City Research Online

Hot ductility of steels

Abu Shousha, R. I. (1991). Hot ductility of steels. (Unpublished Doctoral thesis, City, University of London)

Abstract

The hot tensile test has been used for to examine the influence of such factors as precipitation, composition, inclusions, phase transformation and grain size on hot ductility. Tests were performed mainly in the as-cast condition but also after solution treatment (1330° C) so as to obtain a coarse grain size similar to that found in the continuously cast slab process. Test temperatures in range of 750-1000°C and strain rates between 10-3s-1-3x10-3s-1 inclusive were examined, these being the temperature range and strain rate in which the straightening operation is performed. The ductility trough arises as a result of stress and strain concentrations at the austenite boundaries caused by the presence of thin films of the softer deformation induced ferrite enveloping the austenite grains, or by grain boundary sliding in the austenite. Both failure mechanisms are encouraged by the presence of grain boundary precipitation and inclusions.

The depth and width of the trough is dependent on the size and volume fraction of these precipitates. Dynamic recrystallization on is also found to be a very important factor controlling the ductility trough at the high temperature end of the trough. Precipitation and inclusions can retard the dynamic recrystallization so extending the trough to higher temperatures. Reheating the C-Mn-Al low S level steels after casting can be beneficial to the hot ductility as a result of refining the grain size. For C-Mn-Nb-Al and C-Mn-Al high S level steels reheating causes the precipitation of NbCN and FeMnS in the latter, which are finer and have a closer inter-particle spacing than in the as-cast state. These particles precipitate on the austenite grain boundaries and within the matrix during the tests leading to a delaying of the onset of dynamic recrystallization. Grain boundary sliding and cavitation are enhanced leading to poor ductility.

Improved hot ductility has been achieved by adding Ca to steels which reduces the amount of sulphides inclusions precipitated on cooling from the as-cast condition to the test temperature. Ti additions give rise to only a slight improvement in hot ductility in the as-cast condition when the cooling rate to the test temperature is 100 °C/min but is very beneficial when the cooling rate after casting is reduced to 25°C/min. This probably arises because coarse Ti rich particles allow the Nb to precipitate out at high temperature leaving less available to precipitate out in a detrimental form at lower temperature. Raising the C level was found to move the ductility troughs to lower temperatures as a result of lowering the transformation temperature.

These troughs are controlled by the presence of deformation induced ferrite which forms in the temperature range between the Ar3 and Ae3. It is found that removal of deformation induced ferrite leads to dynamic recrystallization. Ductility is found to deteriorate when Al is added to steels, and the AIN solubility product is high, 2 to 3x10-4. Increasing the solution treatment time probably allows the AlN to precipitate out in a finer form giving inferior ductility. Addition of Cu to the C-Mn-Nb-Al steels can be detrimental to the hot ductility especially when the as-cast steels were tested in air. The segregation of Cu to the boundaries and the oxidation of Fe at the surfaces leads to Cu enrichment and hot shortness.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TJ Mechanical engineering and machinery
T Technology > TN Mining engineering. Metallurgy
Departments: Doctoral Theses
Doctoral Theses > School of Science & Technology Doctoral Theses
School of Science & Technology > Engineering > Mechanical Engineering & Aeronautics
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