T he influence of small additions of titanium on the hot ductility of C-Mn-Nb-Al steels has been examined. T itanium and nitrogen levels varied in the ranges 0•014-0•045 and 0•004-0•011 wt-%, respectively, so that a wide range of T i/N ratios could be studied. T he tensile specimens were cast and cooled at average cooling rates of 25, 100, and 200 K min−1 to test temperatures in the range 1100-800°C and strained to failure at a strain rate of 2×10−3 s−1. It was found that ductility in the titanium containing niobium steels improved with a decrease in the cooling rate, an increase in the size of the titanium containing precipitates, and a decrease in the volume fraction of precipitates. Coarser particles could be obtained by increasing the T i/N ratio above the stoichiometric ratio for T iN and by testing at higher temperatures. However, ductility was generally poor for these titanium containing steels and it was equally poor when niobium was either present or absent. For steels with ~0•005 wt-%N ductility was very poor at the stoichiometric T i/N ratio of 3•4 : 1. Ductility was better at the higher T i/N ratios but only two of the titanium containing niobium steels gave better ductility than the titanium free niobium containing steels and then only at temperatures below about 950-900°C. One of these steels had the lowest titanium addition (0•014 wt-%), thus limiting the volume fraction of fine T i containing particles and the other had the highest T i/N ratio of 8 : 1. However, even for these two steels ductility was worse than for the titanium free steels in the higher temperature range. T he commercial implications of these results are discussed.MST /4232

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A beneficial effect of the copper on the hot ductility was observed in Ti-Nb microalloyed steels over the temperature range 800-1,000 °C at the cooling rate of 0.4 °C/s, but no influence at the cooling rate of 4 °C/s. Precipitates containing Nb and Ti were present whose size was coarser in the Cu-bearing grade as cooled at 0.4 °C/s. Cu-bearing precipitates were not found. In the C-Mn-Al steel, no influence of the copper on the hot ductility was recorded, but CuS particles were detected. Two mechanisms are proposed to explain the positive influence of the copper in the microalloyed steel. The first is that the copper atoms in the solid solution affect the activity of the carbon and the nitrogen analogically to the previously observed effect of the silicon, enhancing the precipitation at high temperatures, and another mechanism that the copper atoms can prolong the lifetime of vacancies generated by straining assisting the formation of TiNb-vacancy complexes and thereby coarsening the precipitates.

A novel approach on the problem of hot cracks in Cu-containing steels is proposed to prevent the hot shortness. Cu-containing steels have been hot tensile tested to failure in argon or vacuum at different cooling rates and temperatures, followed by Scanning Electron Microscopy. EDX analysis both in fractures and cross section show that inclusions are very important in causing the Cu concentration. Although CuS precipitates slightly deteriorate the hot ductility between the Ae 3 and Ar 3 temperatures, the Cu concentration around inclusions can highly contribute to hot shortness by the formation of Cu-containing liquid phase. Laboratory results combine with thermodynamic calculations show that a Cu-containing liquid associated with inclusions at %1200 8C is clearly the cause of the hot shortness. A distinction of hot ductility and hot shortness is proposed, concerning to their very different cracking mechanisms. The interpretation of the hot tensile test for each testing condition and temperature range is discussed. It is concluded that the oxidation is not the principal cause of the increase of Cu concentration, so its importance is relative in the laboratory assessment of hot cracking. Consequently, inclusions cannot be ignored in assessing and preventing the hot shortness, and commercial implications are drawn from the results.

An extensive investigation into the role of low copper contents on the hot ductility of C-Mn-Al steels has been carried out in order to better understanding the problem of "hot shortness". Previous work has suggested that this problem results from the build up of Cu that occurs at the surface of the steel as a consequence of the preferential oxidation of iron. This causes the formation of a Cu-rich film of low melting point. A nickel addition has been reported as a solution to the problem, since it increases the solubility of copper in the austenite. After hot tensile testing to failure, samples of Cucontaining steels have been examined using optical, scanning and transmission electron microscopes. Recently published results indicate that copper, in addition to precipitating out as CuS, also segregates to MnS inclusions forming a shell around them. This does not seem to impair the hot ductility under an inert atmosphere, but may have serious consequences under an oxidising environment and lead to "hot shortness". The influence of nickel in improving the hot ductility seems to be due to it forming a higher melting point alloy with the segregated copper. Data from this incomplete work also suggests that Ni reduces the precipitation of CuS particles. Current investigation carried out using confocal microscopy still to be completed will improve the understanding of the role of copper and nickel on the problem of "hot shortness".