Influence of isothermal treatment on MnS and hot ductility in low carbon, low Mn steels

Hot ductility tests were used to determine the hot-cracking susceptibility of two low-carbon, low Mn/S ratio steels and compared with a higher-carbon plain C-Mn steel and a low C, high Mn/S ratio steel. Specimens were solution treated at 1623 K (1350 °C) or in situ melted before cooling at 100 K/min to various testing temperatures and strained at 7.5 x 10-4 s -1, using a Gleeble 3500 Thermomechanical Simulator. The low C, low Mn/S steels showed embrittlement from 1073 K to 1323 K (800 °C to 1050 °C) because of precipitation of MnS at the austenite grain boundaries combined with large grain size. Isothermal holding for 10 minutes at 1273 K (1000 °C) coarsened the MnS leading to significant improvement in hot ductility. The highercarbon plain C-Mn steel only displayed a narrow trough less than the Ae3 temperature because of intergranular failure occurring along thin films of ferrite at prior austenite boundaries. The low C, high Mn/S steel had improved ductility for solution treatment conditions over that of in situ melt conditions because of the grain-refining influence of Ti. The higher Mn/S ratio steel yielded significantly better ductility than the low Mn/S ratio steels. The low hot ductility of the two low Mn/S grades was in disagreement with commercial findings where no cracking susceptibility has been reported. This discrepancy was due to the oversimplification of the thermal history of the hot ductility testing in comparison with commercial production leading to a marked difference in precipitation behavior, whereas laboratory conditions promoted fine sulfide precipitation along the austenite grain boundaries and hence, low ductility

Delta-Ferrite Recovery Structures in Low Carbon Steels

The development of delta-ferrite recovery sub-structures in low carbon steels has been observed in-situ utlhsing laser scanniJlg confocal microscopy (LSCM). Well developed subboundaries with interfaCial\u27 energies much smaller than that of delta-fen\u27ite grain boundaries fOfmed following transformation from austenite to delta-fen\u27ite on heating. Transfonnatioll stresses associated with the austenite to delta-fenite phase transt\u27onnation generate dislocations that subsequently recover into sub-boundaries by a process of polygonisation. Experimental evidence in suppol1 of this proposal was found in a ferritic stainless steel. Thermal cycling through the high temperature delta-ferrite/austenite/delta-fel1\u27ite phase b\u27ansformation leads to the development of a strong recovery substructure, wh.ich in turn. modifies the low temperature austenite decomposition product from WidmansUilten/bainite to polygonal fen\u27ite, with a commensurate chauge in hardness

Transverse surface cracks in continuously cast steel slabs, oscillation marks and austenite grain size

Transverse surface cracks in low carbon steel slabs are invariably inter-granular and follow the soft ferrite films outlining the grain boundaries of exceptionally large prior-austenite grains often found at the roots of oscillation marks in continuously cast low-carbon steel slabs. Plastic deformation is concentrated in the ferrite films and cracks initiate in the ferrite films, leading to crack propagation along the austenite grain boundaries. Hot-ductility is significantly reduced by an increase in austenite grain size and in situ observations revealed that depending on the cooling rate, austenite can nucleate and grow by diffusional mechanisms or forms by a massive type of reaction. The delta-ferrite transformation has also been studied by using neutron diffraction techniques and high-energy X-rays in a synchrotron

High-temperature phase transitions and mechanical properties in steel of peritectic composition

Continuous casting of steels of peritectic composition has proven to be problematic mostly because of surface crack occurrence. We have studied in situ the peritectic reaction as well as the kinetics of the subsequent peritectic transformations using the so-called concentric solidification technique in a high-temperature laser-scanning confocal microscope. We have also assessed grain growth in reheated as well as in situ cast Fe-C specimens in a Gleeble 3500 thermo-mechanical simulator. Our findings will be discussed with reference to the observation in practice that so-called \u27blownout\u27 grains are associated with oscillation marks in continuously cast slabs

Strain-induced phase transformation during thermo-mechanical processing of titanium alloys

Strain-induced phase transformation studies have been conducted in a Ti-6Al-2Sn-4Zr-6Mo alloy. This alloy was subjected to b sub-transus deformation upon slow cooling from the b-phase field and the effect of strain on the extent and morphology of the phase transformation during hot-deformation was determined. In addition, the transformation kinetics and the morphology of the newly formed a-phase were studied following deformation under different cooling conditions. By applying strain, the rate of the b-to-a phase transformation increased significantly during deformation as well as during slow cooling following deformation. This increase in the kinetics of the b-to-a transformation can be ascribed to straininduced phase transformation. Also, the extent of deformation of the b-phase had a marked effect on the resulting morphology and size of the newly formed a-phase. Transformation of un-deformed b-phase rendered a-phase of acicular morphology only. However, following deformation of the b-phase, acicular as well as globular a-phase morphologies have been observed upon cooling