Fig. 4.Optical photographs of the microstructure of as-cast strips followed by cold rolling and annealing treatment at 625°C for 1 h for steels (a) A2 (0.1 mass% P), (b) A4 (0.3 mass% P), and (c) A5 (0.7 mass% P). with river markings (indicated by the arrow) are the main feature of fracture morphology, showing that cleavage cracks may have passed grain boundaries to form crack steps. 12) Also, micro voids can be found in the fractured area in this sample. In the sample of A5 (0.7 % P), Fig. 9(e), only typical inter-granular fracture morphology can be observed. The composition analysis by EDS shows that P has segregated at grain boundaries, Fig. 9(f), showing the evidence that P segregation along grain boundaries was the main reason for the brittle inter-granular rupture.With increasing the content of P in steel, atomic bonding could have been weakened to generate more cleavage cracks during the tensile testing, Fig. 8(c). With more P having been added in steel, segregation of P along grain boundaries and inverse surface segregation took place, which has weakened grain boundaries to cause inter-granular rupture to occur, Figs. 8(f), 8(d) and 8(e). Therefore, the ductility of the strips has been severely deteriorated when the addition of P was beyond a critical content although their strengths could be increased.
ConclusionStrips of low carbon steels containing high content of P have been made by using a twin roll strip casting process, which led to the following conclusions and might provide ISIJ International, Vol. 47 (2007) primary guidance for recycling dirty steel scraps.(1) Obvious inverse surface segregations have been formed in the as cast strips containing more than 0.1 mass% P, which could not be eliminated by the subsequent annealing treatments.(2) After cold rolling and annealing treatment, the strip containing 0.1 mass% P exhibited the same plastic and rupture behaviors with the strip containing 0.01 % P, showing no deteriorating effect of P on its ductility, and implying that segregation of P could have been prevented to some extent due to the high cooling rate during thin strip casting.(3) More cleavage cracks, micro voids and inter-granular rapture were formed in the fractured areas with increasing P content in steel, resulting from the segregation of P in as cast strips.
AcknowledgementThis work was supported by the "973" project under a contract of 2004CB619108 and the NECT-04-0278 of the Ministry of Education China. , A-river markings on cleavage facets, and B-micro voids, and (e) A5 after tensile testing, the arrow indicates the spot for composition analysis; (f) EDS composition analysis of the spot indicated by the arrow in Fig. 9(e).
