Influence of Dynamic Recrystallisation on the Tensile Ductility of Steels in the Temperature Range 700 to 1150.DEG.C..

Abstract

The role of dynamic recrystallisation (DRX) in influencing the hot ductility of plain C-Mn and microalloyed steels was examined by comparing the critical strain for Dynamic recrystallisation with the fracture strain in a hot tensile test. The temperature range examined was 700 to 1150°C and the strain rates were varied from 3×10–2 to 3×10–4 s–1.For coarse grained plain C-Mn and C-Mn-Al steels solution treated at 1330°C and cooled to the test temperature, the presence at the γ grain boundaries of thin films of deformation induced ferrite at temperatures between the Ae3 and the undeformed Ar3, leads to strain concentrations which give rise to poor ductility. The presence of these thin films prevents the occurrence of DRX. For these steels, the Ae3 temperature, which marks the onset of good ductility is generally high enough to lead to DRX, so that it is not possible to assess its independent contribution to restoring the hot ductility. In coarse grained C-Mn-Nb-Al steels, DRX and the full recovery of ductility are often not observed until the test temperature is higher than 1000°C. This is related to the strain-induced precipitation of NbCN below 1050°C. However, even when recrystallisation is not possible, the ductility can be improved if the amount of strain-induced NbCN is reduced.For fine grained plain C-Mn and microalloyed steels heated directly to the test temperature, DRX often occurs in the trough. Grain boundary migration rates have to be sufficiently high to prevent crack linkage from occurring, and this often necessitates the resolution and coarsening of particles so that they are no longer effective in pinning the boundaries. Finally, of interest in this work was the observation that as the initial grain size do is refined, its influence in encouraging DRX becomes more marked than that given by the simple do1/2 relationship in the equation εp=Ado1/2Zn, where εp is the critical strain to the peak stress, Z is the Zener-Hollomon parameter and A and n are constants.