The influence of thermal history on the hot ductility of Ti containing C Mn Al and C Mn Nb Al steels
A series of C-Mn-Al and C-Mn-Al-Nb steels having nominal composition 0.1 %C, 1.4%Mn, 0.3% Si and 0.005%N with Ti additions from 0 to 0.013% have had their hot ductility determined over the temperature range 700-1100°C. Tensile specimens were cast (melted) in situ and cooled at a rate of 100°C/min to the test temperature. They were subsequently strained to failure using a strain rate of 3x 10-3 s '. The influence of the addition of a 100°C undercooling step into the test cycle with a subsequent re-heat to the test temperature (at 500°C/min) was investigated. It has been shown that Ti additions, both to C-Mn-Al and C-Mn-Nb-Al steels impair hot ductility. Also, Nb containing steels give worse hot ductility with or without Ti additions for the steels examined. Thermal history was seen to have a small but significant effect on the hot ductility of steel. The addition of a 100°C undercooling step generally resulting in worse ductility due to additional precipitation of AIN and in the case of Nb containing steels probably both AIN and NbCN. TiN precipitation has been shown to have a more significant detrimental effect on hot ductility than AIN precipitation when one thermal cycle is introduced. Two regression equations have been obtained which may show that P is beneficial to ductility in Nb, Ti containing steels but detrimental to ductility in CMn-AI-Ti steels. Results have shown that increasing the test temperature (which will encourage coarser particles) generally leads to an increase in the dimple size and to better ductility. Interestingly, adding Ti to steels causes a larger dimple size to occur even though ductility deteriorates as the formation of TiN removes small AIN particles from solution. Thus the dimple size increases as it relates more and more to the presence of the larger MnS inclusions. The addition of an undercooling step in the test cycle reduces the dimple size in accordance with more precipitation taking place, most likely from AIN. During the work program it was discovered that there was potential for Ti to be lost should tensile samples be melted within silica tubes. Re-testing with an alumina based sheath ensured no Ti loss could occur.