Spheroidization behavior of cementite in a low carbon steel processed by the equal channel angular pressing technique was investigated. The effects of annealing temperature and time, and accumulated strain on the morphology of cementite and mechanical properties were studied. The results indicated that the application of the severe plastic deformation can improve the kinetics of spheroidization significantly. In this study, the enhanced spheroidization kinetics was discussed in terms of carbon dissolution from cementites and defects induced in cementites by the severe plastic deformation. In addition, the softening of the steel after the spheroidization treatment was evaluated.
Experiments were conducted on a plain low carbon steel with an initial grain size of -30 11m to investigate the changes of microstructure and mechanical properties by repetitive equal channel angular pressings. Under the pressing conditions of giving a strain of -1 and rotating samples 180°between each pass, the yield strength significantly increases from 310 to 750 MPa after single pass, and it reaches 1050 MPa after 12 passes. The increment of yield strength gradually decreases as the number of passes increases. The examination of microstructure by transmission electron microscopy shows that ferrite consists of parallel bands of elongated subgrains having a width of 0.3 11m and a length of 2 11m after a single pass. The subqrains are further divided by boundaries with low angle misorientation on subsequent passages. Low angle boundaries turn to high angle boundaries without noticeable grain refinement as the number of pass increases. In addition, lamellar cementites in pearlite are broken up into fragments within a pearlite colony. Analyses of structural and mechanical changes in a plain low carbon steel by equal channel angular pressing indicate that the strength enhancement is mainly due to the grain refinement of ferrite. Der Effekt des Winkelstrangpressens auf die Struktur und die mechanischen Eigenschaften eines Stahls mit 0,15% C. Die Experimente wurden an einem Stahl (0,15%C) mit einer AusgangskorngrbBe von 30 11m durchgefUhrt, um die Anderung der Gefuge und der mechanischen Eigenschaften beim wiederholten Winkelstrangpressen zu untersuchen. Mit dem Dehnungsgrad von 1 und einer 180-°-Drehung bei jedem Versuch steigt die Streckgrenze von 310 auf 750 MPa nach dem erstem Versuch und erreicht 1050 MPa nach 12 Versuchen. Die Zunahme der Streckgrenze sinkt stetig, wahrsnd die Versuchszahl steigt. Die GefUgeuntersuchung mit dem Transmissionselektronenmikroskop zeigt, daB Ferrit nach dem ersten Versuch aus parallelen Bandern ausgedehnter Subkbrner mit einer Breite von 0,3 11m und einer Lange von 2 11m besteht. Die Subkbrner werden weiterhin durch Korngrenzen unterteilt, die in den folgenden Passagen eine Kleinwinkel-Misorientierung aufweisen. Wenn die Versuchszahl steigt, wandeln sich Kleinwinkel-Korngrenzen zu GroBwinkel-Korngrenzen ohne eine erwahnenswsrte Kornfeinung um. AuBerdem werden Zementitlamellen im Pearlit zu Fragmenten innerhalb einer Perlitkolonie aufgebrochen. Die Analyse der strukturellen und mechanischen Anderung in einem Stahl mit 0,15%C beim Winkelstrangpressen zeigt, daB die Festigkeit hauptsachllch von der Kornfeinung des Ferrits abhanqt,
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