Year

2003

Degree Name

Master of Engineering by Research

Department

Department of Materials Engineering - Faculty of Engineering

Abstract

The high temperature ductility of steel increases as the grain size is decreased. However, in microalloyed steels this interdependence of grain size and hot-ductility is generally less pronounced because of the overriding effect of precipitation of carbo-nitrides of alloying elements at grain boundaries. Nevertheless, many types of crack have been shown to be associated with coarse austenite grains, and the tendency to crack is reduced when the formation of coarse austenite grains is prevented by the use of suitable secondary cooling. To our knowledge, a systematic study, to isolate the contribution of grain size on hot-ductility has not been reported and hence, under these circumstances, it is important to define clearly the contributing role of grains size to hot-ductility and therefore it is necessary to separate the effect on hot ductility of microalloying precipitates from that of grain size. The materials used for study were Fe-0.05%C, Fe-0.18%C and Fe-0.45%C alloys, prepared in an experimental facility. Carbon was the only alloying element deliberately added to the melt and tramp elements and impurities were kept to as low a value as experimentally possible. A GLEEBLE 3500 thermomechanical simulator was used to conduct hot-tensile tests and from these results the relationship between austenite grain size and hot-ductility could be determined. The hot-ductility tensile tests in this study were performed in the temperature range 1100 to 700 degrees celsius. Hot-tensile test specimens were either solution treated or melted in-situ (direct cast) and cooled to the test temperature in order to simulate the microstructural characteristics of commercially as-cast slab which contains coarse austenite grains at the base of oscillation marks. In order to obtain different grain sizes, the specimens were solution treated at temperatures between 1100 degrees celsius and 1350 degrees celsius for 10minutes. The specimens were then rapidly cooled to the test temperature, in the range 1100 degrees celsius to 700 degrees celsius at a rate of 200 degrees celsius min (superscript �1) and then pulled to fracture at a low strain rate of 7.5�10(superscript �4)s(superscript �1). Specimens that were cast in-situ (referred to as the �direct casting condition�) were cooled at rates of 100 degrees Celsius min (superscript �1) and 200 degrees Celsius min-1 respectively in order to study the effect of cooling rate on hot ductility. It was found that grain size increased almost linearly with increasing solution treatment temperature in all the Fe-C alloys and in case of specimens solution treated at 1350 degrees celsius an average austenite grain size of ~4mm in diameter was obtained. Increasing the grain size resulted in ductility loss under all testing conditions. The existence of a ductility trough between Ar(subscript 3) and Ae(subscript 3) temperature was considered to be due to the formation of deformation induced ferrite as evidenced by a constant peak stress region between these two temperatures. However, convincing experimental evidence of the ductility trough extending beyond the Ae(subscript 3) temperature well into the austenite was found. The mechanism of this interesting, and important observation for low carbon alloy below 0.3%C has not been explored as yet and at present it has to be assumed that it is related to the occurrence of grain boundary sliding because in the high austenite temperature region the grain boundary sliding is favored in a coarse grained structure. For specimens cast in-situ, the largest grains were found in the Fe-0.18%C alloy for both cooling rates. This important observation is attributed to the higher austenitizing temperature of a Fe-0.18%C alloy compared to that of the other Fe-C alloys studied. Moreover, the formation of columnar austenite grains were observed in this alloy on cooling, whereas equi-axed grains were formed in the other two Fe-C alloys. By such columnarization, the surface cracking susceptibility of the peritectic grade steel will be accelerated. The hot-ductility of the Fe-C alloy of near-peritectic composition was the lowest of the alloys studied and the inferior ductility in this alloy is attributed to the coarse grain size and columnar shape of the grains. The Fe-0.45%C alloy had the smallest grain size at any specimens cast in-situ but the hot-ductility was much lower than would have been expected in an alloy of such small grain size. This much reduced ductility may be due to increased grain boundary sliding. At a higher cooling rate of the in-situ melted specimens, smaller grains were produced in all the Fe-C alloys resulting in ductility improvement. This grain refinement obtained at higher cooling rates have important implications for near-net shape casting operations such as thin-slab casting or strip-casting where much higher cooling rates are realized than in the conventional casting process, if the factors related with precipitation are removed. An important insight derived at through this study was that enlarged grain contributed more to reduced ductility at high temperature than did cast structure, at least under the pertaining experimental conditions. This observation has important practical implications because it means that efforts in industry could be concentrated on reducing the chances of forming large austenite grains, such as at the roots of oscillation marks due to a decreased cooling rate, without undue regard to the effect of these measures on cast structure. This study has provided convincing new experimental evidence of the extremely detrimental effect of large austenite grains on hot-ductility in plain carbon steels. The very large columnar shaped grains that can form in alloys of near-peritectic composition is particularly disconcerting. However, the influence of AlN precipitation on austenite grain boundaries on hot-ductility was not studied and it is recommended that this important topic should be included in subsequent investigation. The experimental data on these Fe-C alloys, provided in this study, may now be used to benchmark and further analyse the much larger body of information on low-alloyed steels available in the literature.

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