The pearlite spheroidisation and grain refinement of the C45 steel was investigated. The ASR (accelerated carbide spheroidisation and refinement) process allows us to achieve a microstructure made of fine ferrite grains and globular carbides in seconds or minutes. This paper deals with the ASR process realization via the thermomechanical processing of controlled rolling. Hot rolling is a common part of structural steel processing. A conventional structure after the hot rolling of the C45 steel

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... onsists of lamellar pearlite and ferrite. During controlled rolling with a deformation at temperatures around critical temperature A1, it is possible to achieve a microstructure of a fine ferritic matrix and globular carbides. The deformation causes a recrystallization of the ferritic matrix -a grain refinement. Furthermore, a high dislocation density during the deformation enhances the diffusion and promotes carbide spheroidisation. The aim of the experimental program was to achieve a microstructure of the C45 steel consisting of fine ferritic grains and homogeneously dispersed cementitic globular particles by processing bars in a laboratory rolling mill. The morphological changes of the cementite after the thermomechanical treatment in comparison with the conventional hot rolling were investigated as well as the ferritic grain refinement. An image analysis was performed to determine the spheroidisation level of the cementite (the aspect ratio of cementite particles) and the ferritic grain size. A proper examination of a cementite morphology can be carried out only by observing the entire cementite particle. A common metallographic analysis relies on a 2D section observation. Thus, deep etching and cementite extraction were performed to study the cementite-particle shape formed during the ASR process. Preiskovana je bila sferoidizacija perlita in zmanj{anje zrn v jeklu C45. Proces ASR (pospe{ena sferoidizacija perlita in rafinacija) omogo~a v nekaj sekundah ali minutah dose~i mikrostrukturo iz drobnih feritnih zrn in globularnih karbidov. Tã lanek predstavlja izvedbo ASR-procesa s termomehansko obdelavo pri kontroliranem valjanju. Vro~e valjanje je znan postopek pri preoblikovanju jekla. Navadno je mikrostruktura po vro~em valjanju jekla C45 iz lamelarnega perlita in ferita. Pri kontroliranem valjanju, z deformacijo pri temperaturah okrog kriti~ne temperature A1, je mogo~e dose~i mikrostrukturo iz drobnozrnate feritne osnove in globularnih karbidov. Deformacija povzro~i rekristalizacijo feritne osnove in zmanj{anje velikosti zrn. Poleg tega velika gostota dislokacij med deformacijo pospe{uje difuzijo in spodbuja sferoidizacijo karbidov. Namen programa preizkusov je bil dose~i pri jeklu C45 mikrostrukturo iz drobnih feritnih zrn in homogeno razporejenimi globularnimi cementitnimi delci z valjanjem palic pri laboratorijskem valjanju jekla. Preiskovane so bile spremembe v morfologiji cementita po termomehanski obdelavi v primerjavi z navadnim vro~im valjanjem, kot tudi udrobnjenje feritnih zrn. Stopnja sferoidizacije cementita (razmerje med dol'ino in debelino cementitnih delcev) in velikosti feritnih zrn je bila izvr{ena z analizo slik. Pravilen pregled morfologije cementita je lahko izveden samo z opazovanjem celotnega cementitnega delca. Navadne metalografske analize se opirajo na 2D-opazovanje prereza. Za {tudij oblike cementitnih delcev, ki nastajajo med ASR-procesom, je bilo zato izvr{eno globoko jedkanje in ekstrakcija cementita.