Influence of Micro-alloying Additions on the Hot Ductility of Steels Heated Directly to the Test Temperature
1987 Volume 27 Issue 5 Pages 366-375
Details
1987 Volume 27 Issue 5 Pages 366-375
The hot ductility of a series of micro-alloyed steels has been obtained after heating directly to test temperatures in the range 600 to 1100°C with strain rates between 3×10-2 and 3×10-4s-1.
At a strain rate of 3×10-2s-1, ductility as measured by reduction of area was generally high for all the steels examined.
At a strain rate of 3×10-3s-1, all the steels which showed some grain boundary precipitation, i.e., the C-Mn-Al, C-Mn-Al-Ca, C-Mn-V-Al, and C-Mn-Nb-Al steels exhibited ductility troughs in the temperature range 750 to 1000°C. It is believed that these troughs are caused by grain boundary precipitation retarding the onset of dynamic recrystallisation. The depth of the troughs was generally dependent on the degree of grain boundary precipitation and matrix precipitation, except in the case of the C-Mn-Al-Ca steel which gave only a shallow trough, even though extensive precipitation occurred at the boundaries. Steels in which no grain boundary precipitation occurred, i.e., C-Mn and C-Mn-Ti-Al steels did not show a ductility trough giving excellent hot ductility throughout the temperature range examined.
Decreasing the strain rate further to 3×10-4s-1 deepened and broadened the troughs in the Al, V, and Nb containing steels and introduced a ductility trough in the temperature range 800 to 900°C for the C-Mn steel. Failures in the troughs in all these cases were intergranular by grain boundary sliding in the γ and in the case of the plain C-Mn steel, dynamic recrystallisation was observed even at the temperature giving the minimum ductility. It would appear that the adverse effects of decreasing the strain rate on hot ductility is due to strain concentration at the boundaries, and to reduced grain boundary migration rates which favour intergranular failure. Again at this low strain rate the C-Mn-Ca-Al steel although showing a ductility trough, gave very good ductility compared with these other steels. The C-Mn-Ti-Al steel also gave excellent hot ductility, no ductility trough being formed even at the lowest strain rate of 3×10-4s-1. Possible explanations to account for the superior hot ductility of the Ca and Ti containing steels are discussed.
