Influence of Al and P additions on hot ductility of steels

Abstract

The hot ductility curves of a series of P containing, TRIP type steels (0·08–0·1%P) with Al levels from 0·03 to 0·87% have been determined. The tensile specimens were cast in situ, cooled at 60°C min⁻¹ to the test temperature in the range of 1200–700°C and strained at 3 × 10⁻³ s⁻¹ to failure. The reduction of area (R of A) was taken as the measure of ductility. Whereas the high P addition was found to have little influence on the hot ductility behaviour, the high Al additions resulted in a widening of the trough to higher temperatures. Thin films of deformation induced ferrite were observed throughout the wide temperature range from the Ae₃ to the Ar₃. Ductility did not fully recover at the high temperature end of the trough until the temperature was high enough for there to be little precipitation of AlN particles and to when ferrite films can no longer form making it easy for the austenite grains to dynamically recrystallise. Which of these possibilities is responsible for this extension of the trough at the high temperature end is not clear, although there is some evidence suggesting it may be the ferrite film forming at temperatures above the Ae₃, due to segregation of Al to the γ grain boundaries. Steels with 0·2–0·4%Al also gave R of A values <40% in the temperature range of 900–750°C indicating that they may be susceptible to transverse cracking during continuous casting. Nevertheless, the steel with the highest Al content (0·87%) gave R of A values comfortably in excess of 40%. This is due to the AlN precipitates being so coarse as to not have any significant influence on ductility. The troughs for these steels are also extended at the low temperature end, ductility only recovering when the temperature falls below the Ar₃ temperature. This extension of the trough at the low temperature end is probably a consequence of the high Mn level (1·5%) and high Al levels, preventing growth of the deformation induced ferrite films. The Al containing steels generally had very low N levels (0·002%) and higher N levels would be expected to extend the troughs to even higher temperatures, because the increased volume fraction of AlN may lead to deeper troughs.

Keywords:

• TRIP STEELS
• HIGH AL AND P
• HOT DUCTILITY