Abstract:
The aim of this project is to investigate and show the differences in the hot ductility and high temperature fracture behaviour of vanadium containing steels in the temperature range of 600° C to 1000° C. The steels are continuously-cast as slabs by BHP-New Zealand Steel Ltd and all contain small amounts of vanadium carried over from the iron sand ore. The steels investigated are low-vanadium C-Mn steels (0.0 7wt% V, 0.0 3 5-0.19 5wt% C) and vanadium microalloyed steels (0.012wt% V). An understanding of this behaviour is important to avoid the formation of transverse cracks on the surface of the cast slabs. Within the temperature range investigated most steels are expected to show a drop in ductility and it is also within this temperature range that the slabs are unbent. The unbending or straightening of the slabs in this low ductility range is believed to propagate or initiate surface cracks. To obtain a measure of the hot ductility of the steels, high temperature tensile tests were done on samples cut from as-cast slab blocks. All of the apparatus required for this testing had to be designed and built. Following fracture the specimens were sectioned and examined via optical and scanning electron microscopy. Specimens were also quenched from the test temperature to examine the microstructure prior to testing. Apparatus was built to measure the thermal expansion of steel rods to obtain the austenite to ferrite transformation temperatures. All of the steels showed a drop in ductility when they existed as a two-phase mixture of α-ferrite and γ-austenite. Accompanying the change in the ductility behaviour were changes in the fracture mode. At low temperatures, below the ductility drop the steels fractured in a transgranular ductile manner, at the highest temperatures when the ductility was greatest, fracture was via ductile rupture. Within the low ductility region the steels fractured in an intergranular manner. The intergranular fracture and the drop in ductility resulted from the low rate of deformation and the presence of a proeutectoid ferrite layer, which formed on cooling along the austenite grain boundaries. The low strain rate promotes grain boundary sliding, while as the steel is deformed the strain is concentrated in the soft ferrite layer. Fracture along the soft ferrite layer is via microvoid coalescence. As the fraction of ferrite increased at low temperatures, decreasing the strain concentration at the grain boundaries, the ductility improved. At higher temperatures, in the austenite region, the ductility improved in the absence of a ferrite layer and with the onset of dynamic recrystallization. The V-microalloyed steels showed low ductility within the austenite region as the onset of dynamic recrystallization was delayed. In the absence of a ferrite layer in the austenite region, the action of grain boundary sliding resulted in decohesive intergranular fracture of the V-microalloyed steels. Models of the intergranular fracture behaviour were developed for the different types of intergranular fracture. Using the values of the tensile strength of the steels it was shown that the change from transgranular to intergranular fracture could be related to an old concept of the "equi-cohesive temperature", a temperature above which the grain boundary becomes strength limiting rather than the grain bulk. The equi-cohesive temperature corresponds to the temperature at which a thin proeutectoid ferrite boundary layer forms. This is discussed and related to the hot ductility behaviour. Reduction of area curves obtained for all of the steels show the temperature range of low ductility and the variation of the depth of the ductility trough with composition. These can be used to gauge the susceptibility to cracking at a given temperature. The low vanadium, C-Mn steels all had ductility minima between the Ae3 and Ae1 temperatures, which can be determined empirically. These steels also show a strong ductility dependence on carbon content, which is shown in the hot ductility curves. This behaviour is not shown by higher Mn and Si steels. The low vanadium content of the C-Mn steels did not reduce the ductility of these steels. Therefore the transverse cracking experienced by these steels must be related to other matters to do with the operation of the casting machine, rather than the metallurgical considerations of this investigation. The vanadium micro-alloyed steels showed extended regions of low ductility into the austenitic range, which can not be empirically determined as easily as the C-Mn steels. The hot ductility curve can be used to indicate regions of low ductility or high crack susceptibility.