Abstract
This thesis studies the factors that govern transverse cracking during continuous casting
of low carbon, niobium microalloyed and boron microalloyed steels. Crack susceptibility
in the thick slab, billet and thin slab casting processes are compared by using typical
conditions in laboratory hot ductility tests.
There is limited published literature on hot ductility in aluminium-killed and siliconkilled
boron microalloyed steels and the proposed mechanisms of failure by transverse
cracking are contradictory. Few published papers specifically compare hot ductility
behaviour of any steels between thick slab, billet and thin slab continuous casting
processes. Thus, the basis of this research is to assess the influence of casting parameters
and compositional variations on hot ductility behaviour in low carbon steels, niobium
microalloyed steels, aluminium-killed boron microalloyed steels and silicon-killed, boron
microalloyed steels.
The typical temperature ranges, cooling rate and strain rate conditions of the continuous
casting processes were used in reheated and in situ melted hot tensile tests performed on
steel specimens. Solidification, transformation and precipitation temperatures were
calculated using solubility equations and modelled using the Thermo-CalcTM
thermodynamics program. Scanning electron microscopy and transmission electron
microscopy were used to determine the modes of failure in the tested specimens.
In the low carbon steels, hot ductility was improved by increasing the strain rate; by
calcium treatment, which minimises copper sulphide and iron sulphide formation; and by
maintaining a nickel to copper ratio of 1:1. It was shown that thin slab casting conditions
provided the best hot ductility results for the low carbon steels.
All the niobium steels showed poor ductility in the single-phase austenite temperature
region, indicating that intergranular precipitation of fine niobium carbonitrides was the
cause of the poor ductility. It was shown that the hot ductility was greatly improved by
calcium treatment, by decreasing the cooling rate and by increasing the strain rate. Slow
iv
thin slab and thick slab casting conditions provided the best hot ductility results for the
niobium steels.
Hot ductility was substantially improved in the aluminium-killed boron steels by
increasing the boron to nitrogen ratio from 0.19 to 0.75. The results showed that, at
cooling rates generally associated with thick slab, bloom and slow thin slab casting, a
boron to nitrogen ratio of ≥0.47 was sufficient to avoid a ductility trough altogether.
However, under conditions typically experienced in fast thin slab and billet casting, a
boron to nitrogen ratio of 0.75 was required to provide good hot ductility. The mechanism
of the ductility improvement with increasing boron to nitrogen ratio was found to be
enhanced precipitation of boron nitride, leading to a decrease in nitrogen available for
aluminium nitride precipitation.
In the silicon-killed boron steels, it was found that the boron to nitrogen ratio had the
overriding influence on hot ductility and hence on crack susceptibility. Excellent hot ductility
was found for boron to nitrogen ratios above 1. Additionally, analysis of industrial casting data
showed that the scrap percentage due to transverse cracking increased significantly at
manganese to sulphur ratios below fourteen. An exponential decay relationship between the
manganese to sulphur ratio and the average scrap percentage due to transverse cracking was
determined as a tool to predict scrap levels in the casting plant.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/6499 |
| Date | 26 February 2009 |
| Creators | Chown, Lesley H. |
| Source Sets | South African National ETD Portal |
| Language | Spanish |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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