loma cepanjem na temperaturi tečnog azota za ovu strukturu. Čelici su isporučeni kao toplovaljane šipke. Analizom mikrostruktura koje su postignute pri kontinuiranom hlađenju na mirnom vazduhu, direktno sa različitih temperatura austenitizacije, određen je termički režim pripreme uzoraka za ispitivanje. Pripremljeni uzorci čelika sa strukturom acikularnog ferita ispitivani su na temperaturi tečnog azota jednoosnim zatezanjem i savijanjem u četiri tačke. Za ispitivanje savijanjem u četiri tačke korišćene su epruvete sa zarezom po Grifitsu i Ovenu. Temperatura tečnog azota odabrana je radi ispitivanja u uslovima potpuno krtog loma i određivanja kritičnog napona loma. Određivanje kritičnog napona loma obuhvata fraktografiju i modelovanje savijanja u četiri tačke metodom konačnih elemenata. Rezultati jednoosnog zateznog ispitivanja na temperaturi tečnog azota poslužili su kao ulazni podaci za modelovanje. Modelovanjem je izračunata raspodela napona i deformacija u uzorku, u trenutku loma, pri savijanju u četiri tačke. U opsegu temperatura austenitizacije između 850°C i 1300°C, za pripremu uzoraka za ispittivanje izabrana je temperatura od 1250°C. Nakon hlađenja na mirnom vazduhu sa 1250°C, struktura čelika legiranog vanadijumom i sa nižim sadržajem ugljenika (čelik V19) sastoji se od ferita, perlita i acikularnog ferita, dok se struktura čelika legiranog titanom i vanadijumom, sa višim sadržajem ugljen

У раду су испитиванана два средњеугљенична челика за ковање, микролегирана ванадијумом, који се међусобно разликују по садржају титана и угљеника. V– микролегирани челик садржи 0,26%C, означен je као „V19“, и Ti–V– микролегирани челик микролегиран је титаном и садржи 0,31%C, означен као „TiV22“. Циљ рада био је да се испитају трансформационе карактеристике средњеугљеничних микролегираних челика при континуираном хлађењу на ваздуху, директно са температуре аустенитизације, да се одреде услови постизања структуре ацикуларног ферита и да се одреде критични параметри лома цепањем на температури течног азота за ову структуру. Челици су испоручени као топловаљане шипке. Анализом микроструктура које су постигнуте при континуираном хлађењу на мирном ваздуху, директно са различитих температура аустенитизације, одређен је термички режим припреме узорака за испитивање. Припремљени узорци челика са структуром ацикуларног ферита испитивани су на температури течног азота једноосним затезањем и савијањем у четири тачке. За испитивање савијањем у четири тачке коришћене су епрувете са зарезом по Грифитсу и Овену. Температура течног азота одабрана је ради испитивања у условима потпуно кртог лома и одређивања критичног напона лома. Одређивање критичног напона лома обухвата фрактографију и моделовање савијања у четири тачке методом коначних елемената. Резултати једноосног затезног испитивања на температури течног азота послужили су као улазни подаци за моделовање. Моделовањем је израчуната расподела напона и деформација у узорку, у тренутку лома, при савијању у четири тачке. У опсегу температура аустенитизације између 850°C и 1300°C, за припрему узорака за испиттивање изабрана је температура од 1250°C. Након хлађења на мирном ваздуху са 1250°C, структура челика легираног ванадијумом и са нижим садржајем угљеника (челик V19) састоји се од ферита, перлита и ацикуларног ферита, док се структура челика легираног титаном и ванадијумом, са вишим садржајем угљеника (челик TiV22), претежно сас, Two medium carbon vanadium microalloyed foring steels with different content of carbon and titanium were investigated. Broad goals were to examine transformational behavior of the steels on continuous cooling at still air directly from austenitisation temperatures, to determine optimal regime to produce predominantly acicular ferrite structure and to examine it both by means of mechanical behavior and fracture properties at liquid nitrogen temperature. Main goal was to establish micromehanism of cleavage fracture of the steels with predominantly acicular ferrite structure, and to measure critical parameters of cleavage fracture – critical fracture stress and effective fracture surface energy. The influence of vanadium and nitrogen on microstructure and mechanical properties of medium-carbon steels has been studied by means of metallography and mechanical testing. Vanadium addition to the low nitrogen steel suppresses the formation of ferrite-pearlite following the low reheating temperatures and microstructure consists of bainitic sheaves. For high reheating temperatures, dominantly acicular ferrite structure in both the low nitrogen and the high nitrogen vanadium steels is obtained. The results suggest that vanadium in solid solution promotes the formation of bainite, whereas the effect of nitrogen is related to the precipitation of VN particles in austenite with high potency for intragranular nucleation of acicular ferrite and to the precipitation of V(C,N) particles in ferrite with high potency for precipitation strengthening. Addition of both vanadium and nitrogen considerably increases the strength level. The aim of this work was to establish deformation behaviour of two vanadium microalloyed medium carbon steels with different contents of carbon and titanium by tensile testing at 77 K. Samples were reheated at 1250°C/30 min and continuously cooled at still air. Beside acicular ferrite as dominant morphology in both microstructures, the steel with lower content